Cable Fables–Confusion and Misconceptions about NFPA-79

The 2007 change to the NFPA-79 electrical code states that AWM-style single-conductor wire or multi-conductor cable is not permitted on machinery unless it is part of a UL-listed assembly. In other words, machine wiring requires UL-listed cable. The question for many is, which UL listing is appropriate for particular applications? Different interpretations of NFPA-79 have generated discussions and debates. 

Tom Collen of Northwire, Inc., provides information to sort out misperceptions perpetuated about NFPA-79.  “Some integrators and customers are being misled about what is the best cable for their application because of confusion, misinformation, or lack of knowledge about the code changes among cable manufacturers,” he says. “However, the mystery about NFPA-79 isn’t necessary. A careful reading of the code by a discerning cable expert reveals clear guidelines.”

A few examples are mentioned here, but you can find more detailed information on Northwire’s website.

AWM is permitted in several cases. Table 12.2.7.3 of the NEC says:  “… Exception:  when part of a listed assembly suitable for the intended application, Type AWM shall be permissible.”

It is also permissible to use cable that has multiple listings and recognitions¾listed type TC (Tray Cable) and AWM-recognized cable, for example. In this instance, the printed legend on AWM may be ignored in deference to the cable’s listing type, TC.  UL (or other agency) listed cables are required.   

Although table 12.3.1 of the NEC specifically lists other types, such as UL-listed conductor types THHN, THW, THWN, RHH, and permitting MI (Mineral-Insulated wire), NFPA-79 allows any listed conductors and cables. Table 12.2.7.1 states: “Other listed conductors and listed cables shall be permitted.”  This statement is not new. It has been part of previous NFPA-79 standard editions. The point of the NFPA-79 change was to disallow AWM-only types that were inappropriate to the installation. Other UL-listed types, such as UL type TC must be used under their own rules. In the example above, the use of TC is governed by NFPA-70, article 336.

NFPA-79 does not specifically address 600V cable insulation rating. Table 12.3.2 specifies insulation thickness for 600V-rated conductors according to UL 1063¾a specification for MTW (Machine Tool Wire).  Some have interpreted that specification to infer a 600V rating requirement for the cable.  UL 1063 does not define cable types such as low-voltage communication or instrumentation cables, nor does NFPA-79, section 12.3, “Insulations”¾perhaps a carry-over from the standard originally released in 1943.  The section needs to be revised (e.g., applies to power circuits only) to eliminate inference and differences of interpretation. 

Careful, precise reading of NFPA-79 can affect a company’s bottom line by helping to ensure you select the appropriate allowable cable that meets your need without unnecessarily exceeding it. Copies of NFPA-79 are available for purchase from www.nfpa.org. True understanding of the code helps to ensure that you will not be influenced by hearsay and hyperbole, and helps you make well-informed decisions about what is required for your cable situations.

Northwire, Inc.
www.northwire.com/nfpa