How to choose circuit breakers for electric motors

Written by michael logan
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Electric motors convert electrical energy to rotating mechanical energy, and the amount of work they are capable of doing is rated in horsepower. It is important to know that the motor will consume more energy than it puts out, because some energy is lost to heat and friction. During start-up, the motor may consume considerably more energy, especially if it starts under load conditions. For these reasons, the circuit breaker chosen to run a non-appliance motor circuit will probably be larger than the motor requires to run.

Skill level:

Things you need

  • NEC table 430.148

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  1. 1

    Determine the horsepower rating of the motor from the motor nameplate. Look up the full-load current rating of the motor on National Electric Code table 430.148.

  2. 2

    Multiply the full-load current rating by 2.5. The National Electric Code allows circuit breakers for motors to be up to 250 per cent of the full-load current rating specified on the motor nameplate. This NEC rule takes into account the special circumstance of the high-current requirements of a motor starting under full load.

  3. 3

    Choose the next lowest available circuit breaker for the circuit. For example, if the full-load current on the nameplate is 21 amperes, the maximum sized circuit breaker is 21 x 2.5 = 52.5 amperes. The next lower sized circuit breaker is 50 amperes. Install a 50 ampere circuit breaker for this motor.

Tips and warnings

  • The 250-percent rule for motor circuit breaker amperage seems to contradict the NEC rule that the circuit breaker rating must not exceed the wiring ampacity used in the circuit. However, the motor overload protection device required by NEC 430.6 cannot exceed 125 per cent of the motor full-load current amperage. The overload protection device will allow a very high current for a short period of time, such as during start-up, but will disconnect the motor if the current remains high. The 125-percent rating of the overload protection device is in agreement with the NEC, in that conductors for motors must not be less than 125 per cent of the full-load current listed in table 430.148. Therefore, if the motor current remains over 125 per cent for too long, the overload protection device will disconnect the motor.
  • It is rarely sufficient to simply provide the correct circuit breaker for a motor circuit. You must have a motor controller device for stopping and starting the motor, an insight disconnecting means, and an overload protection device, and you must use the correct conductors rated for not less than the 125 per cent of the full-load current specified in table 430-148.

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