Ran into a strange one this morning. A 120v 20 amp Multiwire branch circuit in my Niece's garage. Line one went to GFCI receptacle. Line 2 went to receptacles on the other side of the garage.

Nothing was connected to the GFCI receptacle. Both breakers had tripped. The Neutral going to the GFCI receptacle had melted and charred insulation and the area where the Neutral post on the receptacle was also severely melted. In fact the heat was so bad, that the mounting screw was fused in the melted plastic on the recap.

I could not find any obvious shorts with either leg of the circuits nor could I find any obvious damage to the wiring.


I replaced the GFCI recep, and reset the breaker and there were no problems.

The GFCI recap is the only outlet for the branch circuit. There is no down stream load.

Any ideas on what the cause was?

Damage was likely caused by a loose connection between the neutral wire and the receptacle. Is the house wired with aluminum wire? I ask because that is a know hazard of aluminum wire. There are specialized connectors designed to prevent aluminum wire from causing a loose connection.

Nope, the wiring is copper clad aluminum. The receptacle is less that 18" away from the panel. I checked the connections at the main neutral and they were tight with no discoloration.

I found the compromised Neutral.

The second leg of the Multiwire circuit also fed an outside pool pump, gas water heater and lights. The cable to the pool heater is a 14 AWG NMB inside FNT. The Neutral of that cable grounded against the frame of the heater and fried.

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Originally Posted by donf

I found the compromised Neutral.

The second leg of the Multiwire circuit also fed an outside pool pump, gas water heater and lights. The cable to the pool heater is a 14 AWG NMB inside FNT. The Neutral of that cable grounded against the frame of the heater and fried.

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Doesn't sound right. The frame and the neutral share the same path. It's far more likely an overload was / is your problem.

True, but that is the only issue I found. so far. I guess it is back to the circuit. I have to change out the 14 AWG cable anyway.

I asked them if they had anything plugged in when the recep went dead but "No one knew anything about it"