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Understanding Circuit Breaker Labeling: What Does “20AF 20AT 3P” Mean?

by John Carter | Jul 17, 2023 | Design

As a vital safety mechanism in every electrical circuit, circuit breakers play a crucial role in preventing damage caused by overcurrent conditions like short circuits and overloads. But the nitty-gritty details inscribed on the labels of these devices can seem like a foreign language to the uninitiated. One such intriguing label you might come across is “20AF 20AT 3P”. Let’s break down this code and look at some potential variations.

Interpreting the Label “20AF 20AT 3P”

The inscription “20AF 20AT 3P” provides specific details about the circuit breaker’s operational capabilities.

• “20AF” refers to the Ampere Frame (AF) size. The AF size is essentially the physical size of the breaker and its associated interrupting capacity. In this case, the circuit breaker is designed for a 20-ampere frame size. This defines the highest continuous current load that the breaker can handle under normal operating conditions.
• “20AT” indicates the Ampere Trip (AT) rating, which is the current level at which the breaker will trip to interrupt the current flow. It acts as a protective measure to prevent equipment damage from excess current. In this scenario, the breaker will trip if the current exceeds 20 amperes.
• “3P” denotes that this is a three-pole circuit breaker. In three-phase power systems, this means the breaker controls three different circuits that share a common trip mechanism. If a fault occurs on any of the three lines, the breaker will trip and disconnect all three phases.

Variations in Circuit Breaker Ratings

These ratings can and do vary widely depending on the specifics of the electrical circuit they’re designed to protect.

For instance, circuit breakers for large industrial applications might have an AF rating of several hundred or even thousands of amperes to accommodate higher load equipment. Conversely, a circuit breaker designed for residential applications might have an AF rating as low as 15 amperes to protect small household appliances.

Similarly, the AT rating can vary to match the load’s characteristics it protects. A piece of equipment with a large inrush current (like motors) may need a breaker with a higher AT rating, allowing a brief surge of current without tripping. However, sensitive electronics may need a lower AT rating to ensure the breaker trips at a lower current, providing finer protection against overcurrent.

Pole count (“P”) can also differ based on the application. Single-phase systems typically use 1P or 2P breakers, while three-phase systems use 3P breakers. A 1P breaker controls a single line, a 2P controls two lines, and a 4P breaker would control three lines plus a neutral in some specific scenarios.

The seemingly cryptic labels on circuit breakers actually provide a great deal of information about the device’s capabilities and applications. Understanding these labels and the ways they can vary will help electrical engineers and electricians select the right breaker for any given application.

Every installation requires a delicate balance between safety and operational requirements, and these labels are the keys to striking that balance. In the end, the choice of a circuit breaker, be it a 20AF 20AT 3P or any other rating, comes down to understanding the needs of the system it protects.

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How to Calculate Circuit Breaker Trip Settings

Each circuit breaker in your breaker panel is labeled with the maximum amperage (current) capacity for that circuit. This may differ among circuits, so always check each breaker's capacity individually when calculating the electrical load that will cause that breaker to trip. Standard household circuits in the U.S. are 120 volts, but some circuits have double that capacity for appliances such as stoves and air conditioners. These breakers have 240-volt capacities and will be approximately twice the size of 120-volt breakers. Wattage is the easiest measure of power load to calculate and monitor so you don't accidentally trip your breakers.

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Look for the amperage notation on the breaker switch. This will generally be 15 or 20. Also look for the voltage notation, which may be on the breaker switch as well, and will be 120 or 240. If you cannot locate the voltage, assume that breakers that take up one panel slot are 120 volts and breakers that take up two slots are 240 volts.

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Multiply the amps by the volts. In most circuits, this will be 20 x 120 = 2400 or 15 x 120 = 1800. The number resulting from this equation is the maximum wattage load you can place on the circuit before tripping the breaker.

Apply the same calculation to 240-volt circuits. For example, a 240-volt circuit with a 30-amp capacity would allow 7200 watts (30 x 240 = 7200).

Check the wattage for all electrical fixtures and appliances on the circuit. If the total wattage is over your maximum calculation, the breaker will trip.

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Sizing A Circuit Breaker: Breaker Size Calculator + Amp Chart

Sizing a circuit breaker is never easy. But it’s also not all that difficult . Everybody knows that we need an adequately sized circuit breaker that allows for sufficient electric current. If we undersize a breaker, the breaker will likely catch on flame. No pressure here.

How do you go about picking the correct circuit breaker size? Do you need a 10A, 15A, 20A, 30A, 40A, 50A breaker, etc?

Standard breaker sizes are 15A, 20A, 25A, 30A, 35A, 40A, 45A, 50A, 60A, 70A, 80A, 90A, 100A, 110A, 125A, 150A, 175A, 200A, 225A, 250A, 300A, 350A, 450A, 500A, 600A, 700A, 800A, 1000A, 1200A, 1600A, 2000A, 2500A, 3000A, 4000A, 5000A, and 6000A.

Sizing a circuit breaker is actually quite easy. You just need to know a couple of rules. These are:

• 80% NEC breaker rule. This is the most basic NEC (National Electric Code) rule that states that you can’t push the current over 80% of its specified ampacity. Example: If you have a 20 amp breaker, you can only allow for a 16A current. 16A is 80% of the max. the specified ampacity of the circuit breaker. This is a safety measure; you better have a bit of overhead to prevent the circuit from frying. You can read the full Article 240.4(B) in NEC 2014 on this here .

If you know how to calculate the amps and account for the 80% breaker rule, you can calculate the size of the breaker yourself.

To help everybody sizing these breakers out, we will explain how to determine the right size of a breaker. On top of that, we include a Circuit Breaker Size Calculator further on (just insert watts and volts, and you get the correct breaker size).

At the end, we also included the ‘just tell me the breaker size I need’ Breaker Size Chart that tells you what breaker size you need for devices with different wattages (from 50W units to big 20,000W devices).

