NFPA 79 Requirements: Are Your VFD Cables Compliant?
Written September 2018 | Lee Perry, Engineering Manager at Service Wire Company
The increase in variable frequency drives (VFDs) over the past decade has resulted in the need for specifically engineered wire and cable solutions to achieve long-term performance and operation.
Many industry groups, standards, and specifications exist to advise VFD cable system designs and use. One important tool used is NFPA 79: Electrical Standard for Industrial Machinery, which is part of the standards portfolio of the National Fire Protection Agency (NFPA). The latest version of NFPA 79 establishes very specific wire and cable requirements for VFD applications, accumulating decades of industry experience into a design document.
NFPA 79 Recommended Insulation
NFPA 79 now discourages the use of thermoplastic insulations (THW, THWN-2) and encourages the use of thermoset insulations (XHHW-2, RHW-2).
Thermoplastics exhibit a higher dielectric constant and a lower resistance to corona inception voltage. High dielectric constant results in higher cable capacitance and higher charging currents, which can be electrically demanding to the insulation over time. Lower resistance to corona inception can lead to premature dielectric breakdown, which results in cable failure.
In addition to these issues, thermoplastic insulations do not perform well in DC applications or in applications such as VFDs that mimic the function of a DC circuit. Also, thermoplastic insulations can deform and melt when exposed to excessive temperatures, such as those that occur electrically when standing voltage waves exist within a VFD cable circuit.
Flexible Motor Supply Cable
NFPA 79 now specifies flexible motor supply cable, referring to the stringent requirements of UL 1277 (Power and Control Tray Cable) as well as UL 2277 (Outline of Investigation for Flexible Motor Supply Cable and Wind Turbine Tray Cable), not to be confused with flexible motor lead wire or hook-up wire.
The combination of demanding requirements of UL 44 (Rubber Insulated Wire) and UL 1277/2277 results in a VFD cable of outstanding performance if the proper cable design/system design is also used. Shielded cables, multiple grounds, tight construction geometry tolerance, and termination methods are all critical to long-term viability and performance of the VFD circuit.
When specifying your cable systems for VFD applications, keep the following NFPA 79 requirements in mind:
Avoid Thermoplastic Wires
(THW, THW-2, THHN, THWN, THWN-2, UF)
Specify Thermoset Wires
(printed RHW-2, XHHW, XHHW-2)
Require Tray Rated Products Listed to UL 1277/2277
(type TC, CT rated jacketed cables)
Ensure Cable System Includes:
- Multiple Grounds
- Proper Shielding
- Proper Shield Termination
ServiceDrive® VFD Cable designs have always met the new 2018 NFPA 79 cable requirements.
The ServiceDrive® system for VFD applications includes proper overall cable design and shield termination methodology (cable, connectors, and termination kits) to ensure long-term viability of VFD circuit requirements.
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