# VFD Line and Load wires separate



## MDShunk

New one on me. They're quite often in the same gutter if there's a bunch of VFD's mounted above a gutter (typically a retrofit situation, but not necessarily). Occasionally they're in the same raceway, but that's a little more rare (not because it's prohibited, but because it's not a really functional way to wire things). If the VFD(s) are inside a control panel, the line and load are ordinarily in the same Panduit. Of the hundreds and hundreds of VFD's I've dealt with, of many different brands, I can think of zero problems ever associated with this practice, nor can I come up with any manufacturer prohibition against such a wiring scheme. 

I have a funny feeling the person that told you that believes it, but they're either going on gut instinct or still have in their head some job spec from a job long since completed. Maybe there is some old (maybe current) manufacturer prohibition against this practice (I doubt it), but I'm here to tell you there's nothing wrong with it. 

Control power things for the I/O and the network connections (if any), naturally, are a different story. Probably most of us know that in our gut even if we've never wired a VFD before.


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## Southeast Power

I was just thinking how much of an awkward design it would be to have both line and load in a raceway for much more than inside of a cabinet or possibly a retrofit where a local drive needed to be relocated.


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## paulengr

There are problems with multiple VFD loads in the same raceway. Had a customer with 100 feet of conduit going to pairs of motors which is almost the length limit for that VFD using inverter duty motors. Both motors typically at the same speed, THHN wiring. It kept shredding motors from overvoltage but you wouldn’t normally see it. When both VFDs pulsed on just with the right timing they would induce enough of a pulse in the second motor cable to double the output voltage. Lowest cost fix was inserting dv/dt filters. This won’t happen in a short run like a control cabinet or a gutter but over a long line it does.

It’s common practice but not Code to put in two gutters with discrete combination starters with line in one and load in the other fir two reasons. First is because combination starters are sort of built top/bottom feed but VFDs are usually all bottom feed. Second is separation...load side faults in the wiring can’t bypass the starter protection and tear up the line side. Depending on space though not a hard rule and running separate conduits up to the top with a 90 entrance works just as good but has a harder pull since ultimately you have more bends instead of one 90 and a straight run into the disconnect. Controls go anywhere.

The same layout works well on larger soft starts because you have the 6 big rails although it is usually easy to lug them bottom or top. Small soft starts usually adopt more of a VFD style layout with power at the bottom and control at the middle or top.

With VFDs the thing is most have line and load terminals at the bottom and controls some place in the middle. Since it’s common practice to separate the controls and required for Ethernet (power limited cable only, even though you can get 600 V AWM CAT 5E), using the space above the VFD fir control and the space below for power makes the most logical layout. So common conduits at the bottom are common especially on small drives. Top feeds always require some sort of side conduit or gutter.

There really isn’t a reason for VFDs to be all bottom fed. The majority of the cabinet is an input side, DC link, and output side. The IPMs are usually mounted somewhat top/bottom and the fans are top or bottom. It would be easy to relocate at least the line side to the top and honestly except for control cabinets where they are packed in tight having the DC bus and brake chopper terminals to the side would make a much cleaner panel layout with the protection above the drive instead of somewhere not logically laid out. Bypass contactor panels are always a messy layout with bottom feeds.


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## gpop

Its always a good idea to keep them separated as a safety precaution. Its the same as incoming and outgoing from a disconnect should always be in separate conduits.

There may be a arguments that a skilled electrician would never mix the wires up when replacing a drive then again this is real life.


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## eddy current

One of the first ones I installed many years ago was a medium sized VFD on an existing air handling unit in a parking garage and the inspector requested that the line and load be in separate conduits. He quoted a code we have in our code book (CEC) that does not allow voltages from different sources in the same conduit or raceway. 

I did as he requested but since then have installed many and kept the wires all in one raceway without problems.


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## MDShunk

gpop said:


> Its always a good idea to keep them separated as a safety precaution. Its the same as incoming and outgoing from a disconnect should always be in separate conduits.


Love ya, bro, but I think you're way out in left field with that statement.


