National Electrical Installation Standards

Standards as High as Your Own

Monday, February 27, 2023


Hello Code Guru, I am working on a bid for an emergency generator upgrade project. The prime mover on generator unit is natural gas, the requirements in NEC 700.12(D)(2) require 2 hours of fuel onsite. The engineer is insistent that we must provide a 2-hour supply of natural gas onsite to meet this requirement. I say that we can apply the exception found in 700.12(D)(2)(c) because the Generator is natural gas, and the natural gas supply is from the utility in a class one (1) city and should be considered reliable. Is utility supplied natural gas considered reliable? Any guidance on this would be helpful. Thanks, Casey Gilbane



Hi Casey, Section 700.12(D) requires any combustion engine prime mover (regardless of the fuel type) must have 2 hours of fuel supply on site. The exception in Section 700.12(D)(2)(c) considers the off-site fuel supply sufficient when the following are met:

  • Approved by the AHJ
  • There is a low probability of a simultaneous failure of both the off-site fuel supply and electric power supply from the utility.

The first step to using this exception would be discussing the installation with the local AHJ. The AHJ will likely have a protocol in place for this situation, which will likely include some type of affidavit from the natural gas utility, attesting to the reliability of the natural gas supply.

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