Inductive heating of neutral wire

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sw_ross

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Had a service call today in a commercial building that I have done some work in previously.
The concern was the owner smelled some hot electrical issue at the panels location. He had taken the panel cover off and found some neutral and ground wires with severely burnt insulation (this panel was 1 of 4 service panels, side by side). He said before taking the panel cover off it felt hot in the area of the neutral bar.

This is your typical older medium sized industrial type of building that is an auto body/ paint shop operation with many evolutions of changes to the electrical setup.

When I was looking at the panel and the conduits feeding in and tracing the wires that terminated onto the neutral bar there was a neutral and 2 hots (black and red), I assumed it was a typical MWBC, the neutral appeared to be the most damaged of the wires on the neutral bar.
Funny thing was, the 2 hots instead of terminating on breakers went through a 10" chase of 1/2" EMT over to the next panel.
Not only that but the 2 wires terminated on 1 breaker (1-pole 20 amp) so it wasn't a MWBC, at least currently...

Could that type of routing create enough to inductive heating to cause that neutral (and the 3-4 adjacent ones) to melt? One of the neutral screw terminals was basically welded; I wasn't able to get the set screw to loosen up!

Thanks!
 
180519-2102 EDT

sw_ross:

Normally you would not talk about inductive heating of a copper or aluminum wire. Usually the discussion is about a current carrying wire of any material carrying an AC current having its changing magnetic induce a current in something else producing heating in that something else. Usually in a magnetic material, steel or iron

In your case it is most likely the screw at your neutral termination was loose causing a high resistance joint and lots of IR heat at that interface point.

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Quick calculation is that the conduit is generating < 1/2 watt of heat due to induction in the conduit.
 
Aside from a loose connection, or corrosion being the culprit, there is also a fair chance that a previous multiwire branch circuit now has both hots on the same leg, so instead of the neutral carrying the imbalance of the two, it is carrying the sum of the two legs.

Even if the panel looks correct,* I would put in amp clamp around that neutral and find out how much current it is carrying... it sounds like it's overloaded, probably jumped to another circuit somewhere in a junction box.

As others wrote, it is not inductive heating causing the problem, it is a plain old overloaded neutral.

*eta: unless the grounds were right next to the burnt neutral, they should not be burnt up.
 
Thanks for input!

Once I cleaned things up with the overheated neutral(s) I amp'd the neutrals with their "typical" loads going. Everything seemed ok. Mostly 20 amp circuits all appeared to be running about 3-7 amps on the neutrals.

Thinking about it more since I left there yesterday, I want to look at a little more at a machine that's labeled "Solvent Recycler". According to the owner it's a process that takes about 9 hours, so definitely a continuos load. It's neutral was fluctuating between 10-17.5 amps.

It's the thing that was running when he initially smelled the burning electrical smell. I think that might have been the neutral that was burnt the most and like all of you said might have had a loose neutral connection.
 
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