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How to size a circuit breaker

Updated March 23, 2017

The size of a circuit breaker you need to run electrical appliances and equipment depends on a number of factors. Circuit breakers operate as heat switches. When too much current runs through a circuit because of faulty electrical equipment or an overload in the 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.

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  1. Recognise that a circuit breaker is usually sized to operate at no more than 80 per cent of its rated capacity. For example, a circuit breaker rated at 100 amps should only be installed in applications where a maximum of 80 amps is required. Circuit breakers are tested in controlled environments under ideal conditions. In most field applications, conditions are less than ideal, and factors such as ambient temperature, the number of circuit breakers in the panel, ventilation and other environmental conditions all affect the circuit breaker's practical load capacity.

  2. Understand that a circuit breaker is designed to carry its rated circuit load under continuous operation for three hours or more. For example, a 100 amp rating means the circuit breaker will carry 100 amps of current for three hours or more without overloading and tripping off.

  3. 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 compnonet, usually on an attached sticker or plate. For example, if you are planning operate a refrigerator rated at 8 amps, a toaster rated at 6 amps and a can opener rated at 6 amps on the same circuit, you need a breaker that that will safely carry a 20 amp load.

  4. 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. Specialised circuits can be found in other increments of 5 amps if necessary. For the load calculated in step 3, 2 0 amps, the smallest circuit breaker you could install would be a 20 amp breaker. However, following the 80 per cent rule, the proper breaker for this application would be calculated as follows: 20 (amps) times 1.25 (125 per cent) = 25 (amps). This application requires a 25 amp circuit breaker.

  5. Tip

    Amps (amperage) is the measurement of current required to power an electrical appliance or piece of equipment. Wire size is also important in sizing a circuit breaker. 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.

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