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#110604 - 05/27/06 09:59 AM 300.3(B) Violation?  
Admin  Offline

Joined: Oct 2000
Posts: 3,460
Happened to see this while doing some work at a municipal water pumping station. The conductors in the picture are on the load side of a VSD feeding a 400HP 600V pump motor. The circuit consists of two parallel runs of 350kcmil CU THHN per phase. Both conductors of phases B and C are in the left 2”EMT and both conductors of phase A are in the pipe on the right which also has a 4 AWG EGC. At the time pics were made each conductor was carrying about 125A. Wire was cool but pipe on left was noticeably very warm to the touch.

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#110605 - 05/27/06 05:32 PM Re: 300.3(B) Violation?  
Rick Kelly  Offline
Joined: Jan 2006
Posts: 60
Iqaluit, NU, Canada
Can anyone say "Circulating Ground Current"...

#110606 - 05/30/06 07:21 AM Re: 300.3(B) Violation?  
RSmike  Offline
Joined: Nov 2002
Posts: 134
Holland, MI USA
Would you expect the one on the left to be warmer since it had more net current in it. There wouldn't happen to be another EGC buried in those wires on the left? Can these wires simply be considered motor leads?

I'd also like to hear more about "circulating ground currents" with respect to being used on a VSD. I thought this type of problem wasn't an issue with a VSD.

Is it possible that the VFD is listed for this type of installation?


#110607 - 05/30/06 01:42 PM Re: 300.3(B) Violation?  
Jonno  Offline
Joined: May 2006
Posts: 20
Just thought that I'd offer a bit of insight on this topic. I am a Power Engineer, not an EC, but this is my area of specialisation.

The VFD is likly not doing anything to cause the conduit heating (unless it is malfunctioning). The two conduits are carying the same net current:

Left Conduit:
125<120 A + 125<-120 A = 125<180 A

Right Condit:
125<0 A

If you were to total the two current you would get 0, this is why we combine all 3 phases (and neutral if aplicable) in the same conduit.

The metalic conduit is acting as a curent transformer, the net current in the conductors is inducing a current in hte conduit and because there is a second ground path (the EGC) this current can flow and heats the conduit by P=I^2R.

As you all know the correct way to do this is to split the conductors into two ABC groups. I hope some one will be fixing this.

The only reasons that I can come up with for one being hotter is either a bad bond an one end of the cold counduit or the EGC is carying current.

#110608 - 05/30/06 03:27 PM Re: 300.3(B) Violation?  
RSmike  Offline
Joined: Nov 2002
Posts: 134
Holland, MI USA
How stupid am I. VERY! I can't believe I made that previous post.

Makes perfect sense now. Right hand rule stuff. Thanks Jonno.

What do you expect from a guy that only deals with 1HP motors?


#110609 - 05/30/06 08:39 PM Re: 300.3(B) Violation?  
Rick Kelly  Offline
Joined: Jan 2006
Posts: 60
Iqaluit, NU, Canada
Ahhh... but you asked the question.

#110610 - 06/02/06 02:19 PM Re: 300.3(B) Violation?  
electricman2  Offline
Joined: Nov 2003
Posts: 269
Thomasville, NC USA
The strange thing is it would have been so easy to do it correctly. Three conductors in one pipe and three in the other.


#110611 - 06/03/06 09:56 AM Re: 300.3(B) Violation?  
resqcapt19  Offline
Joined: Nov 2000
Posts: 2,148
Do you really need a ground path to heet the metal in a case like this? I thought it was just a result of a magnetic circuit and not an electrical circuit.


#110612 - 06/05/06 09:23 AM Re: 300.3(B) Violation?  
Jonno  Offline
Joined: May 2006
Posts: 20

You do need a second ground path, it does not need to be a good one though.

Current will flow along the conduit and needs a path back. This is why we can perfecly safely use single conductor TEK as long as you gorund only ONE end of the sheild.

In the case of this setup, the second conduit would be the return path, because the primary currents are 180 deg out of phase, the induced currets would also be.

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