New Standard Dictates Design Requirements for VFD Applications

One of these—NFPA 79, The Electrical Standard for Industrial Machinery—has recently been updated from its most recent prior 2012 publication. NFPA 79 provides safeguards for industrial machinery to protect operators, equipment, facilities, and work-in-progress from fire and electrical hazards. The potential impact for users of Variable Frequency Drives (VFDs) can be profound, especially when it comes to the type of cable that is allowed to be specified for their use.

Providing a Convenient “List” of Approved VFD Cable Types

As with many regulations, NFPA 79 2018 is lengthy and detailed, but the passage of most vital importance to VFD cable users can be found in Section 4.4.2.8:

“Electrical conductors and equipment supplied by power conversion equipment as part of adjustable speed drive systems and servo drive systems shall be listed flexible motor supply cable marked RHH, RHW, RHW-2, XHH, XHHW or XHHW-2.”

The new regulation flows from concern over the proliferation of cable constructions that run the risk of not being able to handle the demands of VFDs as compared to those of standard motors, such as cables with overly thin insulation material, or those that cannot otherwise operate effectively in the face of the increased output voltages or temperature demands of VFD applications. Such cables can lead to issues ranging from wasted energy to equipment failure or even significant safety concerns.

Helpfully, the regulation conveniently lists the designations of the insulators that are approved for use in VFD machinery applications, so all that operators really need to do in any new installation in order to avoid fines or the need to tear out and replace their new wiring is to look for any of these six designations printed on the cable jacket and specify accordingly.

However, there are a number of attributes inherent to these six designations and absent in others. Understanding this often unstated, “reading between the lines” information may be valuable for operators to more fully grasp what is now specifically being “outlawed,” and help them achieve optimum safety and performance. As a bonus, you’ll stay on the right side of inspectors:

• All VFD cable must use conductors of crosslinked polymers—thermoplastic wiring material such as those designated THHN, THWN and THWN-2, considered more prone to melting, are not allowed. (Note the first letter “T” designates non-allowed thermoplastic.)
  
  
• All VFD cable must utilize UL approved conductors; these six designs incorporate thermoset material approved under UL 44 for the application.
  
  
• All VFD cable must exceed certain thicknesses, putting an end to the disturbing trend toward manufacturers thinning their inner conductors to save money.
  
  
• All VFD cable must be specifically marked as a flexible motor lead cable; all six listed cable designs listed fulfill this demand.
  
  

Buyer Beware

Although operators accustomed to working with quality VFD cable suppliers should have little cause for concern, there is, especially in the short term, an increased danger of being the victim of “misleading” cable claims. Some manufacturers and distributors of “outlawed” thermoplastic and similar cables may strive to unload their excess inventory—still deemed acceptable for many non-machine applications, but now significantly curtailed in their allowable scope of usability.

One thing to look out for are claims using abbreviated phrases such as “NFPA 79 Compliant” or “Meets NFPA 79 Standards”—claims now rendered vague by the update of the standard. Operators should be aware to instead now be on the lookout for the phrase “Meets NFPA 79 2018 Standards” to ensure that the cable doesn’t simply meet the prior NFPA 79 2012 standards and get them in hot water with an astute electrical inspector. If in doubt, ask for documentation or get guaranteed specifications in writing for your own protection.

Known as the originator of the product category, Belden has been supplying VFD cables for more than 20 years. Always intent on providing nothing but the highest quality cables, the company has not had to make any changes to its design to be fully compliant with the stringent new NFPA 79 2018 specs. All of Belden’s VFD cables are and always have been cross-linked, flexible motor lead cables that provide optimum wall thickness, are UL listed and likewise meet all the new demands of NFPA 79, stated and otherwise. Belden stands behind that promise with a full 10-year factory warranty—otherwise unheard of in the industry.

We believe that the new standard is of vital importance in optimizing safety for all VFD machinery applications. If you have questions, we are glad to be a technical resource to help you ensure that what you are getting will protect your people and your equipment, and keep you in the safe zone for the long term.