Why is my neutral overheated at the neutral bar?

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I have a 3phase, 120-208volt sub-panel with separate neutral and ground terminal bars. The neutral bar has clearly overheated where the main neutral conductor is landed and has burnt about 4 branch circuit neutrals as well. Also in that same area of the neutral bar was a bonding wire from the feeder bonding bushing, which should have been landed on the ground bar instead. This panel has been in operation for years and supposedly the only work done recently to this business (an automotive lube center), was an upgrade to their neon signage (which is fed out of this sub-panel). Could the sign installer have somehow caused this overheating (possibly sending neon voltage back or ???) in combination with the incorrectly landed bonding wire or could the bonding wire alone have caused the heat? Also, the sign circuit may be part of a multi-wire branch circuit and I can not be positive the branch circuits are on different phases. I was inside this panel months ago and the neutral was definitely in good shape then. Can anybody help me determine this heat issue so that I can replace this panel and know with confidence the issue will not reappear?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Also, the sign circuit may be part of a multi-wire branch circuit and I can not be positive the branch circuits are on different phases.
If two ungrounded conductors from the same phase share a neutral, it would not comprise a MWBC, and it is not allowed. But that could cause the overheating. It also could be caused by a loose connection at the neutral bar.


In general, when something that has been working suddenly stops working, the prime suspect would be whatever work was recently done. I would check whatever the sign installer might have done.
 

cadpoint

Senior Member
Location
Durham, NC
Walk into the shop when their working!

Put on the PPE Gear and amp-clamp some wires...

Check all the mulitwired circuits and the respective neutrals.

I'd be sure to measure an active neon sign, if they left the ballast floating inside
it's in error of the NFPA. And will cause the amps to go up, Article 600.21.
Also make sure that there is a bond and is with-in inches of the ballast, Article 600.7.

Maybe it's the wires seving the lift motors is the problem and are all to small?

Is the size of the neutral or the sub-panel line side circuits correct or is it (now) undersize to it's usuage?

Hope that helps, come back and do tell of your dectective work! :)
 
I've checked for the obvious problems: loose connections (everything was very tight), checked ampacity with signage energized (with multiple signs on this main sign a total of about 14amps), checked feeder amperage (between 10amps and 18amps between the three phases), checked wire sizes (all correct).

As far as the sign goes, apparently the installer did a horrible job and he was scheduled to return to correct his sloppy work. The ballast was floating at the time I was there and actually had rain water dripping on it, but it was not working. I was originally called out because the circuit was tripping after the sign installer was there, so this sign when it was energized did not stay on very long. The tripping circuit was due to being shared with a kitchenette, which I corrected. I was concerned that maybe he sent some neon voltage back into the panel or something like that and maybe the addition of the bonding wire being terminated on the neutral bar helped to create the heat somehow. If he did that, I won't be able to prove it or I won't be able to say with certainty that if I replace this panel the issue won't arise again. Also, if he did do something wrong it went through an active coffee machine, a juice/punch machine, a water cooler, and a refrigerator with no damage to that equipment.

As far as checking the multi-wire branch circuits, without spending a ton of hours tracing each circuit out, I can't tell which neutrals are being shared with which circuits. This panel has been around for a while and is well beyond using 2 or 3-pole breakers and bundling the MWBC together as the newer code requires. Of the 30 or so circuits, 80% are ran in a 2-1/2" conduit and disappear into the inaccessible drywall ceiling.

Any other ideas?
 

G._S._Ohm

Senior Member
Location
DC area
If it's within a few inches of a connector it's probably a bad connection causing very local overheating. The insulation will tell you.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I've checked for the obvious problems: loose connections (everything was very tight), checked ampacity with signage energized (with multiple signs on this main sign a total of about 14amps), checked feeder amperage (between 10amps and 18amps between the three phases), checked wire sizes (all correct).
Try a fall-of-potential test. Using a voltmeter, measure across each material interface: wire-to-terminal, terminal-to-bus, etc. Start with a high setting, but look for a drop in the fractional-voltage range.
 

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
I wonder if the feeder bonding jumper which creates a parallel path could have heated up from a large load on the neutral and the heat transferred to the neutral buss.
 

renosteinke

Senior Member
Location
NE Arkansas
The previous suggestions will cover 99% of such situations. Here's the one that remains:

Do their lights ever flicker? Specifically, do they actually get brighter when a certain machine runs? Or, have they fried multiple surge protectors and electronic things (like cash registers)? Well, in that case, the neutral bar may be cracked or broken.
 
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