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## bill39

BravoJSierra said:


> It was recently brought to my attention that VFD's need to have their line and load wires in separate raceway. The reason given to me was "that's just what you do with VFD's." My question I guess is is there a code rule to back this? I mean I understand that with the VFD's there are altering voltages and therefore a chance for induction but still curious if there's a code rule to back up separating the wires. Any info would be greatly appreciated. Thanks for your time.


I would ask the VFD manufacturer if they require or recommend separating the wires. I am not aware of any code requirement.


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## micromind

I've installed over 100 VFDs with line and load in the same pipe. Never had any sort of trouble. 

Usually, the pipe is less than 50' long and often it will contain 120V start-stop or on-off controls. 

I've never had 24DC in pipe with line/load though, but it's common to have it in the same gutter.


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## Wiresmith

do you all never use vfd cable either? i'll try to find it but i believe some manufacturers tell you to have segregation. maybe you won't see problems when the frequencies are close but i figured it was because you would cause harmonics and shorten the life of the drive. and add more harmonics to your system. i often see line reactors, filters and shielded drive cables as well. i've seen it gotten away with with small and somewhat isolated set-ups but i think at least some situations you might want segregation. looking at manufacturers info, looks like they just say avoid it on long runs.


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## micromind

Wiresmith said:


> do you all never use vfd cable either? i'll try to find it but i believe some manufacturers tell you to have segregation. maybe you won't see problems when the frequencies are close but i figured it was because you would cause harmonics and shorten the life of the drive. and add more harmonics to your system. i often see line reactors, filters and shielded drive cables as well. i've seen it gotten away with with small and somewhat isolated set-ups but i think at least some situations you might want segregation. looking at manufacturers info, looks like they just say avoid it on long runs.


I've never actually seen VFD cable. 

Basic THHN seems to work just fine but I have heard of instances where a thin spot in the insulation has been blown through on the load side of a VFD. 

I've heard that XHHW (I think......) is a better choice.


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## paulengr

micromind said:


> I've never actually seen VFD cable.
> 
> Basic THHN seems to work just fine but I have heard of instances where a thin spot in the insulation has been blown through on the load side of a VFD.
> 
> I've heard that XHHW (I think......) is a better choice.


The main difference between let's just say MTW, THWN, and XHHW-2 is that MTW is easier to pull and withstands flexing a little more. THWN is the thinnest but has that irritating nylon sheath that helps make it easy to pull but gets in the way when you're terminating, and XHHW-2 is the thickest insulation because it doesn't have that sheath but the improvement is slight. In terms of VFD cables none of this really matters because the motor insulation even on the more expensive "inverter duty" motors is way less than even plain THHN.

I looked into VFD cable. It took me quite a while to look into it because even if the vendors stuck to the truth, it's 99% a sham. But then they make all these really, really wild claims that well...you'll just have to read it. I had some fun with it. You wouldn't not believe the claims they make.

https://www.linkedin.com/pulse/vfd-cable-snake-oil-paul-campbell/


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## MDShunk

paulengr said:


> https://www.linkedin.com/pulse/vfd-cable-snake-oil-paul-campbell/


You've posted that link before a couple times, but it actually goes to nothing. I think it's not a public link, but rather connected to your personal login somehow.


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## paulengr

It cut it off. Or at least the stupid browser did.

https://www.linkedin.com/pulse/vfd-...cs/urn:li:linkedInArticle:6413702271661592576


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## John Valdes

I have never mixed line and load. And if I did, I would make sure I used a line reactor.


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## MDShunk

John Valdes said:


> I have never mixed line and load. And if I did, I would make sure I used a line reactor.


Well, good for you, but... what benefit does that provide or what problem does it solve by never mixing line and load? I'd suggest.... absolutely nothing based on my experiences with drives of every flavor installed in both the best and worst of conditions.


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## John Valdes

MDShunk said:


> Well, good for you, but... what benefit does that provide or what problem does it solve by never mixing line and load? I'd suggest.... absolutely nothing based on my experiences with drives of every flavor installed in both the best and worst of conditions.



