Shared Neutrals between separate sources creating induced heat?

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Jmlucas199

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Maryland USA
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Journeyman electrician
Hey guys; I had a very strange incident occur today. I’m hoping somebody on here will have some insight into how/why this happened.
This is kind of a convoluted story so bear with me. I’ll be as detailed as possible.
I was on a service call today; in a medical lab adding 120v receptacles into existing plastic wiremold. As I was pulling my aluminum jacketed 12/2 MC down through the drywall wall and out of the wiremold; the MC jacket became extremely hot abruptly. I was shocked (figuratively) and let go of the cable. Moments later a circuit in the wiremold tripped. (I could tell from the suitcase UPSs plugged into that circuit started beeping). There was no arc or pop or nothing. I knew something was very wrong; so I started investigating the circuitry in the wiremold.

As it turns out; there was a 12/4 MC feeding the wiremold; with 2 hots (Black and Blue) being fed from a normal panel and 1 hot (Red) being fed from a separate emergency panel. And the white wire was tied into both neutrals from both sources. Now; this much I know as fact as I traced this problem out myself……..but my question is; why did the MC jacket get so hot? It WAS in direct contact with the white wire of the 12/4….could there have been some weird whacko induced heat from a neutral that’s shared from different sources? Or should I just start wearing my tinfoil hat?
 
You have s secondary neutral to ground connection somewhere. Have fun finding it! :p

The MC jacket is now carrying neutral current and 120v would be present if you touch the ungrounded side, say if you break the jacket in half.

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The armor of my MC cable wasn’t “connected” to anything but it was setting on a copper water line in the ceiling as I was in the middle of pulling it. So it was grounded I suppose.
 
DrSparks said:
You have s secondary neutral to ground connection somewhere. Have fun finding it! :p

The MC jacket is now carrying neutral current and 120v would be present if you touch the ungrounded side, say if you break the jacket in half.

Sent from my SM-A326U using Tapatalk
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I may have not been very clear in my description; but the Mc jacket wasn’t in contact with the actual neutral conductor; merely the white wire. Meaning; it was resting on the THHN jacket of the white wire.
 
Jmlucas199 said:
Hey guys; I had a very strange incident occur today. I’m hoping somebody on here will have some insight into how/why this happened.
This is kind of a convoluted story so bear with me. I’ll be as detailed as possible.
I was on a service call today; in a medical lab adding 120v receptacles into existing plastic wiremold. As I was pulling my aluminum jacketed 12/2 MC down through the drywall wall and out of the wiremold; the MC jacket became extremely hot abruptly. I was shocked (figuratively) and let go of the cable. Moments later a circuit in the wiremold tripped. (I could tell from the suitcase UPSs plugged into that circuit started beeping). There was no arc or pop or nothing. I knew something was very wrong; so I started investigating the circuitry in the wiremold.

As it turns out; there was a 12/4 MC feeding the wiremold; with 2 hots (Black and Blue) being fed from a normal panel and 1 hot (Red) being fed from a separate emergency panel. And the white wire was tied into both neutrals from both sources. Now; this much I know as fact as I traced this problem out myself……..but my question is; why did the MC jacket get so hot? It WAS in direct contact with the white wire of the 12/4….could there have been some weird whacko induced heat from a neutral that’s shared from different sources? Or should I just start wearing my tinfoil hat?
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What exactly does that mean? That "white wire" should be within the sheath of it's cable.
 
You basically have a parallel neutral and you have magnetic field created . When the wiring is within nm cable it isn't an issue but within mc there will be heating. Not sure why your mc cable was hot other than it made another path back thru the water pipes.
 
Dennis Alwon said:
You basically have a parallel neutral and you have magnetic field created . When the wiring is within nm cable it isn't an issue but within mc there will be heating. Not sure why your mc cable was hot other than it made another path back thru the water pipes.
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That’s about as far as I can gather. I was hoping someone would come along and lay some science down on me to explain the heat.
 
I think that your MC sheathing that was draped on the grounded water pipe just made contact with a nicked hot conductor inside the wire mold. And the short had enough resistance to not trip the magnetic element of the circuit breaker, only the thermal. And the improperly shared neutral is coincidental.
 
Neither inductive heating nor an overloaded neutral would be likely to trip a breaker. (In fact, that's why such a wiring mistake is dangerous.) Also inductive heating would likely have occurred elsewhere. You just caused a short. That said, now that you found the improperly shared neutral, it would be negligent of you not to fix it. Especially since we also have reason to believe you damaged conductors in the wiremold.
 
I would say either the waterline, or the wire mold metal case was energized, and the al mc completed the path, and was hearing from the current created by the now short to ground.
 
jaggedben said:
Neither inductive heating nor an overloaded neutral would be likely to trip a breaker. (In fact, that's why such a wiring mistake is dangerous.) Also inductive heating would likely have occurred elsewhere. You just caused a short. That said, now that you found the improperly shared neutral, it would be negligent of you not to fix it. Especially since we also have reason to believe you damaged conductors in the wiremold.
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I also agree, there are issues that need to be fixed.
 
The truth: MC armor lacks a bonding strip and is not an effective ground fault current path. This is just one example of where article 250 could have lead to a fire that would have been blamed on arcing.


With that said, there needs to be change where all metal armor cable has a bonding strip.
 
Jmlucas199 said:
The THHN was exposed inside of the wiremold.
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So you were pulling a MC cable through a wiremold ( apparently a larger wireway of some sort and not your typical 500 or 700 series surface raceways or similar?) with other conductors also in said wiremold?

If you contacted something to create a fault current path, the sheath without internal bond strip isn't exactly a low impedance path like others have mentioned and this does make more sense
 
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