VFD "cables"

Electromatic

Member
Location
Virginia
I don't do a lot of work with VFDs but enough to know the basics. While looking at a job to replace some MCCs with VFDs, I discovered that NFPA 79 in 2018 states, “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.” We don't do enough industrial work to follow updates to NFPA 79.

I know the principles behind using R- and X- types of insulation with VFDs. My main question here is the use of the word "cable." I think of cable as a multi-conductor assembly. Is this recommendation (or mandate) meant only to apply when using pre-manufactured "cables" with conductor symmetry, symmetric grounds, etc. selling points, or does it apply to wiring with individual conductors in conduit also?

We're on NEC 2014, and I personally have never seen NFPA79 called out or enforced. I'm more just curious as to the extent to which this standard is meant to apply.
 

kwired

Electron manager
Location
NE Nebraska
first there isn't 2018 states, just 50:)

Next do you have any requirement to follow NFPA 79?

Most places have some requirement by law to follow some version of NFPA 70 but not many other electrical related NFPA publications
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
NFPA79 tells you what situations it applies to, so if you claim to meet those requirements you have to see where NFPA79 says it would apply.

In general it is not a standard like NEC that there is a legal requirement to abide by. there are good reasons to do so, but it is not a legal requirement.

Personally, I look at the millions of drives wired up with regular wire that have worked fine for decades and wonder why we need VFD cables. OTOH, clearly using appropriate VFD cables instead of regular separate conductors is a "better" solution. Whether is is better enough to actually use it should be a design choice. I personally think over time there will be a migration to more cabling and less conduit for most systems, not just VFDs. In many cases it is more cost effective solution as well. But conduit is a tough thing to kill off in the US despite most of the rest of the world going away from it.

I think there is a fear in some corners that the skilled trade aspect of electrical installation will go away if cabling and wire ways other than conduit replace conduit. People do not much like change anyway. It is certainly true that in many places you can install cable trays that allow for a lot of future flexibility and eliminate a lot of electrical labor over time. It is also a lot simpler to reconfigure manufacturing spaces if you use less conduit. Simpler meaning less labor. That is not met with relish in some quarters.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
IMHO, using "VFD cable" AND putting it in STEEL conduit all the way from VFD to motor are redundant. But if you use PVC conduit, or no conduit, or a portion of your run is in tray, PVC, NM Flex or generally mixed with other cables, then you must use the VFD cable.

But even if you use steel conduit all the way from the VFD to the motor, you want to use the RHH/XHH cable; i.e. XLPE insulation.

But as mentioned, NFPA 79 is a "voluntary" standard, however if your customer says they want you to follow it, then follow it.
 

tom baker

First Chief Moderator
Staff member
Yes using XLPE insulation is worth the extra cost over PVC. I have seen many cases where PVC insulation has failed - when installed outdoors in wet locations. Our specs called for XLPE outdoors when underground. This is from the perspective of working maintenance for 40 years at treatment plants, pump stations, etc
 

Electromatic

Member
Location
Virginia
There are at least 2018 states...of confusion when the NFPA is involved!

I have no instructions to follow NFPA 79. I was just researching and didn't know such a recommendation/edict had been issued. It seems by at least Jraef's interpretation, NFPA 79's use of the word "cable" is consistent with my definition of multi-conductor assembly. Wire in conduit SHOULD be R- or X- insulation but isn't called out in the particular quote in my original post.

For the job I'm currently looking at, the organization is usually overboard on their requirements. For this one, though, they call for THWN in EMT between the drives and the motors. I will recommend XHHW.

One other thing these plans call for (more typical of this organization) is new shaft grounding rings onto the existing motors. I had to look them up, but I have heard of VFD/motor bearing issues before. Anyone have experience installing these?
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
...

For the job I'm currently looking at, the organization is usually overboard on their requirements. For this one, though, they call for THWN in EMT between the drives and the motors. I will recommend XHHW.
Good conscientious plan, looking out for the long term success of the installation. Kudos.
One other thing these plans call for (more typical of this organization) is new shaft grounding rings onto the existing motors. I had to look them up, but I have heard of VFD/motor bearing issues before. Anyone have experience installing these?
There are several different types, I like the Aegis retrofit ring kits, because they come as a split assembly that you fit around an existing shaft without having to de-couple the load.

But it also depends on the way the motor frame looks, there is no "one size fits all" solution. What I have done is have someone take photos of all of the motors at the load shaft end, then determine in advance what it going to work. If they are really old motors, consider recommending replacing them or having them rewound for inverter use, then when they do, they can have the bearing seals replaced with ones that have a shaft grounding ring as part of the seal. Inpro-Seal makes a good one for that.
 

Electromatic

Member
Location
Virginia
Thanks for the advice! The Aegis rings were some of the first to come up when I looked. How crucial is it to know the shaft diameter AND the NEMA frame?
 

paulengr

Senior Member
Aegis rings are vastly overpriced and don’t work very well and need to be maintained ((cleaned) and lubricated with colloidal silver.

Vastly cheaper and more reliable is just a brush.


We sell both. The Aegis rings are easier to mount for the motor shop, Helwig for field installs. Motor shops make better money on Aegis.

Shaft grounding ONLY addresses damage caused by common mode which is usually caused by crappy grounding. Load reactors, dc/day, line termination, and sine wave filters only incidentally help by making the output more sine wave like and only sine wave filters really do much. These are really intended to address line length issues (reflected wave). The best filters are common mode filters which are toroidal rings. VFD cable addresses EMI. It might address cable unbalance at 500 feet or so, I’ve never measured unbalanced currents due to cable unbalance so it’s a scam as far as I can tell,

NEMA did some good work on cable types. XHHW is marginally better than THHN but it doesn’t matter because both vastly exceed motor surge voltage limits when dry. When wet THHN and keep in mind this is old style THHN not moisture resistant THWN or THWN-2 loses half of its surge resistance when wet...which of course makes it an improper installation (dry location only) while XHHW only loses about 15%. For the small 12-14 gauge sizes it is less than motor surge ratings so far those sizes only XHHW makes sense. At 20 mils or more it doesn’t matter.
 
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