I guess I don't have a good answer other than thats how we did it and thats how the manufacturers recommended we did it.


I am certain unless you are using VFD cable, load conductors leaving the drive have the capability to induce unwanted currents in the line conductors and these variants would be choked with the line reactor.


I have dealt with numerous nuisance tripping in the early years of AC drives. Some manufacturers like ABB installed them onboard later on. They still might. Not sure.

Many problems we faced were corrected and or helped by the addition of line reactors.
Baldor gave a free reactor with a drive and motor purchase. There was a reason.


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## gpop

Abb 550 size drives still come with a line and load reactor built into them.


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## tates1882

I know for sure that siemens v20 series, ad pf 40, abb 325, yaskawa, and franklin all recommend that line/load isn't installed in the same conduit. I believe it has to do with carrier wave frequency, induced voltage, and noise. I've done it several times and the only time I have trouble is when the vfd controls a load driven motor like a closed loop circ pump or air blower.


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## tates1882

Ive used Belden Vfd cable, comes with a 10 year warranty. I don't use THHN unless the install is in a dry location. RTW/MTW when its a wet location. AB recommends not using THHN on the load side in a wet location.


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## paulengr

Line reactors are there to block noise generated by the line side. Some cheap drives only use a free wheeling diode bridge or passive rectifier. At BEST the THD off of this thing is around 20%. By inserting a filter in the DC link you can significantly improve noise on both the line and load sides. THD can be further reduced in a couple ways. First, you can put an isolation transformer in to feed the VFD. This was the way it was done back "in the day" in the 1990's for instance and even earlier, and still works great if you need a transformer in the first place. With a delta-wye you get rid of all even harmonics and also handle drives that aren't designed to handle ungrounded or resistance grounded systems. Also all ground faults and all even harmonics from the drive are blocked. Second thing is using an active front end. This significantly reduces THD by itself down to if I recall correctly around 6%. Even more fancy is using a drive with multiple intermediate voltages off a tapped transformer (12, 18, 24, 36...pulse drives). This can get you to essentially any desired THD but each stage is effectively doubling the number of drive components so the price is equal to double, triple, etc...so this is a nice idea but basically dead end technology due to costs. Finally you can meet IEEE THD requirements using just basic filtering which is a line reactor when the noise off the drive is unacceptable.


As to when it's "unacceptable" this is something of a moving target. THD will suck very bad when the drive is lightly loaded (off more than it's on). Since THD is more of a "ratio" thing and the effect has more to do with absolute values (how many volts/amps of noise), in my mind this is kind of a meaningless metric. What we really want to know is if we have harmonics at full load, how much additional stress on the transformer does it create and at what point do we cause overheating? Generally this is pretty easy to figure out and don't forget that if you have a bunch of other (non harmonic generating) loads on the same transformer, it's pretty easy to guarantee that THD never really becomes a problem.



Similarly and a little harder to quantify, drives can mess with each other's controls. If you have 2 or 3, no problems but when you start to have more than that, the amount of harmonics can cause control problems on occasion with adjacent drives on the same bus. Most of the time though the big problem is that even though the manual says in big bold letters to ALWAYS run a separate ground wire directly from the motor frame to the drive (common mode noise), I still constantly see installations without one. This is a far worse problem than having too many drives on a common bus.


Load reactors are a totally different issue. That has to do with reflected waves and filtering off the high frequency harmonics that cause the issue. Not sure why anyone would actually use a load reactor anymore though. A full blown dv/dt filter is so inexpensive even though it costs more than a load reactor that it's not even worth it to chance it on a load reactor...go for the real protection every time.


