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.

Example:
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.