The National Electrical Code specifies how to select circuit breakers for electric motors based on the full load current draw of the motor. A motor draws considerably more as it starts than when it runs at full speed. The code takes this into consideration and circuit breakers are designed not to trip during this short period of high current draw. Follow the NEC code while selecting circuit breakers for a safe installation the protects the motor, the wires and your home.
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Things you need
- NEC table 430.148
Read the motor nameplate to find the horsepower and full-load current ratings given by the motor's manufacturer. Use National Electrical Code table 430.148 to determine the full load current in amps for the motor if it is not listed on the motor nameplate.
Calculate the maximum circuit breaker size using the formula: 2.5 x full-load current = maximum circuit breaker rating. High starting currents for motors dictate using a circuit breaker that won't trip during a momentary surge of power.
Install a circuit breaker rated as the maximum size allowed for the motor, or the next size smaller. For example, a motor with a full load current of 13 amps has a maximum circuit breaker size of 13 x 2.5 = 32.5 amps. Since you can't purchase a 32.5 amp circuit breaker, choose one rated at 30 amps -- the next lower size circuit breaker available and the correct size for this motor.
Tips and warnings
- The National Electrical Code allows the use of a circuit breaker up to 2-1/2 times the full load current rating of the motor. The motor's built-in overload protection device -- another NEC requirement -- protects the wires from being overloaded. Circuit breakers only provide protection against shorts in a motor branch circuit.
- The NEC table is an estimation of the full load current. Most motor nameplates have the full load current listed. Use the full load current listed on the motor nameplate instead of the NEC table whenever possible.
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