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## JRaef

When you put line and load conductors of a *starter *in the same conduit, it's irrelevant because the line and load are always at the same frequencies relative to each other; 60Hz, so a cancellation effect on the magnetic fields around the adjacent conductors takes place. But with a VFD, the line side is always at 60Hz (here) but the load side is always going to be at a different frequency, or even if at the same frequency, not synchronized any longer. So the magnetic fields around the line and load conductors are no longer cancelling each other out and you _will _have mutual induction taking place onto the conductors, increasing the voltages. In the case of the line side this is basically irrelevant, but on the load side, it can severely exacerbate any standing wave or reflected wave effects taking place. I have seen a 250HP motor destroyed in 6 months because the line / load conductors shared a common cable tray for just 25ft of a 200ft run. The motor was a Baldor Inverter Duty motor with 2200V insulation and the Yaskawa VFD had a load reactor installed. But because of that doubled run for just a portion of the cabling, we saw spikes of 2800V getting to the motor. We separated the conductors and the highest spike was only about 1400V. 



Even if you don't have the line conductors in the same conduit, it's not a good idea to run multiple VFD outputs together for the same reason; different frequencies. I had another one years ago at an aggregate plant where there were 8 x 5HP VFDs in a panel, all feeding into the same conduit going out to separate gravel piles to the feeder motors in the traps under the piles, the closest was about 200ft, the furthets was about 1200ft from the control room. The one 4" conduit held all 24 conductors from the panel then as it hit a J-box at each pile, one set was pealed off to that trap, the rest continuing on to the next, etc. etc. The last 4 motors would fry every 2 months, the others would take 6-8 months and the drive transistors fried about once per year. We added load reactors and the drives survived, but the motors still fried until their down time losses were so great that they decided to bite the bullet and run separate conduits, which solved the problem. 



So yes, I always tell people that they should separate line and load conductors when VFDs are involved and also not run multiple VFD outputs in the same conduit (unless you use the shielded VFD cable now).


There have been several in depth studies on using THHN or THWN on VFD outputs and the general consensus is that it's not the best insulation to use. The thermoset PVC insulation is extruded onto the wire as a liquid and therefore has microscopic "bubbles" inside of it that weakens the dielectric strength of it. At 600V, you never know the difference but at 650-850VDC, which is what the pulses of a 480V drive are, that is a weakness. Then when you get reflected wave spikes of 1400-2000V, the insulation breaks down. I have pulled old THHN conductors out of conduits and have seen the wave length because you can see burn marks at regular distances along the conductors. These are also often line-to-line shorts so they don't show up on a basic megger test where you look for L-G. I recommend conductors with XLPE insulation, like RHH/RHW or now, XHHW. It's far better. 



There is nothing more expensive than having to do something twice...


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## paulengr

JRaef said:


> When you put line and load conductors of a *starter *in the same conduit, it's irrelevant because the line and load are always at the same frequencies relative to each other; 60Hz, so a cancellation effect on the magnetic fields around the adjacent conductors takes place. But with a VFD, the line side is always at 60Hz (here) but the load side is always going to be at a different frequency, or even if at the same frequency, not synchronized any longer. So the magnetic fields around the line and load conductors are no longer cancelling each other out and you _will _have mutual induction taking place onto the conductors, increasing the voltages. In the case of the line side this is basically irrelevant, but on the load side, it can severely exacerbate any standing wave or reflected wave effects taking place. I have seen a 250HP motor destroyed in 6 months because the line / load conductors shared a common cable tray for just 25ft of a 200ft run. The motor was a Baldor Inverter Duty motor with 2200V insulation and the Yaskawa VFD had a load reactor installed. But because of that doubled run for just a portion of the cabling, we saw spikes of 2800V getting to the motor. We separated the conductors and the highest spike was only about 1400V.


This is a well documented problem. The issue is not the normal frequencies and voltages. Even if they are at different frequencies DC voltage is 1.45 x line voltage so with two motors in a conduit at worst we’d get 1.45x480x2=1392 V. That’s pushing motor limits but not bad. BUT with reflected waves where we’re already pushing close to the motor limit if not beyond with your 200 feet then we induce a transient so we get some fraction of 200% of that and 2800 V is the direct and obvious consequence. Should have cut your book line length limit in half or used separate conduits or shielded wire. It’s not a different frequency thing. My customer that shredded motors at 100 feet on 29 HP motors using power flex drives did it with both motors running at exactly the same frequency.



