why do wires melt in junction box

[ddk2b408]

at a job power went out in the living room so I thought maybe a breaker tripped. breaker was fine, I pulled all outlets in room out the wall, wiring was fine. I traced circuit to a junction box. Wires were burned and came apart. Why breaker didnt trip. and what causes a wire to melt that way. by the way It was a 12/3 so the circuit was sharing a neutral...

 

[George Stolz]

You didn't happen to take any pictures of it, did you?

Bad connections generate heat, among other things that do too. I've seen loose connections at breakers melt the sheathing tag onto the conductors in a big glob.

 

[celtic]

..... by the way It was a 12/3 so the circuit was sharing a neutral...

were they on the same phase?

 

[don_resqcapt19]

Poor connections create heat, but also limit the load so it is unlikely that the breaker will trip. Just look at things like toasters...they make a lot of heat but the breaker does not trip. This is one of the classic cases used to support the AFCI requirement. I am not yet convinced that the fancy arc signature recognition chip in the AFCI will see a poor connection, but all AFCIs have GFPs and it does not take long for a high heat condition to create a low level ground fault that will cause the AFCI to trip.
Don

 

[hardworkingstiff]

I'm with Celtic, were the two circuits on the same or opposite phases?

Which wires were stressed, all or just the white wires?

 

[Inspectorcliff]

not good.

not good.

at a job power went out in the living room so I thought maybe a breaker tripped. breaker was fine, I pulled all outlets in room out the wall, wiring was fine. I traced circuit to a junction box. Wires were burned and came apart. Why breaker didnt trip. and what causes a wire to melt that way. by the way It was a 12/3 so the circuit was sharing a neutral...

As already stated, Two items that creat this, 1) heat and 2) overloading the circuit. BUT, not a dead short. I have seen a number of these over the years.:roll:

 

[don_resqcapt19]

I really doubt that having two hots of the same phase on a common neutral would really make enough heat to melt the insulation. Remember that the conductors in the NM are rated at 90?C and #12 has an ampacity of 30 amps at this temperature rating. Yes it is a code violation and will contribute to heat build up, I don't think that alone would result in melting.
Don

 

[ptonsparky]

That is a plus for the J box. I have seen several melted charred globs but when the box is covered and cable clamps are in use, the charring stays in the box.

 

[lpelectric]

at a job power went out in the living room so I thought maybe a breaker tripped. breaker was fine, I pulled all outlets in room out the wall, wiring was fine. I traced circuit to a junction box. Wires were burned and came apart. Why breaker didnt trip. and what causes a wire to melt that way. by the way It was a 12/3 so the circuit was sharing a neutral...

Were they Federal Pacific breakers?

 

[goindowner]

--------------------------------------------------------------------------------

Poor connections create heat, but also limit the load so it is unlikely that the breaker will trip. Just look at things like toasters...they make a lot of heat but the breaker does not trip. This is one of the classic cases used to support the AFCI requirement. I am not yet convinced that the fancy arc signature recognition chip in the AFCI will see a poor connection, but all AFCIs have GFPs and it does not take long for a high heat condition to create a low level ground fault that will cause the AFCI to trip.
Don







I helped a friend wire his new house a few years ago. He and his brother inlaw put the recpts in the bedrooms. I could not figure out why one afci was tripping. We changed the breaker with one that was not having any problems to make sure we did not get another bad breaker. Problem still was happpening. We then took out every recpt. in this one particular bedroom and found the guy did not tighten the screws good. Problem solved.

 

[raider1]

Problem still was happpening. We then took out every recpt. in this one particular bedroom and found the guy did not tighten the screws good. Problem solved.

Loose connections are a serious problem, but I really doubt that the current generation of branch-circuit feeder type AFCI breakers would trip due to the loose connection as Don has already pointed out. It is more likely that there was an inadvertant neutral to ground connection in one of the receptacle boxes that resulted in the AFCI breaker tripping. The standard branch-circuit feeder type of AFCI breaker also has GFPE protection which will trip at between 30 and 50 milliamps.

Chris

 

[goindowner]

I never tought of that. That is why I like this site. Thanks
Bill

 

[raider1]

Your very welcome.

Chris

 

[andinator]

I saw a beauty this morning---One of the guys I work with brought the mess in for show and tell in a plastic baggie. 1 #4 cu stranded and 1#6cu stranded= meltdown.

 

[celtic]

I really doubt that having two hots of the same phase on a common neutral would really make enough heat to melt the insulation. Remember that the conductors in the NM are rated at 90?C and #12 has an ampacity of 30 amps at this temperature rating. Yes it is a code violation and will contribute to heat build up, I don't think that alone would result in melting.
Don

I must disagree.
I have personally seen 12/3 NM baked (neutral only if I recall) as a result of both ungrounded conductors being on the same phase. We had an entire panel running temp. lighting stringers, "someone" liked the "all black one side, all red the other side" appearance. It took about a year for the charring on the sheath to become visible OUTSIDE the panel.

Another job I wish I had a camera with me....

 

[don_resqcapt19]

Celtic,
Unless you are running more than 15 amps of each ungrounded conductor, the grounded conductor would be operating at or below its rated 90C ampacity. If this is causing the insulation to fail, then it isn't really 90C insulation.
Don

 

[ptonsparky]

Is it the poor connection that causes the overheating or the overheating that causes the poor connection? The chicken or egg question.

 

[weressl]

Is it the poor connection that causes the overheating or the overheating that causes the poor connection? The chicken or egg question.

Of course it start with a poor connection.

More importantl;y I do not believe this issue belongs to the Hazardous Area Classification group.

 

[charlie b]

More importantly, I do not believe this issue belongs to the Hazardous Area Classification group.

Good point. I have moved it to the Safety Topic Area.

 

[LawnGuyLandSparky]

were they on the same phase?

Did some work for a furniture store where miles of 2-circuit track was added by the previous tenant (also a furniture store.) Every track had both hots landed on the same phase via piggyback breaker, ALL of the neutrals were either dark yellow or brown. And this was THHN in EMT.