Circuit Breaker Sizing

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.

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 Circuit Breakers are required not to trip at rated current.

However, a Circuit Breakers 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 Circuit Breakers correctly.

Since the current path, both the Circuit Breaker and the wire, reacts to heat, the overall operating temperature of the equipment becomes a factor in sizing a Circuit Breaker in an enclosure. Our Electricians in Orange County can install and inspect any electrical projects you may have.