> Even if you don't have the line conductors in the same conduit, it's not a good idea to run multiple VFD outputs together for the same reason; different frequencies. I had another one years ago at an aggregate plant where there were 8 x 5HP VFDs in a panel, all feeding into the same conduit going out to separate gravel piles to the feeder motors in the traps under the piles, the closest was about 200ft, the furthets was about 1200ft from the control room. The one 4" conduit held all 24 conductors from the panel then as it hit a J-box at each pile, one set was pealed off to that trap, the rest continuing on to the next, etc. etc. The last 4 motors would fry every 2 months, the others would take 6-8 months and the drive transistors fried about once per year. We added load reactors and the drives survived, but the motors still fried until their down time losses were so great that they decided to bite the bullet and run separate conduits, which solved the problem.


Distance is way too long plus doubled down or more on reflected waves.



> So yes, I always tell people that they should separate line and load conductors when VFDs are involved and also not run multiple VFD outputs in the same conduit (unless you use the shielded VFD cable now).


I haven’t seen major line/load problems, just mixing loads when there is inadequate filtering. A good rule of thumb is cut line length limits by the number of motors.



> There have been several in depth studies on using THHN or THWN on VFD outputs and the general consensus is that it's not the best insulation to use. The thermoset PVC insulation is extruded onto the wire as a liquid and therefore has microscopic "bubbles" inside of it that weakens the dielectric strength of it. At 600V, you never know the difference but at 650-850VDC, which is what the pulses of a 480V drive are, that is a weakness. Then when you get reflected wave spikes of 1400-2000V, the insulation breaks down. I have pulled old THHN conductors out of conduits and have seen the wave length because you can see burn marks at regular distances along the conductors. These are also often line-to-line shorts so they don't show up on a basic megger test where you look for L-G. I recommend conductors with XLPE insulation, like RHH/RHW or now, XHHW. It's far better.


See

https://www.nema.org/Standards/Pages/Application-Guide-for-AC-Adjustable-Speed-Drive-Systems.aspx

It’s free to non members. See page 50. There is your test data. Now compare to motor insulation rated for 1450 V max on inverter duty. You can buy motors rated higher but if someone switches brands later you are creating future issues. The thinnest, weakest THHN tested is in excess of 2000 V. If you did something like what you described above then yes it shreds both motor and cable but the motors with much lower CIV limits die first. If you design not to damage the motor, THWN is fine. If you’re going to ignore it and shred motors, cables due too but take longer before they fail. XHHW-2 lasts even longer so I guess you can make the cable survive a 2800 V transient this way while replacing motors constantly.




> There is nothing more expensive than having to do something twice...



Because it costs 300% on average.




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## gnuuser

tates1882 said:


> Ive used Belden Vfd cable, comes with a 10 year warranty. I don't use THHN unless the install is in a dry location. RTW/MTW when its a wet location. AB recommends not using THHN on the load side in a wet location.


ive used it as well (vfd cable) it also works good for shielded extension cables (emergency supply cables) that can be used in a computer control room shielding must be bonded at one end only same as you would with shielded communication cables.


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## Wiresmith

does anyone know of any literature on running multiple long load conductors in cable tray? or a spacing you would recommend?


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## MechanicalDVR

tates1882 said:


> Ive used Belden Vfd cable, comes with a 10 year warranty. I don't use THHN unless the install is in a dry location. RTW/MTW when its a wet location. *AB recommends not using THHN on the load side in a wet location.*


THHN is never used in a wet location so why would VFD use be any different?


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## tates1882

MechanicalDVR said:


> THHN is never used in a wet location so why would VFD use be any different?


 thhn/thwn most is dual rated.


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## tates1882

Wiresmith said:


> does anyone know of any literature on running multiple long load conductors in cable tray? or a spacing you would recommend?


see page 76 
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

from the manual also "Within Conduit
Do not route more than three sets of motor leads (three drives) in the same conduit. Maintain fill rates per applicable electrical codes.
If possible, avoid running incoming power leads and motor leads in the same conduit for long runs.
IMPORTANT Do not run power or motor cables with control or communications cables in the same conduit."


