How to Size a Circuit Breaker and Wire Sizes

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Sizing conventions for electrical wiring and circuit breakers. Lean all about how to size a fuse box, circuit breakers and sizing wires.

Circuit breakers are over current protection devices (OCPD) that are designed to break the circuit and prevent equipment damage or fire. When too much current runs through a circuit the heat in the circuit exceeds the breaker's load rating and it trips, shutting off the electrical power. Proper sizing eliminates overloads and insures safe electrical operation. Circuit breakers are designed to carry 100% of their rated current while the NEC dictates an 80% application.

A commonly misunderstood fact about circuit breakers (CBs) is related to the percentage of loading permitted by the NEC and the CB design, and why the two may be different. Let's investigate both aspects.

Circuit Breaker Design

A circuit breaker (CB) is designed and evaluated to carry 100% of its rated current for an indefinite period of time under standard test conditions. These conditions, per UL 489, Underwriters Laboratories Standard for Safety for Molded-Case Circuit Breakers and Circuit Breaker Enclosures, include mounting the CB in free air (no enclosure) where the ambient temperature is held at 40 degrees C or about 104 degrees F. Under these conditions, molded-case CBs are required not to trip at rated current.

However, a CB most frequently is applied in equipment at 80% of its rated current under NEC Sec. 384-16(c). If you understand why this requirement is in place, you'll be able to apply CBs correctly.

Since the current path, both the CB and the conductor, reacts to heat, the overall operating temperature of the equipment becomes a factor in sizing a CB in an enclosure.

Other factors that may affect this equipment operating temperature include:

1.) Size and location of the enclosure.

2.) More than one current carrying device housed in the same enclosure.

3.) Level of current each device is carrying.

4.) Environmental conditions in the area of the equipment

A circuit breaker either carries a standard rating (80%) or a 100% rating. The standard rating is subject to the NEC sizing rules. CBs that are 100%-rated are permitted to be loaded continuously at their full rating as long as the assembly is listed and conductors are properly connected.


Determine the total electrical load required for the circuit you are sizing. All electrical appliances and equipment list the operating load they need to operate properly somewhere on the component, usually on an attached sticker or plate. For example, if you are planning operate a refrigerator rated at 8 amps, a toaster oven rated at 6 amps and a microwave rated at 6 amps on the same circuit, you need a breaker that that will safely carry a 20 amp load.

Calculate the size of the circuit breaker needed to safely carry the load. Circuit breakers are most commonly found in 15, 20, and 30 amp sizes. For the load calculated above, 20 amps, the smallest circuit breaker you could install would be a 20 amp breaker. However, following the NEC 80 percent rule, the proper breaker for this application would be calculated as follows: 20 (amps) times 1.25 (125 percent) = 25 (amps). This application requires a 25 amp circuit breaker.

Wire Sizing

Wire size is a very important part in sizing a circuit. For circuit breakers up to 15 amps, use 14 gauge wire or larger. For a 20 amp breaker, use 12 gauge wire or larger. For up to a 30 amp breaker, use 10 gauge wire or larger. These are the most common sizes for household use.

Wire is manufactured to a specific group of sizes that are designated by numbers known as gauges. The gauge of the wires that carry the power from the transformer to your home and within your home are chosen in size to ensure that they do not overheat at their rated amperage. In fact, there should be no noticeable heat on the wires at any time.

For this reason simply increasing the size of a circuit breaker without also increasing the wire size can lead to a potentially dangerous situation that can result in fire and death.

Choosing the correct wire gauge within your home, wires that run from the electrical distribution panel to various appliances and receptacles is crucial. You do not want the wire to act as a fuse and burn should a short circuit occur.

The table provides the correct gauge of wire for electrical circuits in your home depending on the rated load.

It is also very important not to use devices labeled for copper only on circuits with aluminum wire. Look for these markings on your existing circuit breakers and receptacles and if you have aluminum wiring in your home it is best to hire a licensed electrician to make any alterations to your electrical system.

1 comment

Kevin Naderi
Posted on May 26, 2018