Capacitors Switched With A Motor

Where capacitors are switched with the motor, show the maximum recommended capacitor ratings that should be used for two particular motor designs. Refer to Tables RECOMMENDED MAXIMUM CAPACITOR RATING WHEN AN INDUCTION MOTOR AND CAPACITOR ARE SWITCHED AS A UNIT Table A and RECOMMENDED MAXIMUM CAPACITOR RATING WHEN AN INDUCTION MOTOR AND CAPACITOR ARE SWITCHED AS A UNIT Table B in Appendix page 240. Also, the percent reduction in line current due to the capacitors (% AR) is shown. When the capacitor is connected across the motor terminals, motor running protective devices must be selected on the basis of motor full load current reduced by this percentage. If the capacitor is connected between the contactor and the protective device, the selection of the protective device will not be affected by the addition of the capacitor., NEC(460-9).

Example: Motor Capacitor Selection

Choose a capacitor from Table A to use with a motor rated 25 hp. The nameplate ampere rating is 32 amperes. Calculate the line current to the combination when the motor is fully loaded.

From Table A: 5 kVAC
Read: 9% ampere reduction
Thus: Line current = 0.91 x 32A = 29.1 Amps.

Motor overload protection would be based on this reduced line current resulting from the capacitor being connected at the motor terminals.

And furthermore . . .

Selection Of Conductors & Switching Devices