Let’s look at how breaker size can be calculated manually (you can also use the calculator or/and chart below):

Table of Contents

How To Calculate Size Of A Circuit Breaker?

This is the easiest to explain with an example.

Let’s say that we have a simple 1,500-watt space heater running on a standard 120V circuit. What size amp breaker do you need for a 1,500-watt space heater?

First, you need to calculate how many amps does this heater draw like this:

Current (Amps) = Power (Watts) / Voltage (Volt)

In our situation this is:

Current = 1,500W / 120V = 12.5 Amps

Now we know that the 1,500W space heater draws 12.5 amps. We have to account for the 80% breaker rule. This means that these 12.5 amps should represent 80% of the breaker amps. To calculate the size of the circuit breaker needed, we have to multiply the amp draw by 1.25 factor like this:

Minimum Circuit Breaker Size = 12.5A × 1.25 = 15.63 Amps

We can’t use a 15A breaker because the breaker ampacity should be at least 15.63A. The next breaker size is 20 amps; that means we need to use a 20A breaker for a 1,500W space heater running on 120V standard circuit.

Here is the basic step-by-step procedure we did to determine the size of a circuit breaker:

• Calculate the amp draw. We use the basic electric power equation for this. If we know the wattage and voltage, we can quite easily calculate the amp draw.
• Multiply amp draw by 1.25 to account for the 80% breaker rule. The resulting amps are the minimum ampacity a correctly sized circuit breaker should have.
• Choose a circuit breaker size. We usually pick between 10A, 15A, 20A, 25A, 30A, 35A, 40A, 50A, 60A circuit breakers, and so on.

This is how breaker sizing is done manually. The easiest way is to use a dynamic calculator. You simply input that wattage and the voltage, and the calculator will tell you what is the minimum size of a circuit breaker you need. You can use this calculator here:

Circuit Breaker Size Calculator

Here is how this breaker sizing calculator works:

Let’s say you have a big 5,000W air conditioner (this is usually a 5-ton unit). It runs on a 220V circuit. What size circuit breaker do you need?

Just slide the wattage slider to ‘5000’ and voltage slides to ‘220’ and you get ‘28.41 Amps’. Therefore you need a circuit breaker with at least 28.41A ampacity. 25A breaker is too small; you need a 30A breaker .

You can do this for literally any device running on any voltage. You can also play around with numbers to see how the amps change.

If you want the ‘just tell me the circuit breaker I need’ you can consult this chart:

Breaker Size Chart (For 120V And 220V Circuits)

You just need to know the wattage of the device you need a circuit breaker for and you can check what size breaker you need if you run it on a standard 120V circuit or an upgraded 220V circuit:

As you can see, calculating what size breaker you need is not all that hard. Of course, with bigger amp draws, you can connect several 30A or 50A in parallel to increase the total breaker ampacity.

We hope this illustrates how everybody can figure out the size of circuit breaker they need. If you have any questions regarding this breaker sizing, you can use the comments below and we’ll try to help you out.

Related posts:

• 50 Amp Wire Size Details: Gauge, Breaker, 220/240V Example
• Thermostat Not Reaching Set Temperature: 7 Causes + Fixes
• Wire Gauge Wattage Charts For AWG Wires (4/0 AWG To 14 AWG)
• How Many Outlets On A 15 Amp, 20 Amp, 30 Amp Circuit? (NEC 210.21)
• Thermostat Wire Color Codes For 3-8 Wire Thermostats (Color-By-Color)

12 thoughts on “Sizing A Circuit Breaker: Breaker Size Calculator + Amp Chart”

Thank you, this is very help full.

very much help full

Excelente information. Thank you.

The most amazing easy method. very Helpfull.

Thank you, Imran, we try to simplify it as much as possible. It’s nice to see a bit of recognition.

Helpful. 1,500-watt space heater running on a standard 120V circuit. What size amp breaker do you need for a 1,500-watt space heater? how can I use 120 v circuit.

Hi Masum, alright, 1500W heater on 120V draws 1500W/120V = 12.5 amps. To apply the 80% rule, you have to multiply this current by 125% like this: 12.5A × 1.25 = 15.63 amps. So, a 15A breaker won’t cut it, but a 20A breaker will be great for a 1500-watt space heater on a 120V circuit. Hope this helps.

Hi I have a 1200 watt generator , the breaker has gone bad and there is no size printed on the breaker. Im not sure if you can calculate the size the same way you would for a AC unit. What size would you suggest ? Thanks

Hi Renald, the size of the breaker you need for a 1,200 watt generator really depends on the amps it will give out. You can calculate the amps if you know the voltage. Generators can have 12V DC, 24V DC, 110-120V AC, 240V AC voltages. Example: Let’s say your 1200W generator has a 24V DC voltage. You can calculate the amps like this: 1200W/24V = 50 Amps. This is the current in the wire. You can calculate the minimum required breaker size for this generator like this: 50 Amp × 1.25 = 62.5 Amps. In this case you can go for 70A breaker, or 3x30A breakers, since their capacity if north of 62.5 amps. Hope this helps.

This article is flawless thank you!

Great info! Much appreciated but I’m trying to determine what breaker or breakers I need for my bass boat. 3x 12V in series for my 36V trolling motor and 1x 12 V for my electronics and starting battery which also powers the various pumps and aerators. Any help would be greatly appreciated!

Hi Tom, the wattage you need is the key here. Electronics will have lower wattage but pumps and aerators… that should like high wattage stuff. Example: If you would put in 60A amp breaker on 36V voltage, and apply the 80% breaker rule, you will get 1,728W of power. It’s really hard to advise here without wattages but from the looks of it – 36V and energy-demanding pumps – the first presumption would be that you need big breakers, at least 60A. Hope this helps.

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Article 100 Definitions. Interrupting Rating.

Many people are under the misconception that the interrupting rating of an overcurrent device such as a molded case circuit breaker is the marked value on the circuit breaker handle. The value marked on the handle is actually the ampere rating of the device.

