Sharing neutral from separate panel

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Re: Sharing neutral from separate panel

I can tell you that taking a neutral from one panel and a phase from another can cause a nice light show! We were upgrading an emergency backup system which involved a new generator, transfer switch, and panel. We had shut the breaker off that was feeding our panel and started removing the branch circuits. When we started pulling the neutrals off the bar, sparks started to fly. Apparently, through the years, someone had managed to tie into a neutral from the panel we were working on and a phase from another panel. Of course whatever was attached was running at full load and as we loosened the wire up and started to pull it out we got a little light show. Needless to say, our little shut-down got extended for a few hours while we traced the culprit down and corrected the work done by some stinking hack!

[ December 18, 2004, 05:02 PM: Message edited by: kpepin ]
 
Re: Sharing neutral from separate panel

George I'll try my best:

When all of the circuit conductors are installed in the same conduit or other magnetic raceway the current field is 180? out of phase with each other and cancels out the field. But when only one of the current carrying conductors of a circuit is run in a ferromagnetic conduit if causes a current to be induced into the conduit which cause it to be magnetized. If you apply a magnetic field to a ferrous material like steel the steel will become magnetized also. The molecules in the steel will try to align themselves according to the north and south poles of the magnetic field imposed upon it. With an alternating current the magnetic north and south poles alternate with the frequency of the current. The molecules of the steel will alternate back and forth with it. This creates friction in the material and it gets hot. And it can get hot enough to cause a fire.

This is why when we parallel conductors we have to install one of each phase and the neutral in each of the ferromagnetic conduits to keep these fields minimized. Grounding conductors are not current carrying conductors and do not cancel these currents, but in the case of a ground fault since this induced current also rises the impedance of a circuit, the grounding conductor must also be run with the circuit conductors to keep the impedance of the fault circuit low enough to allow the OCPD to operate. This is why grounding conductors must run with the circuit conductors. This is also a requirement of 300.3(B)

Maybe others can chime in to explaine this better.

[ December 19, 2004, 07:08 AM: Message edited by: hurk27 ]
 
Re: Sharing neutral from separate panel

I understand better now.

It's hard to visualize that causing a fire, but I can understand the principle now. Thanks Wayne. :)
 
Re: Sharing neutral from separate panel

Originally posted by georgestolz:
Originally posted by Midget:
Here's the deal...say you pulled a neutral one panel, and a hot from another panel?
Click to expand...
Hey, I finally figured out what your original question was! :D

While it might work, it definutely is not a good idea. But, no worries, I seriously doubt you'd have occasion to.
Click to expand...
LOL...yeah, I couldn't really think of a reason that . I was more asking to see if that article appied to that kind of situation, which it would. :D Now I'm just curious to know if it would work or not. :p
 
Re: Sharing neutral from separate panel

George we had a EC who once paralleled 3 250 MCM's or KCMIL's but put only one phase in each EMT run. We were called out because the conduit was red hot with smoke running out of the JB'S. Lucky the conduit was suspended from pipe hangers and there was no combustibles close to it. The wire insulation was melted to the conduit so we had to replace the whole run. But I have never seen this on a low amperage circuit like a 15 or 20 amp.
 
Re: Sharing neutral from separate panel

Hurk,on a delta 3-phase system wire does not have to be in conduit to get this effect. I used to work for a POCO and the super and assistant manager bypassed the system engineers recommendation for size of quadriplex and paralleled 3 runs of smaller. This was to a sawmill and load caused the quads to smoke and burned wedge clamps off at supports. I was the one sent on the call and when I called the super and AM on it the AM said it would not do it because it was not in conduit. Kept doing it until they un-paralleled them.
 
Re: Sharing neutral from separate panel

You are correct as it can happen where only one current carrying conductor is run through any encircling ferrous metal. This is why 300.20(B) requires us to cut slots between hole where the conductors are run though.
 
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