WIRES, FUSES AND DISCONNECTS - WHAT SIZE DO I USE?
Wiring power factor capacitors in your electrical system involves determining the proper size wire, fuse and disconnecting means when the power factor capacitor is not part of a motor circuit. The following are some requirements from the National Electrical Code, Article 460:
NEC Article 460-8-b
- An overcurrent device shall be provided in each ungrounded conductor for each capacitor bank.
- The rating or setting of the overcurrent device shall be as low as practicable.
NEC Article 460-8-c
- A disconnecting means shall be provided in each ungrounded conductor for each capacitor bank and shall meet the following requirements:
- The disconnecting means shall open all ungrounded conductors simultaneously.
- The disconnecting means shall be permitted to disconnect the capacitor from the line as a regular operating procedure.
- The rating of the disconnecting means shall not be less than 135 percent of the rated current of the capacitor.
NEC Article 460-8-a
- The ampacity of capacitor circuit conductors shall not be less than 135 percent of the rated current of the capacitor.
Following the requirements set forth in NEC Article 460, the wire, fuse and disconnect sizes for a 400 kVAR capacitor bank at 480 volt, 60 Hz would be as follows:
400 kVAR x 1.2 amps per kVAR = 480 amps
480 amps x 135% = 648 amps
Disconnect or circuit breaker size = 800 amps
Fuse size = 800 amps
Wire size = (2) 500 mcm per phase
Here is a convenient chart of recommended wire, fuse and switch sizes for 3-phase 60 Hz capacitors - 240, 480 and 600 volts.