UL 489 is the Standard For Safety For Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures . According to UL 489, the ampere rating of a circuit breaker rated 100 amperes or less shall be molded, stamped, or etched on the handle or the escutcheon area of the circuit breaker so as to be visible without removing the trim or cover of the enclosure.

The ampere rating is the lowest current, that if exceeded, will initiate the overcurrent device to trip.

The interrupting rating is commonly referred to with several terms that mean the same thing:

• Interrupting rating (IR)
• Ampere interrupting capacity (AIC)
• Ampere interrupt rating (AIR)

The interrupting rating is defined in the NEC ® as “ the highest current at rated voltage that a device is identified to interrupt under standard test conditions .”

A circuit breaker with a 200- ampere rating for example will not trip unless more than 200 amperes of current is drawn through the circuit breaker. The same 200-ampere rated circuit breaker might also have an interrupting rating or ampere interrupting capacity (AIC) of 35,000 amperes which means that if the breaker is subjected to up to 35,000 amps of current during a fault condition, the device will interrupt the fault condition without blowing up.

See the actual NEC ® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2014 - 2017 Code Language:

Article 100 Definition:

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• Ashraf Ashour Mahmoud
• 13-December-2016

What is meant by 100AT/125AF in a circuit breaker?

100AT =100 Amperes Trip

125AF =125 Amperes Frame

This means that the circuit breaker is rated for a maximum current of125 Amperes and is currently set to trip (Open Contacts) at100 Amperes.

AT - donates for tripping rate for breaker

AF- donates r frame size , physical dimensions to allow for replacing breakers in place of each others , manufactures , usually make 5 to 6 frame sizes ,

Example : 100 AT / 250 AF , 125 AT/250 AF , 225 AT/250 AF , 250 AT/ 250 AF

the rate of breaker is 160 AMP.

but the frame size is same as 250 AMP. meaning that you can simply replace the 16 A breaker by 250A breaker

From Clarity point of view, I would to recommend Jalal's explanation, however it is a minor  typo perhaps might need to be corrected in the last row of his comment 160A instead of 16A.

For 125 amperes circuit breaker protection , the wire to be use is?

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Home > Infrastructure and Grid > Power Distribution and Management > Why Frame Size Matters With Molded Case Circuit Breakers

Power Distribution and Management

Why Frame Size Matters With Molded Case Circuit Breakers

July 11, 2019

2 min read |  Gilles Lordey

This audio was created using Microsoft Azure Speech Services

Molded case circuit breakers (MCCBs) are the critical building blocks for many electrical panels . But panel builders could be excused for treating these devices as a commodity at times. However, the physical design of these products can make a big difference in how efficiently panel builders are able to do their jobs, because when it comes to MCCBs, smaller can be better.

Starting with the basics, MCCBs are circuit breakers that incorporate all current-carrying parts, mechanisms and trip devices into a molded case manufactured from insulating material. Designed to protect connected circuits in low-voltage distribution systems, MCCBs are comprised of two main components:

• Frame – This is considered the body of the circuit breaker. It is the molded, insulated housing, fabricated from a glass-polyester, thermoset composite resin, or thermo-plastic glass fiber material.
• Trip unit – this is considered the brain of a circuit breaker. It activates an operating mechanism when a short circuit or prolonged current overload occurs. Today’s most advanced trip units are electronic and offer flexible mounting configurations depending on the application.

Building power distribution systems incorporate multiple MCCBs into centralized electrical panels. Assembling these panels is a complicated process. Ensuring long-lasting electrical connections, addressing the possible need for auxiliary circuits, meeting earth-leakage requirements and enabling the option for future upgrades all can be part of the same assignment.

These challenges are multiplied when one considers the critical element of space to consider. In industrial and small commercial settings, especially, wall space can be valuable real estate, so minimizing panel dimensions also can be a customer goal. But wiring a smaller panel can mean a greater time commitment for builders, as adjustments become more difficult to make.

For example, ComPact NSX MCCBs , address panel board real estate issues by incorporating earth leakage protection into the breakers, themselves. The trip units in these breakers are interchangeable, so they can be upgraded over time to include more advanced metering and alarming functions. Even with their reduced footprint, these MCCBs feature breaking capacities of up to 100 kA at 690 V. Watch the video below to learn more!

Now, the smaller ComPact NSXm MCCBs can make panel building even easier. These devices are designed for breaking capacities up to 70 kA at 415 V and also incorporate earth leakage protection. Additionally, panel builders benefit from:

• EverLink ™ connectors, which enable easy cable connections using a patented spring-loaded mechanism that also maintains constant pressure on connected cables over time.
• One-click auxiliaries that simply snap into place and are both field-installable and externally visible.
• Built-in DIN rails , so these breakers simply click into place.

To further learn about to ensure power protection, download our white paper, “ How to ensure a secure, long-lasting power connection for your electrical installation.” Also, be sure to register for our Panel Builder Portal to find even more ways to make panel builders’ work easier and more efficient.

Tags: circuit breaker , ComPact NSX , Earth Leakage , EverLink , MCCBs , molded-case circuit breakers , panel building , Power Protection , white paper

Selective Coordination with Molded Case Circuit Breakers

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How do you calculate ampere trip and ampere frame?

Table of Contents

Multiply the amps by the volts. In most circuits, this will be 20 x 120 = 2400 or 15 x 120 = 1800. The number resulting from this equation is the maximum wattage load you can place on the circuit before tripping the breaker.

How do you calculate ampere trip?

Divide the wattage by the voltage. The answer will be the amperage the device draws on your circuit. For example, a 150-watt device on a 120-volt circuit will draw 150 ÷ 120 = 1.25 amps.

What is ampere frame?

Ampere Frame [AF] it is the rating breaker current [maximum. current which the breaker will withstand for a long. time]. Ampere trip [AT] it is the current set to trip the. circuit [usually from 60% up to 100% of the AF].

What does trip mean on Breakerbox?

A circuit breaker “trips” (shuts off the electrical flow) in order to protect the circuit from overheating. It’s a safeguard that helps prevent damage and electrical fires.

