When it comes to cables between VFDs (variable frequency drives) and AC motors, the rules have changed dramatically over the last few years. When NFPA 79 came into play in the 2018 edition, it was truly a game changer. NFPA 79 firmly dictated how these cables needed to be constructed and labeled.
“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”
Source: NFPA.ORG NFPA79 2018 archived revision information.
Now with the NFPA 79 2021 edition on the streets, there have been some minor changes. We do feel it is our responsibility to make certain we pass these along.
But before we look at the changes, let’s take a real quick review of how VFD cables benefit you. Since we’re talking about NFPA 79, let’s take a quick look at what that is first. According to NFPA 79 itself:
“The standard shall provide detailed information for the application of electrical/electronic equipment, apparatus, or systems supplied as part of industrial machines that will promote safety to life and property”
So, how does VFD-rated cable help meet this outcome? VFD-rated cables minimize ground currents, protect against motor bearing damage (such as fluting and pitting), and protects other equipment by protecting against electric interference caused by the high-frequency switching of modern VFDs and reflected wave bouncing back from the motor to the drive, which can double the voltage.
They do this by having the conductors equally spaced and insulated from each other, a quality shielding system, good grounding conductors, and industrial-grade jacketing.
Note the difference between a standard power cable and a VFD-rated cable. Pay particular attention to the ground conductors!
This picture shows all those components of a VFD-rated cable with the top image also being armored.
So, what changes came along with the NFPA 79 2021 edition? Primarily, the construction stays the same but now the labeling has changed. The 2018 edition dictated cables to be marked RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2. But in the NFPA 79 2021 edition chapter four, paragraph 126.96.36.199, this requirement has been eliminated and replaced with the cables “shall be identified as suitable for the electrical power characteristics and in accordance with any instructions provided by the manufacturer(s)”. It also references chapter 12: conductors, cables, and flexible cords for “general” cable requirements.
While these seem very minor (and they are), cable selection became a bit more of a gray area than in the 2018 Edition. How do you know which cable you need and you’re meeting NFPA 79 2021 Edition? The best way to be certain you are getting the correct VFD cable for your application would be to contact your Power Control Specialist and we can help you make that critical selection.
Do you need help finding the right cable for your application? Whether you’re trying to navigate the NFPA 79 2021 edition or looking for practical advice, we’ve got you covered. Our Power Specialists are here with the knowledge and expertise you need. Contact us today!
Illustrations are © Belden Inc.