# EMT conduit required for VFD??



## EE1 (Nov 17, 2009)

:001_huh:Need some input from someone who knows VFDs well.

Why do VFD manfg require that you pipe in EMT conduit the load side of a VFD to the motor? I'm guessing something about the harmonics created with the VFD, but would MC cable be acceptable too? I notice multilple VFDs loads are to be in separate conduits too.


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## GEORGE D (Apr 2, 2009)

I'm definitely no VFD expert but I recently encountered a situation were multiple circuits were in one conduit. The circuit I was working was dead. When I stripped one conductor I got bit. The other circuits were inducing voltage on my dead circuit. I measured around 60 volts. I imagine this could be one reason for that.


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## CoopElec (May 16, 2011)

*Vfd*

Maybe air infiltration?


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## brother (Nov 25, 2008)

GEORGE D said:


> I'm definitely no VFD expert but I recently encountered a situation were multiple circuits were in one conduit. The circuit I was working was dead. When I stripped one conductor I got bit. The other circuits were inducing voltage on my dead circuit. I measured around 60 volts. I imagine this could be one reason for that.


Did you test it again before you stripped it? I think this is something that all of us can have a reminder about, because some people forget about induced voltage on a circuit that was turned off.


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## Big John (May 23, 2010)

Because of the rapidly changing voltage on the output of a VFD the load conductors make an excellent antenna; they will couple the output frequency and associated harmonics into any parallel conductive object.

The best way to get rid of the capacitive portion of that coupling is by surrounding the conductors with a grounded jacket. It can be a cable shield, the metal jacket on MC cable, or a piece of metal conduit. 

Of course if you run more than one circuit inside a piece of conduit, you lose the shielding benefits of the conduit and you can get induced voltage like _George D_ spoke of.

-John


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## backstay (Feb 3, 2011)

....


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