Confusion Over NFPA Regs About UL-Recognized Cable

OSCEOLA, Wis.—Confusion abounds about the "new" NFPA (National Fire Protection Association) regulations pertaining to UL-recognized (AWM style) cable. Long after the practice became widespread, inspectors became aware of the ubiquitous use of AWM (Appliance Wiring Material) style cables in machine interconnecting sensors, actuators, switches and other components—often as part of premolded connector assemblies.

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. Clearly, UL-listed cable has been available for many years. So the real question is, which UL listing is appropriate for particular applications?

Herein lies the confusion—speculating about the intent of the code change. Different interpretations of NFPA-79 have generated articles in trade publications and on Web sites, in white papers and in discussions and debates.

A Few of the Fables

Tom Collen, marketing consultant for Northwire, Inc., provides information to sort out the misperceptions perpetuated about NFPA-79. "Some integrators and other 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. "This is apparent from just looking at blogs and forums on the subject. However, the mystery about NFPA-79 isn't necessary. A careful reading of the code by a discerning cable expert reveals clear guidelines."

Collen addresses specific misconceptions:

　"AWM is strictly forbidden. If AWM is marked on the cable, I cannot use it."

Not true. 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."

　In another notable exception, it is permissible to use cable that has multiple listings and recognitions—listed 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.

　"The cable must be MTW."

A fable. 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.

　"The cable must be 600V."

A rumor. NFPA-79 does not specifically address 600V cable insulation rating. Table 12.3.2 specifies insulation thickness for 600V-rated conductors according to UL 1063—a 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"—perhaps 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.

　"The smallest conductor size allowed is 22 AWG."

Fiction. This interpretation derives from NFPA-79, section 12.3. However, conductor sizing is subsequently discussed in section 12.6, which states: "Conductors shall not be smaller than 14 AWG for power circuits unless otherwise permitted in 12.6.1.1 or 12.6.1.2." These sections define exceptions allowing 16 AWG and 18 AWG under certain conditions. Section 12.6.4 (1) allows 24 AWG for electronic PLC I/O or static control circuits in a raceway (30 AWG allowed in a multiconductor cable or cord). Section 12.6.4 (2) allows 26 AWG or larger if the wires are inside an enclosure (30 AWG allowed in a multiconductor cable or cord). These are notable exceptions.

　"Cords must be 600V."

A falsehood. Section 12.8.1 states that cord types "… suitable for their intended use …" listed in table 12.8.2. This table includes numerous SJ types, which are 300V-rated cords.

More than MTW

One wire and cable manufacturer has suggested using MTW to ensure compliance with NFPA-79. MTW is a UL-listed 600V-rated single conductor cable with construction and properties that are governed by UL 1063—the UL standard for Machine Tool Wires and Cables. MTW is generally a larger cable suitable for use inside panels or pulled through conduit. It is constructed with larger copper conductors and contains more plastic than a 300V cable. Many sensor, switch and actuator designs use multi-conductor cables, often employing quick disconnects. One of the most common disconnects found in industrial automation is the M12 connector, which has voltage ratings ranging from 30 VDC to 250 VAC and 300 VDC. Lower voltage ratings and smaller cables generally translate to lower cost. A better choice for interconnection of low-voltage controls and instruments is UL-listed ITC/PLTC (Instrumentation Tray Cable/Power Limited Tray Cable).

The ITC designation was added to the NEC in 1996 to meet contemporary demands for small-diameter, lower-cost cable for industrial environments and process control and instrumentation. The newer ITC specification is simpler to interpret than the older UL 13 specification. The NEC code change made it permissible to use a wiring method that had been used effectively for years on offshore oil rigs, an extremely rugged environment. By definition, ITC cable meets all the requirements for PLTC; hence the dual rating.

　ITC is defined in NEC article 727, which classifies cable for remote instrumentation and controls in industrial environments under certain conditions. Most notably, article 727 specifies 300V insulation-rated cable, limited to applications of 150V or less with a 5-Amp maximum current. The conductor is limited to sizes not smaller than AWG 22 and not larger than AWG 12.

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. ITC/PLTC cable is a more economical choice for industrial environments, process control and instrumentation applications. Other acceptable and lower cost cables worthy of consideration for these applications include CM (for communications cables) CL2, PLTC and SJTO (for power cords). This cable construction and testing are described in UL 2250.

A Bonus: Exposed Run

NEC 2008 article 725.154(D) hazardous (classified) locations (2) and article 727.4(5) allow ER (Exposed-Run) listed cables to be installed: "The cable shall be continuously supported and protected against physical damage using mechanical protection such as dedicated struts, angles or channels. The cable shall be secured at intervals not exceeding 1.8 m (6 ft.)." UL 13 PLTC and UL 2250 ITC-listed cables also may have an ER listing. These cables must pass the same crush and impact tests applied to metal-clad cables, (UL 1569 crush and impact tests). UL 13 PLTC-ER-listed cables meet the requirements of NEC article 725.154(D), and cables UL 2250 ITC-ER-listed cables meet the requirements of NEC article 727.4(5). These provisions can result in significant freedom for installers and system designers and significant cost savings compared to armored or conduit-installed cables. For 600V circuits, type TC-ER is also available.

Knowledge Is Power

NFPA-79 is ambiguous and potentially confusing to the casual reader or one who is tricked by others' interpretations of this important document. Copies of NFPA-79 are available for purchase from www.nfpa.org. Cozy up with a copy—read and learn. At least closely examine the sections that apply to your application. True understanding of the code helps to ensure that you will not be influenced by hearsay and hyperbole. True understanding also helps to ensure that you will make well-informed decisions about what is required for your cable situations.

In Aesop's fable about the boy who cried wolf when no emergency was at hand, the villagers stopped believing him. The moral of the fable is to beware of those who cry wolf about NFPA-79. Discriminating readers will differentiate fable from reality.

Contact Northwire for information and samples at +1 715-294-2121, or www.northwire.com/nfpa.