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## John Valdes

MechanicalDVR said:


> THHN is never used in a wet location so why would VFD use be any different?





tates1882 said:


> thhn/thwn most is dual rated.



I don' think I have ever seen THHN without the dual rated THWN on the jacket.


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## sparkiez

I've seen random/intermittent trips due to this before. The company actually installed about 20 VFD's and ran all wire in the same conduit for about 30 feet. These were an after-the-fact install on a hardwired MCC. The trips were completely random and affected all VFD's. Each VFD was feeding four identical motors. Emerson could not give us an answer. On investigation we discovered that the feeders originally going to the motors were not derated either. So there were a few variables.

Here is the thing though, the load side of the VFD has a different frequency than the line side. This can cause undesirable effects via inductance. I don't really know enough to break it all down, but that is the basic idea. Most of the time, there won't be an issue. That being said, who wants to put in all of that work to feed a motor and have it be one of those times when you do have an issue and end up having to run another pipe and pull more wire?


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## Wiresmith

tates1882 said:


> see page 76
> http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf
> 
> from the manual also "Within Conduit
> Do not route more than three sets of motor leads (three drives) in the same conduit. Maintain fill rates per applicable electrical codes.
> If possible, avoid running incoming power leads and motor leads in the same conduit for long runs.
> IMPORTANT Do not run power or motor cables with control or communications cables in the same conduit."


thanks

cable tray if anyone is interested
" Keep a
minimum separation of one cable width between bundles to reduce
overheating and cross-coupling. Current flowing in one set of cables can induce
a hazardous voltage and/or excessive noise on the cable set of another drive,
even when no power is applied to the second drive."


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## gpop

sparkiez said:


> I've seen random/intermittent trips due to this before. The company actually installed about 20 VFD's and ran all wire in the same conduit for about 30 feet. These were an after-the-fact install on a hardwired MCC. The trips were completely random and affected all VFD's. Each VFD was feeding four identical motors. Emerson could not give us an answer. On investigation we discovered that the feeders originally going to the motors were not derated either. So there were a few variables.
> 
> Here is the thing though, the load side of the VFD has a different frequency than the line side. This can cause undesirable effects via inductance. I don't really know enough to break it all down, but that is the basic idea. Most of the time, there won't be an issue. That being said, who wants to put in all of that work to feed a motor and have it be one of those times when you do have an issue and end up having to run another pipe and pull more wire?


Its a common problem on ABB drives. Turn on one drive and the next one over trips on wiring fault. The correct fix is to run each vfd in its own conduit. The lets fix it later way is to disable wiring fault in the program.


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## John Valdes

sparkiez said:


> Here is the thing though, the load side of the VFD has a different frequency than the line side. This can cause undesirable effects via inductance. I don't really know enough to break it all down, but that is the basic idea. Most of the time, there won't be an issue. That being said, who wants to put in all of that work to feed a motor and have it be one of those times when you do have an issue and end up having to run another pipe and pull more wire?



Great post. Not only do VFD and motor manufacturers recommend separation of line and load. To me its also common sense.
If anyone has looked at a drives output on a scope the reason is evident.
Also does anyone know why most motor manufacturers use *Spike Resistant wire? Switching peaks can and do reach 1000 volts on 460 systems.


*Baldor's trademark for higher voltage magnet wire. I thinks it 1500 volt wire. Coated magnet wire.




gpop said:


> Its a common problem on ABB drives. Turn on one drive and the next one over trips on wiring fault. The correct fix is to run each vfd in its own conduit. The lets fix it later way is to disable wiring fault in the program.



I have not had much experience with multiple drive input power being together in conduit. But I will assume its much better than mixing line and load conductors in the same raceway.

I every case, no matter the input source, I use line reactors/filters to lessen the chances of nuisance tripping.


