# VFD motor leads and controls



## Peewee0413 (Oct 18, 2012)

I believe best practice is twisted pairs with some sort of shielding, but saying that I have most definitely done THHN control wire in the same conduit without issues.


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## John Valdes (May 17, 2007)

I always used THHN for motor loads on VFD motors before I was schooled on using VFD motor cables designed for this purpose. I learned that here. I always ran motor and control separate as well. But I have learned that may not be required even though best practice in my opinion is to separate. I would talk to the VFD manufacture and see what they recommend. 
It’s free to ask and does not hurt to get opinions at the job site.


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## Slay301 (Apr 23, 2018)

You can only combine vfd loads in the same conduit if your using vfd cable which you would have to run a 2” conduit for a 1/2 hp motor


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## micromind (Aug 11, 2007)

I've always used basic THHN for VFD to motor and I've run 120AC or 24DC controls (3 wire start-stop or 2 wire on-off) in the same pipe as the motor. Be careful though, if the controls go to an ice cube relay (or some other low current load), there's enough capacitive coupling to hold the coil in if it's a big motor or more than 150' or so. 

I've never run any sort of analog signals or anything directly connected to the VFD or a PLC.


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## bill39 (Sep 4, 2009)

How long would the run of conduit be is one consideration. The longer it is the more likely you are to have trouble with it.
What is the control voltage?
What does the VFD manufacturer’s recommendation? It’s worth a phone call because you can bet that will be the first thing the consulting engineers will do if there is a problem.
IMHO, running the controls in the same conduit will always be a question mark and someplace to point fingers at if there are any problems.


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## 460 Delta (May 9, 2018)

micromind said:


> I've always used basic THHN for VFD to motor and I've run 120AC or 24DC controls (3 wire start-stop or 2 wire on-off) in the same pipe as the motor. Be careful though, if the controls go to an ice cube relay (or some other low current load), there's enough capacitive coupling to hold the coil in if it's a big motor or more than 150' or so.
> 
> I've never run any sort of analog signals or anything directly connected to the VFD or a PLC.


Would it help with interference if the control wires are a twisted pair? I’ve seen mixer mechanics take a cordless drill and two wires off the spools and twist up a section to repair or extend the J1939 data wires on Advance mixers. They never seemed to have any issues doing it like they did.


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## splatz (May 23, 2015)

460 Delta said:


> Would it help with interference if the control wires are a twisted pair? I’ve seen mixer mechanics take a cordless drill and two wires off the spools and twist up a section to repair or extend the J1939 data wires on Advance mixers. They never seemed to have any issues doing it like they did.


The idea with the twist is that both conductors will get the same interference. If they are not twisted, one wire is closer to the source of interference, and one will get stronger interference than the other. Depending on what's on the wires, as long as both conductors get the same interference, it will cancel. 

I don't think you can make a blanket statement about whether VFD noise will disrupt the control wiring, because "control wiring" can be many different things - 120VAC power, 24VDC power, 0-10VDC analog, 4-20ma analog, ethernet, etc., etc., some immune to noise, some sensitive to noise.


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## SteveBayshore (Apr 7, 2013)

I have never done it but if you're pulling twisted pair cables in the same conduit with the power conductors, make sure that the shielded cable has at least the same or greater voltage rating.


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## Slay301 (Apr 23, 2018)

I would say just run separate conduits charge, them for it and avoid a call back and every one will be happy they won’t be happy when production keeps shutting down because they didn’t wanna spend a few more grand. The quality will still be there long after the price has been forgotten


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## CAUSA (Apr 3, 2013)

Check for the maximum reactance resistance of the twisted pair cable that you are using as well.

that the Ohms maximum is not excessive for the distance of linear run. The manufacturer of the drive should be able to state it in the manual.


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## gpop (May 14, 2018)

Last place i worked it was common to see the disconnect aux ran in the same conduit as the vfd output. I honestly think its a bad idea especially if its underground as a failed wire in the conduit generally meant the relay got fried. I also hated pulling wires like #6 with a couple of #14 as the 14 like to tangle at the pull boxes. 

