National Electrical Installation Standards

Standards as High as Your Own

Friday, December 20, 2013



I appreciate the fine responses to these questions every day. I was recently installing THHN/THWN conductors for the branch circuits for an operating room supplied by an isolated power system. The hospital inspector says that these are the wrong type of wires for the system. What types of conductors are acceptable for such installations?

Kurt Spencer




Hi Kurt,

Thanks for the nice comments. Section 517.19(F) requires isolated power systems to be listed and installed in accordance with Section 517.160. Sections 517.160(A)(5) and (6) include the requirements for circuits supplied by isolated power systems. A specific color code according to 517.160(A)(5) is required and 517.160(A)(6) cautions about using wire-pulling compounds (soap or lubricants) that can increase the conductor dielectric constant. Informational Note No. 2 to 517.160(A)(6) contains important information regarding the electrical characteristics of the conductors supplied by an isolated power system. The conductor insulation should have a dielectric constant not less than 3.5 and an insulation resistance constant greater than 6100 meg-ohm meters to reduce leakage from line to ground. Since isolated power systems operate ungrounded and monitor leakage current levels, higher insulation resistance values are recommended. The Code does not specifically address this other than the Informational Note, but it is a requirement because of Section 110.3(B). The manufacturer’s installation instructions for isolated power systems typically indicate maximum length of circuit conductors and the insulation type that should be used. XHHW and XHHW-2 are two types of insulation that would meet the manufacturer’s requirements and provide higher insulation resistance constants as recommended in Informational Note No. 2. The answer to this issue will be found in the installation instructions.


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