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## paulengr

Motor wire insulation is very thin, the thinnest in any industrial plant. The goal is to pack as much energy as possible into the smallest volume for efficiency reasons. Within the coil voltages are only a few Volts turn to turn so insulation is relatively noncritical. This makes it also very fragile. Magnet wire insulation is so thin you can barely see it and can strip the smaller sizes with a finger nail or touch it with Emory to remove.

NEMA has two ratings. Part 30 motors must be at least around 1000 V rated. Part 31 motors jump up to around 1500 V and get labeled inverter duty. Most manufacturers jack this up by a few hundred Volts for advertising reasons with probably 0.1 mm of extra insulation, #14 THHN tests to at least 2200+ Volts. The motor will fail first.



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## Wiresmith

micromind said:


> I've never actually seen VFD cable.
> 
> Basic THHN seems to work just fine but I have heard of instances where a thin spot in the insulation has been blown through on the load side of a VFD.
> 
> I've heard that XHHW (I think......) is a better choice.


i only quoted you because you were the first to talk about it.


just a FYI if you want it


NFPA 79 2018: Electrical Standard for Industrial Machinery 4.4.2.8

“Circuits Supplied From Power Conversion Equipment. 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 type RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2 or selected based
on the equipment manufacturer’s instructions.” 

https://www.rockwellautomation.com/...lautomation/noa/raotm-tech-sessions/ct479.pdf


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## MDShunk

Wiresmith said:


> NFPA 79 2018: Electrical Standard for Industrial Machinery 4.4.2.8
> 
> “Circuits Supplied From Power Conversion Equipment. 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 type RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2 or selected based
> on the equipment manufacturer’s instructions.”
> 
> https://www.rockwellautomation.com/...lautomation/noa/raotm-tech-sessions/ct479.pdf


Interestingly, that would only apply to drives mounted in machine control cabinets and motors on those machines, but I think it's sound advice just the same. I've seen a heck of a lot of THHN on drive load sides with holes eventually popped in the insulation. I'll save a piece next time I tear one out. It won't be long....


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## Wiresmith

https://literature.rockwellautomation.com/idc/groups/literature/documents/td/20b-td001_-en-p.pdf



> Unshielded
> THHN, THWN or similar wire is acceptable for drive installation in dry environments provided adequate free air space
> and/or conduit fill rates limits are provided. Do not use THHN or similarly coated wire in wet areas. Any wire that is
> chosen must have a minimum insulation thickness of 15 mils and should not have large variations in insulation
> concentricity.
> 
> Shielded/Armored Cable
> Shielded cable contains all general benefits of multi-conductor cable with the added benefit of a copper braided shield
> that can contain much of the noise that is generated by a typical AC drive. Strong consideration for shielded cable should
> be given in installations with sensitive equipment such as weigh scales, capacitive proximity switches and other devices
> that may be affected by electrical noise in the distribution system. Applications with large numbers of drives in a similar
> location, imposed EMC regulations or a high degree of communications/ networking are also good candidates for
> shielded cable.
> Shielded cable may also help reduce shaft voltage and induced bearing currents for some applications. In addition, the
> increased impedance of shielded cable may help extend the distance that the motor can be located from the drive without
> the addition of motor protective devices such as terminator networks.


ran across it, just thought i should add it


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## paulengr

Wiresmith said:


> https://literature.rockwellautomation.com/idc/groups/literature/documents/td/20b-td001_-en-p.pdf
> 
> 
> 
> ran across it, just thought i should add it




Most “THHN” on the market today is dual or triple rated as THWN-2 (wet locations and meets the 90 C rating) and often as MTW (high flex ratings, easy to pull but doesn’t hold its shape well in a panel), or as RHW. So AB is right if you can find actual vanilla variety THHN which is indeed dry location only. The issue though isn’t so much anything to do with a VFD as the fact that it is not rated for wet locations. The even insulation issue is an issue with some really cheap off brand cable. I’ve seen it once in a while but it’s rare. The same cable won’t pass a simple megger test at 1800 V either.


