Frequently Asked Question: MZI capacitor installation instructions recommend the use of circuit breakers or fuses to meet NEC overcurrent protection requirements. Why is this necessary when the capacitor system is already fused?

Answer: The fuses installed in capacitor systems protect the capacitors from excessive currents resulting from a short circuit or voltage anomaly. Fuses are sized as high as 250% to 300% of the rated current and are not intended as a substitute for the overcurrent device per NEC requirements.

Operating voltage determines the current drawn by the capacitor. At 480 volts, the capacitor current is 1.2 amps per kVAr. For a 50 kVAr unit or step, the current is 60 amps. Capacitor fuses for a 50 kVAr step typically range from 150 to 200 amps. The fuses are fast-acting and large enough to prevent nuisance fuse blowing on startup. The fuses are designed to interrupt excessive current very rapidly and protect the capacitor cell from catastrophic failure.

The NEC and good practices require overcurrent protection and a means of disconnecting the capacitor system. The conductors, overcurrent device, and disconnecting device must be sized for 135% of the rated current. Compared to the capacitor fuses the response time of the overcurrent device is much slower. The overcurrent device will not trip on momentary surges but will provide the appropriate system protection. NEC requires that the rating or setting of the overcurrent device shall be as low as practicable. The minimum rating for a 50 kVAr step is 81 amps (1.35 x 60).

There is one exception to this requirement: if the capacitor unit is connected to the load side of the motor overload, neither a separate overcurrent device nor a disconnecting means are required. In this case, the reduced current due to improved power factor will impact motor overload rating or setting selection.