250.166 Size of the Direct-Current Grounding Electrode Conductor.

These are utility grade DC solar heat generators that direct sunlight into a Stirling engine.
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These are utility grade DC solar heat generators that direct sunlight into a Stirling engine.

Section 250.166 provides rules on proper sizing of a DC grounding electrode conductor (GEC).

DC grounding electrodes and grounding electrode conductors are sometimes required for specific DC systems such as solar photovoltaic systems and fuel cells.

The code language in the parent text of 250.166 makes it clear that the DC GEC is never required to be larger than 3/0 copper or 250 kcmil aluminum even if the conductors supplied by the DC system exceed those values.

Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC® edition of NFPA 70.

2020 Code Language:

250.166 Size of the Direct-Current Grounding Electrode Conductor. The size of the grounding electrode conductor for a dc system shall be as specified in 250.166(A) and (B), except as permitted by 250.166(C) through (E). The grounding electrode conductor for a dc system shall meet the sizing requirements in this section but shall not be required to be larger than 3/0 copper or 250 kcmil aluminum.

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250.166 Size of the Direct-Current Grounding Electrode Conductor.

Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:

Which of the following is true of a DC grounding electrode conductor terminating from an underground metal water pipe to an enclosure where the DC system supplies 350 kcmil copper circuit conductors?

A: The DC GEC is not required to be larger than 3/0 copper.
B: The DC GEC must be 350 kcmil copper.
C: The DC GEC must be 350 kcmil aluminum.
D: The DC GEC is not required for a grounded system.
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