National Electrical Installation Standards

Standards as High as Your Own

Tuesday, August 7, 2018


Re: CQD answer published Friday, July 27, 2018 -120 volt ungrounded systems


But what about an isolated power system?

Matthew Hermanson



Hey Matthew thanks for your follow-up question. You have a good point, systems that are described in 250.21 are not required to be grounded and those described in 250.22 are not allowed to be grounded. Ground detection as stated in 250.21(B) and marking as stated in 250.21(C) are also required. Other 120 volt systems that supply premises wiring must be grounded unless there is a rule in Chapters 5 through 7 that allows them not to be grounded. If you have a specific example of one that isn't covered by the existing rules consider submitting a Public Input to add it. You might need to wait for the 2023 NEC process if it is a new concept.

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