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## Wiresmith

paulengr said:


> Most “THHN” on the market today is dual or triple rated as THWN-2 (wet locations and meets the 90 C rating) and often as MTW (high flex ratings, easy to pull but doesn’t hold its shape well in a panel), or as RHW. So AB is right if you can find actual vanilla variety THHN which is indeed dry location only. The issue though isn’t so much anything to do with a VFD as the fact that it is not rated for wet locations. The even insulation issue is an issue with some really cheap off brand cable. I’ve seen it once in a while but it’s rare. The same cable won’t pass a simple megger test at 1800 V either.
> 
> 
> Sent from my iPhone using Tapatalk


i'll try to find it but i thought it was because it was hygroscopic and when using with a vfd it could likely see over 600 volts, so you could be more likely to blow pin holes through it if it had water in it, even small molecules.
what do you think the reasoning behind nfpa 79 not allowing even thwn? honestly asking, i don't know if its not the reason above.

i guess it may be more because of thhn lower insulation resistance.

edit, i don't believe it is hygroscopic anymore


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## Wiresmith

https://www.rockwellautomation.com/...lautomation/noa/raotm-tech-sessions/ct479.pdf

page 68



> use thermoset insulation not pvc(thhn)(thermoplastic)


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## paulengr

Keep in mind low voltage class VFDs and this extends up to 690 V (IGBTs have a pseudo-standard of 1400 V rating and you can only use half that in a 6 pulse setup) don’t have much filtering. They rely on the fact that standard ICEA 600 V insulation ratings are tested to 1800 V, and standard NEMA chapter 30 (not inverter duty) motors have a minimum rating of 1200 V. This is very different from medium voltage drives where the insulation doesn’t give you a 200-300% protection, So the drive output filtering is cheap.

The issue with moisture has to do with water treeing and electrical trees. It was thought and there are still believers that in a porous cable material which includes ALL polymer insulation’s that when a water tree penetrates into a cable and collided with an electrical tree you get a failure. Testing has shown that service aged cables have water trees but that the water trees don’t have anything to do with failures, but the rumor persists, google partial discharge and you’ll find all kinds of information about this. AB is just reacting to the partial discharge water tree theory which has been disproven. Electrical trees are all that matters.

If water and moisture were so bad overhead line insulators would explode. They don’t and they prove water tree theories are all wet.

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## Wiresmith

i'll keep looking eventually but what i'm finding is water trees convert to electric trees, just pd doesn't happen in water trees but when they become electric trees they do pd.

http://sites.ieee.org/sas-pesias/files/2016/06/Partial-Discharges-in-Electrical-Insulation.pdf

http://www.sqon.se/wp-content/uploads/2013/02/Partial-Discharge-Course.pdf


http://www.imcorp.com/files/Article...Magazine Mashikian Offline PD Diagnostics.pdf


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## JRaef

It doesn’t have to do with moisture. With THHN (and THWN for that matter), the Thermoplastic, the “T”
In THHN, is PVC, applied to the wire as a liquid and “set” to solidify with heat. As a liquid, it contains microscopic bubbles in it that become air pockets when it sets. At 600V, these are irrelevant to the insulation dielectric, it’s ability to stop current from flowing. But in a VFD where there is the possibility of standing wave that can become voltage spikes of up to 2x the peak voltage and if waves in adjacent conductors converge, that can reach voltage peaks of over 2900V between them. Under these conditions those “bubbles” of reduced dielectric strength become susceptible to a phenomenon called “corona discharge” where pin holes burn between the conductors. Most of the time if there is no moisture involved, the leakage goes phase to phase, so it does not show up as a ground fault nor does it show up on a typical megger test to ground. So a LOT of people have this problem and have no idea, until they pull the conductors and see the pinholes. In one project I did where I suspected this I could see the burn marks at regular intervals, indicating that the wavelength of that standing wave was approximately 9 ft. 

RHH/RHW or now, XHHW cable uses Cross Linked zpolyethylene insulation that is made as a tube and after the wire is put inside, it is heat-shrunk down tight to it. There are no bubbles, it lasts much longer. That’s why it is the recommended type for the output side.


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