What is frame size in Mccb?

Industrial-grade MCCBs are available in frame sizes from 100 amperes to 3000 amperes. ICCBs are available in frame sizes from 400 to 5000 amperes. The 400-ampere-frame ICCB is typically the same size and cost as the 800-ampere frame but is equipped with smaller-ratio current sensors.

What is the frame size of a 30a circuit breaker?

2-Space Square D Homeline Circuit Breaker 30A

How do you read a tripping curve?

The Trip Curve

• The X axis represents a multiple of the operating current of the circuit breaker.
• The Y axis represents the tripping time. A logarithmic scale is used in order to show times from . 001 seconds up to 10,000 seconds (2.77 hours) at multiples of the operating current.

What is breaker frame?

Frame – This is considered the body of the circuit breaker. It is the molded, insulated housing, fabricated from a glass-polyester, thermoset composite resin, or thermo-plastic glass fiber material. Trip unit – this is considered the brain of a circuit breaker.

What is the Kaic rating?

It stands for Kilo Ampere Interrupting Capacity and is sometimes referred to as Thousand Ampere Interrupting Capacity. KAIC in electricity refers to refers to measurements of the ability of a circuit breaker to withstand a short circuit or overload.

What does trip mean in electricity?

A trip unit is the part of a circuit breaker that opens the circuit in the event of a thermal overload, short circuit or ground fault. An open circuit will not conduct electricity because either air, or some other insulator has stopped or broken the flow of current in the loop.

What causes power to trip?

Common reasons for your circuit breaker tripping are because of either a circuit overload, short circuit or a ground fault. Here’s some information about the differences between a circuit overload, a short circuit and a ground fault to help you solve your circuit breaker and electrical systems issues.

What is frame of breaker?

Frame – This is considered the body of the circuit breaker. It is the molded, insulated housing, fabricated from a glass-polyester, thermoset composite resin, or thermo-plastic glass fiber material.

Philippine Electrical Code 2009 Part 1/Chapter 2. Wiring and Protection/Article 2.40 - Overcurrent Protection

Article 2.40 - overcurrent protection.

While the authors have used good faith and efforts to ensure that the information and instructions contained in this work are accurate, the authors disclaim all responsibility for errors or omissions, including without limitation responsibility for damages resulting from the use of or reliance on this work. Use of the information and instructions contained in this work is at your own risk. If any contents or other technology this work contains or describes is subject to open source licenses or the intellectual property rights of others, it is your responsibility to ensure that your use thereof complies with such licenses and/or rights.

• 1.1.1 2.40.1.1 Scope.
• 1.1.2 2.40.1.2 Definitions.
• 1.1.3 2.40.1.3 Other Articles.
• 1.1.4 2.40.1.4 Protection of Conductors.
• 1.1.5.1 2.40.1.6 Standard Ampere Ratings.
• 1.1.6.1 2.40.1.10 Supplementary Overcurrent Protection.
• 1.1.6.2 2.40.1.12 Electrical System Coordination.
• 1.1.7 2.40.1.13 Ground-Fault Protection of Equipment.
• 1.2.1 2.40.2.1 Ungrounded Conductors.
• 1.2.2.1 2.40.2.3 Grounded Conductor.
• 1.2.2.2 2.40.2.4 Change in Size of Grounded Conductor.
• 1.2.2.3 2.40.2.5 Location in or on Premises.
• 1.2.3.1 2.40.3.1 General.
• 1.2.3.2 2.40.3.3 Damp or Wet Locations.
• 1.2.3.3 2.40.3.4 Vertical Position.
• 1.2.4.1 2.40.4.1 Disconnecting Means for Fuses.
• 1.2.4.2 2.40.4.2 Arcing or Suddenly Moving Parts.
• 1.2.5.1 2.40.5.1 General.
• 1.2.5.2 2.40.5.2 Edison-Base Fuses.
• 1.2.5.3 2.40.5.3 Edison-Base Fuseholders.
• 1.2.5.4 2.40.5.4 Type S Fuses.
• 1.2.5.5 2.40.5.5 Type S Fuses, Adapters, and Fuseholders.
• 1.2.6.1 2.40.6.1 General.
• 1.2.6.2 2.40.6.2 Classification.
• 1.2.7.1 2.40.7.1 Method of Operation.
• 1.2.7.2 2.40.7.2 Indicating.
• 1.2.7.3 2.40.7.4 Marking.
• 1.2.7.4 2.40.7.6 Applications.
• 1.2.7.5 2.40.7.7 Series Ratings.
• 1.2.8.1 2.40.8.1 General.
• 1.2.8.2 2.40.8.3 Location in Circuit.
• 1.2.8.3 2.40.8.4 Series Ratings.
• 1.2.9.1 2.40.9.1 Feeders and Branch Circuits.
• 1.2.9.2 2.40.9.2 Additional Requirements for Feeders.
• 2 Other Pages in this Category: Chapter 2. Wiring and Protection
• 3 References

2.40.1 General

2.40.1.1 scope..

Parts 2.40.1 through 2.40.7 of this article provide the general requirements for overcurrent protection and overcurrent protective devices not more than 600 volts, nominal. Part 2.40.8 covers overcurrent protection for those portions of supervised industrial installations operating at voltages of not more than 600 volts, nominal. Part 2.40.9 covers overcurrent protection over 600 volts, nominal.

FPN : Overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in conductors or conductor insulation. See also 1.10.1.9 for requirements for interrupting ratings and 1.10.1.10 for requirements for protection against fault currents.

2.40.1.2 Definitions.

Current-Limiting Overcurrent Protective Device. A device that, when interrupting currents in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance. Supervised Industrial Installation. For the purposes of Part 2.40.8, the industrial portions of a facility where all of the following conditions are met:

Tap Conductors. As used in this article, a tap conductor is defined as a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 2.40.1.4.

2.40.1.3 Other Articles.

Equipment shall be protected against overcurrent in accordance with the article in this Code that covers the type of equipment specified in Table 2.40.1.3.

