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When I was a first year I helped a journeyman pipe a mechanical room in a commercial building. We connected probably twenty 600v motors and drives and I distinctly remember using EMT, metallic liquid tight flex and 600v rated T90 conductors.

From what I learned in school and on this forum since that time, that installation would be less than ideal because the voltage spikes associated with rectifying AC would create corona discharge that pinhole the insulation. By my calculations, on a 600v system the rectified DC would be 846v.

I see that most cable manufacturers sell at least one type of VFD cable. Since VFDs are everywhere now, when is one of these special cables actually a necessity? I ask because I've never actually seen any in person. Whenever I've seen a VFD in an industrial application the motor has generally been connected using 1000v Teck 90. Is this acceptable or are you theoretically supposed to use VFD cable "every time"?
 

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We do the same as Don. We would use FMC or LFMC for the connection to the motor.

VFD cable would only be for open wiring designs.
 

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That's a good question that'd I'd like to know the answer to also.

I realize VFD manufacturers most likely recommend it for every install, but myself, I've really only installed it when I've also used EMI/RFI filters because of potential issues with RFID readers for cow ear tags.

Another thing I've read is to keep VFD motor leads in their own metallic conduit and away from other motor leads. I've been on one job where the VFD leads have been XHHW or THHN in their own pvc conduit, but mixed in with a bank of other conduits, some 400+ feet long, which include other VFD conduits as well.

What's surprising is everything I've read says this type of install should cause VFD issues and the THHN/XHHW insulation should break down, but this particular install is 4+ years old and hasn't had any wire or drives replaced to my knowledge for this issue.

This makes it difficult to justify the cost for me of VFD cable plus oversized conduits unless I'm missing something in the bigger picture that I don't know about yet?
 

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Bilge Rat
motors and controls.........
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I've yet to see this 'special' cable as well. I use basic pipe, flex and THHN. Never had an issue, even on 500' runs.

I've also run 120AC controls in the same pipe as the VFD output.

Seems to me that the 'requirement' for special cable is a lot like the requirement for AFCIs.
 

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Never seen an issue, I've always piped them with emt and lfmc and wired with thhn/thwn...
 

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99% of the time we use metallic conduit and thwn wire. We are starting a job next week and the engineer Speced VFD cable. Not sure why exactly.
 

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I always used pipe and THHN myself. However, special cable was just starting to appear on the market right before I left the business.
I also never had any issues with this wiring method though I can promise you, there will be several who will disagree.

I can't really take a side on this as I only know the conduit and THHN side.
 

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I have started specifying XHHW for VFDs now, just because that is what is used in VFD cables, but it is not based on any type of failure. It is also a tougher cable and I specify it for all underground runs too.

We have also connected may older non-inverter rated motors to VFDs without any issues and we typically don't use load side reactors unless the wiring between the VFD and the motor is over 500'
 

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I think Jreaf taught me vfd cable in a steel raceway is just redundant and unnecessary. Like don said the xhhw and a properly bonded steel raceway is the way to go


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This is something that you should not rely on as "Rule of Thumb" type of thing. If you're wrong and have only pulled single conductors and need to instead pull cable then your conduit may be too small. Plus you'll end up looking bad.

I try to call the VFD's manufacturer and let them tell me. Running the cable drives up the installation cost but at least you've done your due diligence.
 

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I was looking at a installation manual from a major manufacturer, one of their requirements says to run motor cables from multiple vfds in separate conduits.
Do most manufactures have this requirement?
 

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IIRC, the universal standard test for 600 V conductors is TWICE the labeled voltage PLUS + 1000 VAC. ( It's a NEMA thing, IIRC. )

Hence the run-of-the-mill THWN-2/ MTW/ XHHW-2 is routinely tested at 2,200 Volts before it's kicked out the door. (So they say. I've seen the occasional conductor with factory flaws -- which should've busted out into a corona at 2,200 volts.)

So, it's no surprise that such conductors can tolerate ordinary VFD harmonics.
 

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From what I learned in school and on this forum since that time, that installation would be less than ideal because the voltage spikes associated with rectifying AC would create corona discharge that pinhole the insulation. By my calculations, on a 600v system the rectified DC would be 846v.
A regular AC sine wave from your favorite local utility at 600 Vrms will also peak at 846V, so I don't see why that would be any different on a VFD from a purely "peak voltage" standpoint.
 

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Anytime you aren't running a metallic raceway all the way to the motor you should be using drive cable. Or if you absolutely have to run multiple drive outputs in one conduit. Those are the biggies.
 

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I feel compelled to correct some slight misinterpretations here (and confirm other statements). I had a doozy of a reply all typed up on this, and my internet went down before I was able to post it... I hate when that happens. :censored:

This is part of what I do for a living, so I'm pretty sure I have posted all that someplace at one time or another and rather than re-do it now, I'm going to try to find it and link to it.
 

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This is something that you should not rely on as "Rule of Thumb" type of thing. If you're wrong and have only pulled single conductors and need to instead pull cable then your conduit may be too small. Plus you'll end up looking bad.

I try to call the VFD's manufacturer and let them tell me. Running the cable drives up the installation cost but at least you've done your due diligence.

If I called the manufacturer every time I hooked up a vfd I'd get less done than I do now. Of course drive manufacturers will tell you it's best to use their brand drive cable but why wouldn't they. I couldn't even begin to guess the amount of vfds I've hooked up in the last 5 years alone that weren't simple pipe and wire.


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If I called the manufacturer every time I hooked up a vfd I'd get less done than I do now. Of course drive manufacturers will tell you it's best to use their brand drive cable but why wouldn't they. I couldn't even begin to guess the amount of vfds I've hooked up in the last 5 years alone that weren't simple pipe and wire.


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My experience has been if there happens to be something flakey going on and I need to call tech support they will point to any part of the installation that doesn't follow their manual or recommendations. Then my customer gets cranky and starts complaining that is wasn't installed correctly.

Better safe than sorry.
 

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Bilge Rat
motors and controls.........
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My experience has been if there happens to be something flakey going on and I need to call tech support they will point to any part of the installation that doesn't follow their manual or recommendations. Then my customer gets cranky and starts complaining that is wasn't installed correctly.

Better safe than sorry.
This is the reason why a lot of tech support is useless.

Always blame the installer.........
 
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