National Electrical Installation Standards

Standards as High as Your Own

Tuesday, August 14, 2018


Re: CQD answer published Tuesday, August 7, 2018 -

Instead of using THHN conductors (I just assumed that since very popular), use a Machine Tool (MT) conductor with similar electrical properties but with finer strands of softer, or annealed, copper. The THHN and standard types of conductors are not designed for continuous flexing or significant vibratory motion because of stiffness. Machine tool wiring is designed for such applications. If that's not desired, a short length of outdoor rated type S flexible cord could be used. Type SWO comes to mind, but there are many others that even have better environmental ratings.

Best Regards, Nick Abbatiello


Hey Nick thanks for your comment, it's good to hear from you. An installation on a roof would be considered a wet location as that term is defined in Article 100 (Location, Wet). Conductors inside raceways above grade that are in wet locations must be suitable for wet locations as stated in 300.9 and comply with 310.15(C). Conductors that are only marked THHN are only suitable for dry and damp locations as shown in the application provisions column of Table 310.104. Many conductors marked THHN are also marked with insulation types that have a "W".

Machine tool wiring is intended to be used for machine tools as shown in the application provisions column of Table 310.104. The Informational Note references NFPA 79.

Flexible cords could be used as an alternative as stated in 400.10(A)(7) but also need a "W" as part of the marking as shown in the Use column of Table 400.4.

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