2.40.1.4 Protection of Conductors.

Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 3.10.1.15, unless otherwise permitted or required in 2.40.1.4(a) through (g).

(a) Power Loss Hazard . Conductor overload protection shall not be required where the interruption of the circuit would create a hazard, such as in a material-handling magnet circuit or fire pump circuit. Short-circuit protection shall be provided.

FPN : See NFPA 20-2003, Standard for the Installation of Stationary Pumps for Fire Protection.

(b) Devices Rated 800 Amperes or Less . The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(c) Devices Rated Over 800 Amperes . Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device defined in 2.40.1.6.

(d) Small Conductors . Unless specifically permitted in 2.40.1.4(e) or 2.40.1.4(g), the overcurrent protection shall not exceed 15 amperes for 2.0 mm 2 (1.6 mm dia.), 20 amperes for 3.5 mm 2 (2.0 mm dia.), and 30 amperes for 5.5 mm 2 (2.6 mm dia.) copper; or 15 amperes for 3.5 mm 2 (2.0 mm dia.) and 25 amperes for 5.5 mm 2 (2.6 mm dia.) aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied. (e) Tap Conductors. Tap conductors shall be permitted to be protected against overcurrent in accordance with the following:

(f) Transformer Secondary Conductors . Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single- voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 4.50.1.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio.

(g) Overcurrent Protection for Specific Conductor Applications. Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 2.40.1.4(g).

2.40.1.5 Protection of Flexible Cords, Flexible Cables, and Fixture Wires.

Flexible cord and flexible cable, including tinsel cord and extension cords, and fixture wires shall be protected against overcurrent by either 2.40.1.5(a) or (b).

(a) Ampacities . Flexible cord and flexible cable shall be protected by an overcurrent device in accordance with their ampacity as specified in Table 4.0.1.5(a) and Table 4.0.1.5(b). Fixture wire shall be protected against overcurrent in accordance with its ampacity as specified in Table 4.2.1.5. Supplementary overcurrent protection, as in 2.40.1.10, shall be permitted to be an acceptable means for providing this protection.

(b) Branch Circuit Overcurrent Device . Flexible cord shall be protected where supplied by a branch circuit in accordance with one of the methods described in 2.40.1.5(b)(1), (b)(2), (b)(3), or (b)(4).

2.40.1.6 Standard Ampere Ratings.

(a) Fuses and Fixed-Trip Circuit Breakers . The standard ampere ratings for fuses and inverse time circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.

(b) Adjustable-Trip Circuit Breakers . The rating of adjustable- trip circuit breakers having external means for adjusting the current setting (long-time pickup setting), not meeting the requirements of 2.40.1.6(c), shall be the maximum setting possible.

(c) Restricted Access Adjustable-Trip Circuit Breakers . A circuit breaker(s) that has restricted access to the adjusting means shall be permitted to have an ampere rating(s) that is equal to the adjusted current setting (long-time pickup setting). Restricted access shall be defined as located behind one of the following:

2.40.1.9 Thermal Devices.

Thermal relays and other devices not designed to open short circuits or ground faults shall not be used for the protection of conductors against overcurrent due to short circuits or ground faults, but the use of such devices shall be permitted to protect motor branch-circuit conductors from overload if protected in accordance with 4.30.3.10.

2.40.1.10 Supplementary Overcurrent Protection.

Where supplementary overcurrent protection is used for luminaires (lighting fixtures), appliances, and other equipment or for internal circuits and components of equipment, it shall not be used as a substitute for required branch-circuit overcurrent devices or in place of the required branch-circuit protection. Supplementary overcurrent devices shall not be required to be readily accessible.

2.40.1.12 Electrical System Coordination.

Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on the following two conditions shall be permitted:

FPN : The monitoring system may cause the condition to go to alarm, allowing corrective action or an orderly shutdown, thereby minimizing personnel hazard and equipment damage.

2.40.1.13 Ground-Fault Protection of Equipment.

Ground-fault protection of equipment shall be provided in accordance with the provisions of 2.30.7.6 for solidly grounded wye electrical systems of more than 150 volts to ground but not exceeding 600 volts phase-to- phase for each individual device used as a building or structure main disconnecting means rated 1000 amperes or more.

The provisions of this section shall not apply to the disconnecting means for the following:

2.40.2 Location

2.40.2.1 ungrounded conductors..

(a) Overcurrent Device Required . A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor. A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit.

FPN : For motor circuits, see Parts 4.30.3, 4.30.4, 4.30.5, and 4.30.9.

(b) Circuit Breaker as Overcurrent Device . Circuit breakers shall open all ungrounded conductors of the circuit both manually and automatically unless otherwise permitted in 2.40.2.1(b)(1), (b)(2), and (b)(3).

(c) Closed-Loop Power Distribution Systems . Listed devices that provide equivalent overcurrent protection in closed-loop power distribution systems shall be permitted as a substitute for fuses or circuit breakers.

2.40.2.2 Location in Circuit.

Overcurrent protection shall be provided in each ungrounded circuit conductor and shall be located at the point where the conductors receive their supply except as specified in 2.40.2.2(a) through (g). No conductor supplied under the provisions of 2.40.2.2(a) through (g) shall supply another conductor under those provisions, except through an overcurrent protective device meeting the requirements of 2.40.1.4.

(a) Branch-Circuit Conductors . Branch-circuit tap conductors meeting the requirements specified in 2.10.2.1 shall be permitted to have overcurrent protection located as specified in that section.

(b) Feeder Taps . Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 2.40.2.2(b)(1) through (b)(5). The provisions of 2.40.1.4(b) shall not be permitted for tap conductors.

FPN : For overcurrent protection requirements for lighting and appliance branch- circuit panelboards and certain power panelboards, see 4.8.3.7(a), (b), and (e).

(c) Transformer Secondary Conductors . Each set of conductors feeding separate loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 2.40.2.2(c)(1) through (c)(6). The provisions of 2.40.1.4(b) shall not be permitted for transformer secondary conductors.

