Melting wire nuts

[bobb123]

Here is the situation. I can't find the cause. In wiring a bathroom addition the building contractor uncovered many wires in a floor that had been covered up. Some of these were hot some were not. We were able to pull the wires into accessible j-boxes. After all the wiring, the contractor called and said they homeowners smelled smoke. They had plugged in their vacuum cleaner in a receptacle and the junction in the box heated up and melted the wire nuts in one of the j-boxes. I searched all my junctions, and everything looked correct. I found that the hot black wire had no current in the j-box but the white (grounded conductor) did. Just to test I switched the wires in the j-box and turned on the vacuum cleaner for 15 minutes. Nothing heated up and all is well. Then they called again and said the same thing happened in another part of the house that we did not even touch. Can anyone offer some insight as to what may be happening. Note: The house has had several electrical contractors who have done work. There is a new panel with new wire going from the panel but most of the devices are still 2 wire. Thanks for any insight on this.

 

[infinity]

Any MWBC's involved? Could be a single neutral carrying the current of two circuits.

 

[ramsy]

They had plugged in their vacuum cleaner.. and melted the wire nuts in one of the j-boxes.

Rather than ask, how many unqualified contractors it takes to screw in a lightbulb, I'm going with wirenut installation error.

Loose wires at Wirenuts / overtight / undersize, or contractors used a Megger tester & blew out the wirenuts.

 

[qcroanoke]

Just throwing this out there:
Reversed polarity? In different places.

 

[MasonF]

Im with infinity, sounds like some mwbc’s tied together wrong.

 

[Amps]

Any MWBC's involved? Could be a single neutral carrying the current of two circuits.

So the hot conductors are both on the same phase, not on a 2pole breaker?

 

[James L]

So the hot conductors are both on the same phase, not on a 2pole breaker?

It happens

 

[qcroanoke]

So the hot conductors are both on the same phase, not on a 2pole breaker?

Yeah. At one time they were required to be on opposite phases and still are but a 2 pole breaker wasn't required. It is now.

 

[MasonF]

Yeah. At one time they were required to be on opposite phases and still are but a 2 pole breaker wasn't required. It is now.

Going through my old code book collection it appears the 2 pole breaker disconnecting means was in 2008. Prior to that it was worded to be a simultaneous disconnect if both phases were on 1 yoke.

 

[MattG0311]

Loose wires at Wirenuts / overtight / undersize, or contractors used a Megger tester & blew out the wirenuts.

Never heard of a megger damaging wire nuts…also never seen a megger pulled out on a resi job…

 

[Another C10]

The house has had several electrical contractors who have done work.

Careful on that project, make sure to make detailed notes on what you have done, what you have come across and what your suggestions are to resolve the concerns directly on any invoicing. If not you could be responsible for work you had not directly installed or maybe relied upon without knowing of the existing wirings safeguards or practices, if a court order were to be filed with a liability claim or worse, you now are a person of concern.

 

[synchro]

I can only guess, but perhaps poor workmanship in making up the wirenut connections is the problem. Mixing stranded and solid wire can take a little more care to prevent the stranded from spinning around the solid and being pulled up inside of the wirenut. Also, if somehow the spiral spring inside of a wirenut is conducting a significant amount of current due to poor contact between the wires, then that could cause the wirenut to overheat.

 

[ramsy]

Never heard of a megger damaging wire nuts…also never seen a megger pulled out on a resi job…

The Megger tester is best known as a maintenance electrician's tool for medium voltage cable performance histograms, but permits or specs may also require such tests for solar-arrays, or building-wire commissioning.

Megger makes low-voltage testers below 1kv for structured wiring & appliances, but "A Stitch In Time" remains the most referenced Megger test procedure.

The "Stitch In Time" procedure for medium voltage cables => 2kv, was not intended for testing installation errors of wirenuts listed for 300-600v, with bulging-soft shells in contact with metallic boxes & raceways.

 

[bobb123]

Any MWBC's involved? Could be a single neutral carrying the current of two circuits.

From the panel the circuits involved are on separate breakers. I have determined that there are junction boxes covered over by sheetrock in the ceiling. Because the last issue involved 2 different circuits my thought was that they shared a neutral somewhere but just needed a sanity check.

 

[bobb123]

I can only guess, but perhaps poor workmanship in making up the wirenut connections is the problem. Mixing stranded and solid wire can take a little more care to prevent the stranded from spinning around the solid and being pulled up inside of the wirenut. Also, if somehow the spiral spring inside of a wirenut is conducting a significant amount of current due to poor contact between the wires, then that could cause the wirenut to overheat.

I'll double check that.

 

[bobb123]

Careful on that project, make sure to make detailed notes on what you have done, what you have come across and what your suggestions are to resolve the concerns directly on any invoicing. If not you could be responsible for work you had not directly installed or maybe relied upon without knowing of the existing wirings safeguards or practices, if a court order were to be filed with a liability claim or worse, you now are a person of concern.

Yep, I have talked both with the homeowner and the contractor. I have maintained because we installed our circuits correctly, and they are not even connected to these problems, that this is due to poor workmanship when other projects have been done. The homeowner just wants it safe and is going to hire me to start re-wiring after they finish paying for the current bathroom remodel.

 

[Fred B]

Seen many times neutrals of different circuits getting mixed in Jbox connections, and if the hots got mixed also and they happen to be on the same pole you can double the available amperage just like a mixed up MWBC.
The most likely heating issue leading to meltdown was a bad connection only apparent under load (vacuum cleaner).
To add to OP issue it was stated he had wires that didn't seem to have any power on them at the time he tested, these may have added another risk issue, why were these wires dead? Were they decommissioned due to a short of some sort, that might have led to an issue if reconnected into the live system? Or part of a 3way switch system?
OP reports thinking there was a L/N reversal, and simply reversed the connections in a jbox. How did OP make this determination, use a plug in circuit tester showing L/N reversed or simply a volt meter to ground?

 

[bobb123]

Just throwing this out there:
Reversed polarity? In different places.

That is the oddity of it. When I use my receptacle tester to check, all it shows is an open ground, it's a two-wire system, so I am going to buy a new tester to double check that. I used a voltage indicator to see what side of the receptacle has voltage and which does not. It is showing different sides having voltage throughout the circuit. I know that's a crude way to do it, but I'm concerned for the safety so I am trying to do all I can to find the issue for them.

 

[bobb123]

Im with infinity, sounds like some mwbc’s tied together wrong.

Yes, I agree. Trying to find it is going to be the fun but I'll get it.

 

[bobb123]

Yeah. At one time they were required to be on opposite phases and still are but a 2 pole breaker wasn't required. It is now.

These are single circuits. One on the left one on the right. I'll go back and look at the which phase each breaker is on that has been affected.