Motor Disconnect Size

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Motor Disconnect Size

Probably on of the easiest calculations to make concerning motors is the size of the disconnect for a motor. Note that a disconnect has no overcurrent protection if strictly a disconnect.

First find the full load current for the motor in the table. If the motor horse power is not given in the table, then interpolation may be needed. Find instructions on interpolation HERE.

The full load current for the motor from the table is multiplied by 115%. The result is the MINIMUM size disconnect for that motor.

This means that if the calculation does not match a rating or setting of available disconnects, then the next size larger is needed. There is no language in the NEC to permit a smaller size, so it is up you go.

Find practice problems HERE.

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