VFD's and the NEC

Status
Not open for further replies.
K

Karl H

Senior Member
Location
San Diego,CA
I just recently have had two seprate conversations with two different guys saying,That you couldn't mix line and load in the same conduit if supplied by a VFD.I asked both of them for a ART reffrence and both replied,"Well, so and so told me." Is this true? ART.#? I've installed dozens of VFD's and never mixed line and load but,only because I didn't need to. I've searched in the 2008 NEC but, to no avail. Anyone know? Thanks
 
I cannot say that this will answer your question definitively, but according to article 300.3(C)(1), you can run any conductors of voltages 600 or less (ac or dc, it does not matter) in the same raceway. So long as all conductors have an insulation rating equal to the max circuit voltage applied to any conductor in the raceway!

Do I "think" it is a good idea to run line and load together no.
 
I'm questioning the issue because I thought It was odd to hear of this "Mysterious" rule just recently from two different guys.So I thought I'd better look it up.I typically won't debate a code until I've seen it in print myself first. Thanks for your response.
 
Not an NEC issue but check the manufacturers directions for the unit, I bet you will find that the manufacturer says not to run line and load in the same conduit.

As I understand it the load side can introduce all sorts of electrical noise on the line side which may effect that VFD and other electronics supplied by the same power source.

Many of the manufacturers actually recommend a shielded multi-conductor cable on the load side of VFDs, but I have only actually installed 'VFD Cable' once.

vfd.gif
 
Last edited:
I have installed GE, Seimens, and Westinghouse VFD's. They require seperate conduit for the load side conductors and 100' max length @ 460 volts.
 
iwire said:
Many of the manufacturers actually recommend a shielded multi-conductor cable on the load side of VFDs, but I have only actually installed 'VFD Cable' once.
Click to expand...

They do that because in the rest of the world "grounded" conduit is not a normal wiring method. In the US our THHN in grounded raceways offer the result as the shielded cable which is to reduce EMI/RFI transmitted by the VFD leads.
 
krunchtroll said:
Do I "think" it is a good idea to run line and load together no.
Click to expand...
What is it going to hurt? This is pretty common when a VFD is installed on an existing motor. Its easier to run one conduit from the vicinity of the motor to where the VFD is located than two.
 
iwire said:
Not an NEC issue but check the manufacturers directions for the unit, I bet you will find that the manufacturer says not to run line and load in the same conduit.

As I understand it the load side can introduce all sorts of electrical noise on the line side which may effect that VFD and other electronics supplied by the same power source.

Many of the manufacturers actually recommend a shielded multi-conductor cable on the load side of VFDs, but I have only actually installed 'VFD Cable' once.

vfd.gif
Click to expand...

Many drives made overseas state in the installation manuals to use a VFD cable, instead of separate wires. If they do, thats what I put on the drawings. Whether the guy in the field actually does it or not is something else.
 
petersonra said:
This is pretty common when a VFD is installed on an existing motor. Its easier to run one conduit from the vicinity of the motor to where the VFD is located than two.
Click to expand...

Not common here except for short distances.

Why would the VFD not be at the location of the original controller? :confused:
 
OK that makes perfect sense to me. Someone read the Manufacture specs about not combining LINE and LOAD in one raceway.Then told other guys that then all of a sudden it's a new NEC req. lol I was really scratching my head on that one. I read the ARTS. over and over again thinking I missed something.I'm not currently installing any VFD's. It was just two random converstations I had with two diffent guys about VFD's.But, about a month ago I did install two 500HP VFD's for chillers and the Specs. did say to use Sheilded Belden cable for the control wiring. Thanks for clearing this up for me guys! Now next time I see those guys I can tell them they are WRONG!It's not a NEC Req. LOL J/K
 
iwire said:
Not common here except for short distances.

Why would the VFD not be at the location of the original controller? :confused:
Click to expand...

I see a lot of people installing VFDs prety close to the motor, due to the standing wave hype. The original motor starter might be just about anywhere.

Its also a space thing. A VFD often eats up much more space than a motor starter. Its not as if you can take out the MS in an MCC and put a VFD in its place for the same size motor.

Lastly, people put things where they can. Space is often at a premium in a lot of plants these days.
 
Status
Not open for further replies.
Top