National Electrical Installation Standards

Standards as High as Your Own

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Friday, May 1, 2020

Question:

Hello,

What size grounding electrode conductor would be required for a 4 Plex service with 4 100 amp mains? Thanks for any help you might be able to provide.

James Sacry

A

Answer:

Hello James and thanks for the question. There are some key elements missing from the question, but here is an attempt to provide what you are looking for. First, the key elements missing are what type of grounding electrode(s) are present for use as the grounding electrode system, and second, what is the size of the largest ungrounded service conductor supplying the service equipment. Here are the answers using a couple assumptions: The service is a 400 ampere, meter pack (with 4 100 ampere breakers) supplied by 600 kcmil copper service conductors. The grounding electrode system is a water pipe electrode and two ground rods. Using Table 250.66 and Section 250.66 the minimum size grounding electrode conductor for the service would be 1/0 copper or 4/0 aluminum or copper-clad aluminum to the water pipe and a minimum 6 AWG copper to the ground rods. The key in this response is the use of Section 250.66 and Table 250.66 in the NEC to determine the minimum size GEC for the service. Hope this is helpful. 

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