Only came across one problem where someone had run start/stop wires in the same conduit over 250'. There was enough coupling voltage to hold in a large relay so the stop button would not work.


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## Peewee0413 (Oct 18, 2012)

I've been tingled by control circuits wrapped around feeders, never personally encountered a relay staying latched, though I have heard stories.


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## paulengr (Oct 8, 2017)

No induction type issues with DC inputs. Or at least I’ve never seen any. Even with multiple motors and hundreds of feet of conduit. Controls especially DC aren’t really “current carrying conductors”. With AC it depends on lead length and the input but a typical digital AC inout is high impedance. Shielding DC is a waste of time. If you attempt to run say an HOA circuit with 120 VAC in the same conduit it usually works with a starter because the contactor coil is enough of a load to eliminate any induction issues but converting to digital controls usually causes chaos. Adding a heavy machine tool relay as an inter posing relay solves the issue.

The bigger conversion problem when going from AC to DC is this. Essentially low voltage DC relay contacts and AC contacts are different. An AC relay or push button contact is usually rated in amps. Whatever the material it is made of every time the arc that forms when the contacts open or close purposely blows away the thin oxide/corrosion that forms. This is called “wiping” action. DC control circuits are very different. Low voltage DC doesn’t have enough current to do any wiping so the contacts are all gold or gold plated to prevent corrosion. If you apply AC to a DC contact you blow all the gold away quickly. If you apply DC to AC it can’t wipe so eventually you get corrosion problems. There are “universal” contacts out there that are rated AC and DC but they just have an AC contact with a thin gold plating. You can freely use them as AC or DC and you can switch from DC to AC but once you’ve used it in an AC circuit it should never be used in DC. However YMMV. It usually takes a little over a year for this problem to show up and most maintenance guts don’t catch on that their push buttons fail about a year after we upgrade something from AC to DC.

Even with controls 4-20 mA really doesn’t need it. 10 VDC analog does but it’s kind of a waste. Unless you need very high performance (20 KHz bandwidth or more) which a PF40 can’t do, use 4-20 mA for analog. That 4 mA keeps the signals off the noise floor.

If you put multiple VFD motors in the same conduit, watch out! Reflected wave issues put a maximum line length limit on the motor to drive distance. For PF40 I think it’s 100 feet. But if you put multiple sets of motor cables in the same raceway a pulse from one VFD induces a pulse in the cable of another VFD. When they synchronize you get twice the reflected wave voltage, so divide the maximum line length by the number of VFDs sharing a conduit or use shielded cable.

Don’t forget ampacity. With a drive you are already at the 3 conductor limit with one motor. NEC is ambiguous with the ground since a drive most certainly does put current on the ground but it’s less than 10% of the phase current normally. A separate EGC is NOT optional with VFDs. Practice is to make it full size. I don’t count it as CCC but that’s open to debate. Terminals are usually 75 C so you use the 75 C chart. When you use 2 motors you are in the 4-6 conductor category (again depends on how you use the CCC rule for neutrals). So you can use the 90 C rating within the conduit derated to 80% which just happens to be the same as 75 C so two motors do not require derating. At 3 motors though you derate to 70% of the 90 C table,

AB heavily pushes using SHIELDED cable. I agree it has advantages. I’ve done a lot of independent measurements of reflected wave situations so I understand it intimately. But “shielding” means any kind of shielding. Using MC or even running in conduit counts. VFD cable is a total ripoff. They claim reflected wave resistance but even #14 THHN is good to 2850 V surges while the best motors are 1750 V. If you are getting reflected wave damage, replace both motor and cable after installing a filter. VFD cable does not fix reflected waves. It also generally helps very little with bearing fluting, if at all. Now as for THHN vs XHHW, original (not the upgraded THHN-2 and XHHW-2) types when dry with the thinnest allowable motor cable, #14, passes a 2800 V surge for THHN and 3200 for XHHW. That’s with multiple samples and includes concerns about uneven PVC coating. However when THHN gets wet it drops to close to 1000 V. The testing was done with original flavor THHN. No data exists on THHN-2 that I’ve seen. So if you don’t have a wet environment you can ignore this. If moisture is a factor either use #12 minimum (it exceeds 1800 V when wet) or use XHHW. Either one is vastly cheaper than VFD cable. If you just like a nice shielded conductor consider buying PVC coated type MC with 3 conductors and a ground.


