National Electrical Installation Standards

Standards as High as Your Own

Tuesday, January 21, 2020


Is THWN conductor insulation suitable for use for the branch circuits for an operating room that is supplied by an isolated power system?

Anthony Goode



Hey Anthony, thanks for the question.

The dielectric value on THWN is typically not suitable for use on branch circuits supplied by isolated power systems. While not directly prohibited by the Code, Section 517.160(A)(6) not using wire-pulling compounds and informational Note No. 2 discusses minimizing conductor length and using conductor insulation's with a dielectric constant of less than 3.5 and an insulation resistant constant of greater than 6100 megohm-meters. This can reduce leakage from line to ground, thus reducing hazard current in those locations or spaces. Many isolated power system manufacturers require specific conductors with specific insulation be used with there equipment. XHHW is a common insulation specified. Because Isolated power systems are listed, the often include specific installation instructions, so triggering the general requirements in 110.3(B). It's always a good practice to work with the manufacturer of such equipment.  

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