FPN : For overcurrent protection requirements for transformers, see 4.50.1.3.

(1) Protection by Primary Overcurrent Device . Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 4.50.1.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio. Single-phase (other than 2-wire) and multiphase (other than delta- delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

(2) Transformer Secondary Conductors Not Over 3 000 mm Long . Where the length of secondary conductor does not exceed 3 000 mm and complies with all of the following:

(3) Industrial Installation Secondary Conductors Not Over 7 600 mm Long . For industrial installations only, where the length of the secondary conductors does not exceed 7 600 mm and complies with all of the following:

(4) Outside Secondary Conductors . Where the conductors are located outdoors of a building or structure, except at the point of load termination, and comply with all of the following conditions:

(5) Secondary Conductors from a Feeder Tapped Transformer . Transformer secondary conductors installed in accordance with 2.40.2.2(b)(3) shall be permitted to have overcurrent protection as specified in that section.

(6) Secondary Conductors Not Over 7 600 mm Long . Where the length of secondary conductor does not exceed 7 600 mm and complies with all of the following:

(d) Service Conductors. Service-entrance conductors shall be permitted to be protected by overcurrent devices in accordance with 2.30.7.2.

(e) Busway Taps. Busways and busway taps shall be permitted to be protected against overcurrent in accordance with 3.68.2.8.

(f) Motor Circuit Taps. Motor-feeder and branch-circuit conductors shall be permitted to be protected against overcurrent in accordance with 4.30.2.8 and 4.30.4.3, respectively.

(g) Conductors from Generator Terminals. Conductors from generator terminals that meet the size requirement in 4.45.1.13 shall be permitted to be protected against overload by the generator overload protective device(s) required by 4.45.1.12.

2.40.2.3 Grounded Conductor.

No overcurrent device shall be connected in series with any conductor that is intentionally grounded, unless one of the following two conditions is met:

(1) The overcurrent device opens all conductors of the circuit, including the grounded conductor, and is designed so that no pole can operate independently.

(2) Where required by 4.30.3.6 or 4.30.3.7 for motor overload protection.

2.40.2.4 Change in Size of Grounded Conductor.

Where a change occurs in the size of the ungrounded conductor, a similar change shall be permitted to be made in the size of the grounded conductor.

2.40.2.5 Location in or on Premises.

(a) Accessibility . Overcurrent devices shall be readily accessible and shall be installed so that the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, is not more than 2.0 m (6 ft 7 in.) above the floor or working platform unless one of the following applies:

(b) Occupancy . Each occupant shall have ready access to all overcurrent devices protecting the conductors supplying that occupancy.

' Exception No. 1 : Where electric service and electrical maintenance are provided by the building management and where these are under continuous building management supervision, the service overcurrent devices and feeder overcurrent devices supplying more than one occupancy shall be permitted to be accessible to only authorized management personnel in the following:

"(1) Multiple-occupancy buildings

"(2) Guest rooms or guest suites of hotels and motels that are intended for transient occupancy

Exception No. 2 : Where electric service and electrical maintenance are provided by the building management and where these are under continuous building management supervision, the branch circuit overcurrent devices supplying any guest rooms or guest suites shall be permitted to be accessible to only authorized management personnel for guest rooms of hotels and motels that are intended for transient occupancy.

(c) Not Exposed to Physical Damage . Overcurrent devices shall be located where they will not be exposed to physical damage.

FPN: See 1.10.1.11, Deteriorating Agents.

(d) Not in Vicinity of Easily Ignitible Material . Overcurrent devices shall not be located in the vicinity of easily ignitible material, such as in clothes closets.

(e) Not Located in Bathrooms . In dwelling units and guest rooms or guest suites of hotels and motels, overcurrent devices, other than supplementary overcurrent protection, shall not be located in bathrooms.

2.40.3 Enclosures

2.40.3.1 general..

(a) Protection from Physical Damage. Overcurrent devices shall be protected from physical damage by one of the following:

(1) Installation in enclosures, cabinets, cutout boxes, or equipment assemblies

(2) Mounting on open-type switchboards, panelboards, or control boards that are in rooms or enclosures free from dampness and easily ignitible material and are accessible only to qualified personnel

(b) Operating Handle. The operating handle of a circuit breaker shall be permitted to be accessible without opening a door or cover.

2.40.3.3 Damp or Wet Locations.

Enclosures for overcurrent devices in damp or wet locations shall comply with 3.12.1.2(a).

2.40.3.4 Vertical Position.

Enclosures for overcurrent devices shall be mounted in a vertical position unless that is shown to be impracticable. Circuit breaker enclosures shall be permitted to be installed horizontally where the circuit breaker is installed in accordance with 2.40.7.2. Listed busway plug-in units shall be permitted to be mounted in orientations corresponding to the busway mounting position.

2.40.4 Disconnecting and Guarding

2.40.4.1 disconnecting means for fuses..

A disconnecting means shall be provided on the supply side of all fuses in circuits over 150 volts to ground and cartridge fuses in circuits of any voltage where accessible to other than licensed electrical practitioner or non licensed electrical practitioner under the supervision of a licensed electrical practitioner, so that each circuit containing fuses can be independently disconnected from the source of power. A current-limiting device without a disconnecting means shall be permitted on the supply side of the service disconnecting means as permitted by 2.30.6.13. A single disconnecting means shall be permitted on the supply side of more than one set of fuses as permitted by 4.30.9.2, Exception, for group operation of motors and 4.24.3.4(c) for fixed electric space-heating equipment.

2.40.4.2 Arcing or Suddenly Moving Parts.

Arcing or suddenly moving parts shall comply with 2.40.4.2(a) and (b).

(a) Location. Fuses and circuit breakers shall be located or shielded so that persons will not be burned or otherwise injured by their operation.

(b) Suddenly Moving Parts. Handles or levers of circuit breakers, and similar parts that may move suddenly in such a way that persons in the vicinity are likely to be injured by being struck by them, shall be guarded or isolated.