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## JRaef (Mar 23, 2009)

Noise and RF are your enemies in control circuits. Even if the I/O in the VFD or other devices has protection against it, you are CONSTANTLY stressing those protective elements and shortening their life. Running the control conductors in the same conduit as the VFD output conductors is just begging the universe to make your life miserable and force you to redo it later when you get frustrated trying to fix the problems you have created by trying to save a buck. Keep them separate.

Read this, cover to cover. It's provided by Rockwell but is based on experience and research that goes back to when they bought Reliance, so the information is based on 30+ years of VFD application expertise.


https://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf


Top of Page 24 discussed Digital I/O wiring::


> Standard individual conductors that meet the general requirements for type, temperature, gauge, and applicable codes are acceptable if they are routed away from higher voltage cables to minimize noise coupling. However, multi-conductor cable can be less expensive to install. *Separate control wires from power wires by at least 0.3 m (1 ft).*


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## Flyingsod (Jul 11, 2013)

Just out of curiosity what’s the argument for running it all in one conduit? 
In my considerable experience it’s rarely much more difficult or expensive to run two pipes as opposed to one. 


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## gpop (May 14, 2018)

Flyingsod said:


> Just out of curiosity what’s the argument for running it all in one conduit?
> In my considerable experience it’s rarely much more difficult or expensive to run two pipes as opposed to one.
> 
> 
> Sent from my iPhone using Tapatalk


On a new install that makes sense but on a upgrade or a opp's where you only have one conduit available it becomes more about can you rather than should you.


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## Slay301 (Apr 23, 2018)

Flyingsod said:


> Just out of curiosity what’s the argument for running it all in one conduit?
> In my considerable experience it’s rarely much more difficult or expensive to run two pipes as opposed to one.
> 
> 
> Sent from my iPhone using Tapatalk


It can be way more difficult now having to run screw pipe out of a lift 175ft in the air during the winter on the river front


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## Flyingsod (Jul 11, 2013)

gpop said:


> On a new install that makes sense but on a upgrade or a opp's where you only have one conduit available it becomes more about can you rather than should you.


I agree, I hadn’t considered a retrofit. 


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## Majewski (Jan 8, 2016)

Slay301 said:


> It can be way more difficult now having to run screw pipe out of a lift 175ft in the air during the winter on the river front


i go numb with the windchill. BRING HEAT lol


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## Flyingsod (Jul 11, 2013)

Run the conductors live. Lotta heat. 


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## Soilentblue9 (Apr 11, 2014)

Rmc is not cheap right now is why. I put 2 pipes in the job tho and after hearing from a few of ya I will stick with the plan 


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## Slay301 (Apr 23, 2018)

Soilentblue9 said:


> Rmc is not cheap right now is why. I put 2 pipes in the job tho and after hearing from a few of ya I will stick with the plan
> 
> 
> Sent from my iPhone using Tapatalk


Can you get away with imc ?


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## just the cowboy (Sep 4, 2013)

Soilentblue9 said:


> Rmc is not cheap right now is why. I put 2 pipes in the job tho and after hearing from a few of ya I will stick with the plan
> 
> 
> Sent from my iPhone using Tapatalk


I've said this before don't let money overrule design. Downtime and rework cost way more than the cost of parts and labor. Always explain this to the customer that this may SAVE money doing it right. 
Car plant in the 90's was $15,000/ min downtime, with no way to make it up on OT.
Even the napkin plant was $100/minute downtime and could not get made up due to shipping demands..


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