What is the minimum size grounded conductor required for the following installation?

A service consists of a NEMA 3R service-rated fusible disconnect. The load supplied is a 100 hp, 460V, 3-phase motor for a pump in the middle of a field (with appropriate motor controls and equipment). The ungrounded service entrance conductors are 2/0 AWG XHHW. The electric utility supplied service is 3-phase, 4-wire grounded.

  1. 1/0 XHHW (70% of 2/0)
  2. 2/0 XHHW
  3. No. 1 XHHW (2 sizes smaller than 2/0)
  4. No need for a grounded conductor since the motor is 3-phase.


Answer: B

Because the electric utility service is grounded, a grounded conductor must be installed. As noted in 250.24(C)(3), the grounded conductor shall have an ampacity not less than that of the ungrounded conductors. This is a new requirement in the 2011 edition of the NEC.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.