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My barn had a small (6-space) main lug sub panel with four 20A 120V breakers and one 20A 240V breaker. The panel itself was protected by a 50A 240V breaker in the main service panel. It was wired up presumably before 2001 since all of the NM wire going into the panel have white sheathing -- even the 12 gauge. It worked fine until just a few days ago.

On this day, one of the 120V circuits abruptly stopped working without tripping the breaker. I noticed that the panel felt very hot so I cut power to the entire panel until the next day, when I had time to investigate.

It was then that I discovered the carnage inside of the panel. All of the neutral wires had around an inch of melted plastic that was the wire insulation and several inches of blacked/burnt plastic going up the wire. One of the neutrals (the one for the circuit that stopped working) was loose, since it had melted right through -- the hole in the neutral bar is filled with previously molten copper and/or aluminum! The neutral bar itself is very discolored and the plastic support for the bar is thoroughly deformed due to its own melted state.

EDIT: Added photo showing the remaining very-burnt neutrals. The neutral bar lug that is largely black is the one with the fully melted neutral (not pictured). Burnt and melted neutral wires

Notably, none of the circuit breakers ever tripped -- not even the one with the completely melted through neutral. They are all 20+ year old breakers so none are arc-detecting.

I have since ripped the entire panel off the wall and replaced it and all the breakers with a new panel and breakers, just in case. I also cut back the neutrals to leave only non-discolored copper and wire insulation. But what I want to know is why the previous panel failed the way it did, to ensure that something like that doesn't happen again.

One theory is that the melted neutral actually worked itself loose over the decades and at some recent time, was at the right distance from the neutral bar to start arcing to the bar. This would create tremendous heat -- presumably enough to heat up the neutral bar enough to transfer the heat to the other neutrals, causing their insulation to melt and burn... and hot enough to melt the copper conductor in the problem neutral wire itself.

But can a loose neutral wire arc quite that much, especially without ever tripping the breaker?

Is there something else obvious that might be the culprit?

Kurt Granroth
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3 Answers3

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Because NEC 110.14(D) wasn't followed. A torque wrench was not used to set the screw torques. This is why the rule was added to NEC 2014.

Now if the work predates 2014, it was legal and should not have failed, but unfortunately arc faults can't read :)

The circuit you found loose may have been loose before, and that may have caused all of this. Arcing is serious business.

Series arcing is dealing with potentially as much power as the load is designed to draw. And a resistive load will decrease current as arcing consumes some of the current, but a motor load may draw more, and a constant-power switching power supply will definitely draw more current as the arcing gets worse, creating a race condition. EV charging, a common source of such trouble, will draw constant current despite the arcing.

But can a loose neutral wire arc quite that much, especially without ever tripping the breaker?

Yes. And breakers are not magic everything detectors. They only detect overcurrent.

Harper - Reinstate Monica
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A possible way a loose wire can cause this after appearing fine for a while is a glowing connection. The poor contact creates a hot spot which causes thermal cycling of the wire and busbar. This will loosen the connection further over time.

Glowing connections don't tend to cause a large voltage drop, so go unnoticed. Eventually they fail, or melt enough insulation to cause a short circuit first. Increasing the load will make failure more likely due to increased heating.

Once the connection fails, it can start arcing, and the series arc will be like that of a welder, ballasted by the load on the circuit.

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I lack reputation to comment on this post, but I must make the following remark;

1 element in the photo that immediately drew my attention, but is not menioned in any comments or answers, is the burned/melted stranded-wire sticking-out of the neutral bar.

I am in the EU, and I am not an electrician or up-to-date with current regulations here in the EU, but as far as I know, stranded-wires are an absolute no-go in wiring panels and sockets. (I am not at all familair with the relevant code/regulations in the US).

Even without the relevant training, I can think of 3 or 4 different ways stranded wires might pose a safety hazard in this context and I'm sure there are more I didn't think of.

So, although I don't have an answer, maybe it's worth investigating the role of these stranded wires in your situation?

arritjenof
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