Variable Frequency Drives (VFD) are among the most common electrical control devices found in an industrial facility. VFDs precisely control motor speed rpm by changing its output frequency and voltage. Industrial users utilize VFDs to improve their process control, save energy, reduce maintenance costs, and improve reliability. A typical VFD system consists of the drive, a VFD cable, and an AC motor (see the figure below).
Cables connect VFDs to motors, playing a critical role in the safe, efficient operation of the system. In the past, there were no standards for what a manufacturer called a VFD cable. Cable manufacturers used their own judgment to classify their products as VFD cables. This all changed in 2018 with the National Fire Protection Association (NFPA) updating NFPA 79: Electrical Standard for Industrial Machinery.
The introduction of cable construction requirements and recommendations under NFPA 79 article 18.104.22.168 is a good step to ensuring the proper cable is specified and used in these applications. Paragraph 22.214.171.124 of NFPA 79/2018 Edition says:
"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.”
How does this make it easier? Now, all you need to do is confirm that one of these six designations is printed on the cable jacket.
Whether you are specifier, engineer, or installer you want to see “Meets NFPA 79 2018 Standards.” Otherwise, you won’t be assured of receiving a true VFD cable for your application.
Thermoplastic insulation material is no longer allowed. THHN, THWN and THWN-2 designated cable cannot be used for VFD applications. These are considered more prone to melting (the first letter “T” indicates thermoplastic). In the past, THHN cable has been a lower-cost alternative to truly designed VFD cables.
The National Fire Protection Association made choosing a VFD cable easier by adding this new standard to NFPA 79, creating safer environments for all VFD machinery applications. With our cable manufacturing partners, we can assist you in choosing the right VFD cables, a critical component in the complete VFD system. Contact us today.
Want to learn more about VFDs? Check out our popular, on-going blog series from Automation Specialist, Kevin Beach.