2.40.5 Plug Fuses, Fuseholders, and Adapters

2.40.5.1 general..

(a) Maximum Voltage. Plug fuses shall be permitted to be used in the following circuits:

(1) Circuits not exceeding 125 volts between conductors

(2) Circuits supplied by a system having a grounded neutral where the line-to-neutral voltage does not exceed 150 volts

(b) Marking. Each fuse, fuseholder, and adapter shall be marked with its ampere rating.

(c) Hexagonal Configuration. Plug fuses of 15-ampere and lower rating shall be identified by a hexagonal configuration of the window, cap, or other prominent part to distinguish them from fuses of higher ampere ratings.

(d) No Energized Parts. Plug fuses, fuseholders, and adapters shall have no exposed energized parts after fuses or fuses and adapters have been installed.

(e) Screw Shell. The screw shell of a plug-type fuseholder shall be connected to the load side of the circuit.

2.40.5.2 Edison-Base Fuses.

(a) Classification. Plug fuses of the Edison-base type shall be classified at not over 125 volts and 30 amperes and below.

(b) Replacement Only. Plug fuses of the Edison-base type shall be used only for replacements in existing installations where there is no evidence of overfusing or tampering.

2.40.5.3 Edison-Base Fuseholders.

Fuseholders of the Edison-base type shall be installed only where they are made to accept Type S fuses by the use of adapters.

2.40.5.4 Type S Fuses.

Type S fuses shall be of the plug type and shall comply with 2.40.5.4(a) and (b).

(a) Classification. Type S fuses shall be classified at not over 125 volts and 0 to 15 amperes, 16 to 20 amperes, and 21 to 30 amperes.

(b) Noninterchangeable. Type S fuses of an ampere classification as specified in 2.40.5.4(a) shall not be interchangeable with a lower ampere classification. They shall be designed so that they cannot be used in any fuseholder other than a Type S fuseholder or a fuseholder with a Type S adapter inserted.

2.40.5.5 Type S Fuses, Adapters, and Fuseholders.

(a) To Fit Edison-Base Fuseholders. Type S adapters shall fit Edison-base fuseholders.

(b) To Fit Type S Fuses Only. Type S fuseholders and adapters shall be designed so that either the fuseholder itself or the fuseholder with a Type S adapter inserted cannot be used for any fuse other than a Type S fuse.

(c) Nonremovable. Type S adapters shall be designed so that once inserted in a fuseholder, they cannot be removed.

(d) Nontamperable. Type S fuses, fuseholders, and adapters shall be designed so that tampering or shunting (bridging) would be difficult.

(e) Interchangeability. Dimensions of Type S fuses, fuseholders, and adapters shall be standardized to permit interchangeability regardless of the manufacturer.

2.40.6 Cartridge Fuses and Fuseholders

2.40.6.1 general..

(a) Maximum Voltage — 300-Volt Type . Cartridge fuses and fuseholders of the 300-volt type shall be permitted to be used in the following circuits:

(b) Noninterchangeable — 0–6000-Ampere Cartridge Fuseholders . Fuseholders shall be designed so that it will be difficult to put a fuse of any given class into a fuseholder that is designed for a current lower, or voltage higher, than that of the class to which the fuse belongs. Fuseholders for current-limiting fuses shall not permit insertion of fuses that are not current-limiting.

(c) Marking. Fuses shall be plainly marked, either by printing on the fuse barrel or by a label attached to the barrel showing the following:

The interrupting rating shall not be required to be marked on fuses used for supplementary protection.

(d) Renewable Fuses . Class H cartridge fuses of the renewable type shall only be permitted to be used for replacement in existing installations where there is no evidence of overfusing or tampering.

2.40.6.2 Classification.

Cartridge fuses and fuseholders shall be classified according to voltage and amperage ranges. Fuses rated 600 volts, nominal, or less shall be permitted to be used for voltages at or below their ratings.

2.40.7 Circuit Breakers

2.40.7.1 method of operation..

Circuit breakers shall be trip free and capable of being closed and opened by manual operation. Their normal method of operation by other than manual means, such as electrical or pneumatic, shall be permitted if means for manual operation are also provided.

2.40.7.2 Indicating.

Circuit breakers shall clearly indicate whether they are in the open “off” or closed “on” position.

Where circuit breaker handles are operated vertically rather than rotationally or horizontally, the “up” position of the handle shall be the “on” position.

2.40.7.3 Nontamperable. A circuit breaker shall be of such design that any alteration of its trip point (calibration) or the time required for its operation requires dismantling of the device or breaking of a seal for other than intended adjustments.

2.40.7.4 Marking.

(a) Durable and Visible. Circuit breakers shall be marked with their ampere rating in a manner that will be durable and visible after installation. Such marking shall be permitted to be made visible by removal of a trim or cover.

(b) Location. Circuit breakers rated at 100 amperes or less and 600 volts or less shall have the ampere rating molded, stamped, etched, or similarly marked into their handles or escutcheon areas.

(c) Interrupting Rating. Every circuit breaker having an interrupting rating other than 5000 amperes shall have its interrupting rating shown on the circuit breaker. The interrupting rating shall not be required to be marked on circuit breakers used for supplementary protection.

(d) Used as Switches. Circuit breakers used as switches in 120-volt and 277-volt fluorescent lighting circuits shall be listed and shall be marked SWD or HID. Circuit breakers used as switches in high- intensity discharge lighting circuits shall be listed and shall be marked as HID.

(e) Voltage Marking. Circuit breakers shall be marked with a voltage rating not less than the nominal system voltage that is indicative of their capability to interrupt fault currents between phases or phase to ground.

2.40.7.6 Applications.

A circuit breaker with a straight voltage rating, such as 240V or 480V, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the circuit breaker’s voltage rating. A two-pole circuit breaker shall not be used for protecting a 3-phase, corner-grounded delta circuit unless the circuit breaker is marked 1–3 to indicate such suitability.

