We are converting a former garage space into an office. We hired a general contractor to frame out the new office, then I went through and ran the electrical throughout the room, then the contractor came back and put the drywall up and finished the walls. This last week I have been installing all the lights, outlets, ect. When I ran the electrical, I installed a small load center just outside of the office for the various circuits of the new office. On one 20 amp circuit that supplies power to wall outlets on 3 of the 4 walls, I wired and installed all the outlets, then turned the breaker on and it instantly tripped. I then removed all the outlet covers and looked closely for and issues. I found none, nothing even questionable. I then checked the circuit for a short between the hot and neutral/ground wires and found none. There is no longer a short. Puzzled and worried, I pulled out each outlet and inspected all outlets, boxes, and wiring for any possible signs of a short, and found none. I then put everything back together and once again turned the breaker on. This time no problems. I put my clamp meter on the wire coming out of the breaker and measured no current.
Each outlet works properly. I then plugged in 2 electric heaters drawing a total of 16 amps and let them run for 1 hour, no problem. The only thing I can think of is when the contractor hang the drywall he may have screwed into a wire. Either way, there was a short and now there is not, what would you suggest I do.

 

Sometimes a breaker will trip when turned on, even when there is no fault. I have seen it many times. If the breaker reset I suspect it is OK.

If the cables were installed properly and protected by keeping them 1 1/4" away from the stud face, or using nail plates, there should be little chance that a cable was hit. That said, it still could happen if the drywall hanger used overly long screws.

I should ask, is this a standard breaker or a GFCI or AFCI breaker? Did the breaker make a big pop/spark when it tripped?

 

Thanks for the reply.
The breaker definitely made a violent pop.
I was sure to drill the holes through the center of the studs, if not a hair to the rear of the stud, as the back of the studs will not be finished.
Another thought, is it possible to tighten the wire clamps in the boxes to the point they can cause a short? I had a helper that was stripping the wire back and tightening them in the clamps.

 

Standard breaker, not GFCI.

 

"... is it possible to tighten the wire clamps in the boxes to the point they can cause a short?"

Definitely possible.

 

Well, seeing as there is no detectable short at this point, is there any fancy way of testing the circuit?
If there is a damaged wire, will there be a different voltage drop at a load placed at the end of the circuit, then if the wiring was undamaged? I mean, if there was a short initially, and the wire partially burned away, causing the short to open, the wire diameter will be reduced where it is burned. If I placed the heaters on the last outlet on the circuit, and measured the voltage at the heaters, will the voltage drop be greater then on a healthy circuit?

 

I don't know of any sure fire method to find the fault, if indeed there is one. Visual inspection of the clamps and cable should show a burn mark if that was the cause of the breaker tripping initially. That it no longer trips tells me the original fault had to have burnt away about the same time as the breaker tripping. Is it still a hazard? Maybe, I don't know.

Wish I had better news.

 

I told the owner about this problem, and seeing that the walls and finished and painted, he does not want to open them back up.
One thing I did before I left was replace the 20 amp breaker with a 15 amp one. My theory was if the short does return, the breaker would be easier to trip.

 

There is a way to try to find the short. You'd need a megger. It's a piece of test equipment that sends high voltage down the wire and checks for insulation breakdown. It will measure a "near short" location causing an overall low resistance measurement. They are bucks. The one I borrow is over $1500. Usually larger electrical contractors will have them.

 

I thought you were the owner. Are you a just the general contractor?

An AFCI breaker would be better.

If you open each box you might be able to see some black marks on the cable clamp if that was the cause of the short.

 

I am the owners employee.