A circuit breaker with a slash rating, such as 120/240V or 480Y/277V, shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker’s voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker’s voltage rating.

FPN : Proper application of molded case circuit breakers on 3-phase systems, other than solidly grounded wye, particularly on corner grounded delta systems, considers the circuit breakers’ individual pole-interrupting capability.

2.40.7.7 Series Ratings.

Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in (a) or (b), and (c).

(a) Selected Under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.

(b) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.

(c) Motor Contribution. Series ratings shall not be used where

(1) Motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device, and

(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.

2.40.8 Supervised Industrial Installations

2.40.8.1 general..

Overcurrent protection in areas of supervised industrial installations shall comply with all of the other applicable provisions of this article, except as provided in Part 2.40.8. The provisions of Part 2.40.8 shall be permitted only to apply to those portions of the electrical system in the supervised industrial installation used exclusively for manufacturing or process control activities.

2.40.8.3 Location in Circuit.

An overcurrent device shall be connected in each ungrounded circuit conductor as required in 2.40.8.3(a) through (d).

(a) Feeder and Branch-Circuit Conductors. Feeder and branch- circuit conductors shall be protected at the point the conductors receive their supply as permitted in 2.40.2.2 or as otherwise permitted in 2.40.8.3(b), (c), or (d).

(b) Transformer Secondary Conductors of Separately Derived Systems. Conductors shall be permitted to be connected to a transformer secondary of a separately derived system, without overcurrent protection at the connection, where the conditions of 2.40.8.3(b)(1), (b)(2), and (b)(3) are met.

(1) Short-Circuit and Ground-Fault Protection. The conductors shall be protected from short-circuit and ground-fault conditions by complying with one of the following conditions:

a. The length of the secondary conductors does not exceed 30 m and the transformer primary overcurrent device has a rating or setting that does not exceed 150 percent of the value determined by multiplying the secondary conductor ampacity by the secondary-to- primary transformer voltage ratio.

b. The conductors are protected by a differential relay with a trip setting equal to or less than the conductor ampacity.

FPN : A differential relay is connected to be sensitive only to short-circuit or fault currents within the protected zone and is normally set much lower than the conductor ampacity. The differential relay is connected to trip protective devices that will de-energize the protected conductors if a short-circuit condition occurs.

c. The conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors within recognized time vs. current limits for all short-circuit and ground-fault conditions.

(2) Overload Protection. The conductors shall be protected against overload conditions by complying with one of the following:

a. The conductors terminate in a single overcurrent device that will limit the load to the conductor ampacity.

b. The sum of the overcurrent devices at the conductor termination limits the load to the conductor ampacity. The overcurrent devices shall consist of not more than six circuit breakers or sets of fuses, mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard. There shall be no more than six overcurrent devices grouped in any one location.

c. Overcurrent relaying is connected [with a current transformer(s), if needed] to sense all of the secondary conductor current and limit the load to the conductor ampacity by opening upstream or downstream devices.

d. Conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors from overload conditions.

(3) Physical Protection. The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

(c) Outside Feeder Taps. Outside conductors shall be permitted to be tapped to a feeder or to be connected at a transformer secondary, without overcurrent protection at the tap or connection, where all the following conditions are met:

a. Outside of a building or structure

b. Inside, nearest the point of entrance of the conductors

c. Where installed in accordance with 2.30.1.6, nearest the point of entrance of the conductors (d) Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a transformer shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided the primary device time– current protection characteristic, multiplied by the maximum effective primary-to-secondary transformer voltage ratio, effectively protects the secondary conductors.

2.40.8.4 Series Ratings.

Where a circuit breaker is used on a circuit having an available fault current higher than its marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device having the higher rating, the circuit breaker shall meet the requirements specified in 2.40.8.4(a) or (b) and (c).

(a) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.

(b) Selected Under Engineering Supervision. The line-side device is selected under engineering supervision. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.

2.40.9 Overcurrent Protection Over 600 Volts, Nominal

2.40.9.1 feeders and branch circuits..

(a) Location and Type of Protection . Feeder and branch-circuit conductors shall have overcurrent protection in each ungrounded conductor located at the point where the conductor receives its supply or at an alternative location in the circuit when designed under engineering supervision that includes but is not limited to considering the appropriate fault studies and time–current coordination analysis of the protective devices and the conductor damage curves. The overcurrent protection shall be permitted to be provided by either 2.40.9.1(a)(1) or (a)(2).

On 3-phase, 3-wire circuits, an overcurrent relay element in the residual circuit of the current transformers shall be permitted to replace one of the phase relay elements.

An overcurrent relay element, operated from a current transformer that links all phases of a 3-phase, 3-wire circuit, shall be permitted to replace the residual relay element and one of the phase-conductor current transformers. Where the neutral is not regrounded on the load side of the circuit as permitted in 2.50.10.5(b), the current transformer shall be permitted to link all 3-phase conductors and the grounded circuit conductor (neutral).

(b) Protective Devices . The protective device(s) shall be capable of detecting and interrupting all values of current that can occur at their location in excess of their trip-setting or melting point.

(c) Conductor Protection . The operating time of the protective device, the available short-circuit current, and the conductor used shall be coordinated to prevent damaging or dangerous temperatures in conductors or conductor insulation under short-circuit conditions.

2.40.9.2 Additional Requirements for Feeders.

(a) Rating or Setting of Overcurrent Protective Devices . The continuous ampere rating of a fuse shall not exceed three times the ampacity of the conductors. The long-time trip element setting of a breaker or the minimum trip setting of an electronically actuated fuse shall not exceed six times the ampacity of the conductor. For fire pumps, conductors shall be permitted to be protected for overcurrent in accordance with 6.95.1.4(b).

(b) Feeder Taps . Conductors tapped to a feeder shall be permitted to be protected by the feeder overcurrent device where that overcurrent device also protects the tap conductor.

Other Pages in this Category: Chapter 2. Wiring and Protection

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• Philippine Electrical Code 2009 Part 1/Chapter 2. Wiring and Protection