I rewired an old house a few months ago. The owner has not moved in as of yet, since a lot of remodeling had to be done. I wired the dining room circuit on a AFCI. The breaker had been tripping when a wet vac was plugged into the circuit. I traced the problem to one outlet on a shallow wall on the back side of a fireplace. Every time I would disconnect the feed to that outlet, everything worked fine. I went up into the attic and cut the wire just before it went down into the wall to the outlet. I tried plugging something into the other outlets on the circuit then and everything was fine. I then proceeded to fish another wire down to the outlet, assuming that there was a "nick" in one of the wires. This was of course after I had checked the outlet itself. After I fished the new wire down to the outlet and hooked it up, IT STILL TRIPPED THE BREAKER!!. Since I had already spent around 3 hours checking the different outlets etc.... I proceeded to put a regular breaker on that circuit for now. The only thing I can figure is, that when I was pulling the new wire down the wall, it got scraped or something on the way down. It was a rough pull since it was right against the concrete on the back side of the fireplace. Only problem with that thought is, that I checked the wire I pulled out and didn't see any evident signs of anything wrong with it. Does anyone have another idea other than the thought that I'm crazy?? It had to work with a new wire, right??? Only thing I would know to do would be to pull another new wire down and hope it don't scrape the same thing the other two may have come in contact with. The wall was sheetrocked and paneling was put up on the lower half of the wall. Possibly a nail??
After going through with that, I just got my tools up and went home.... Thanks

---tripping when a wet vac was plugged into the circuit.---

Before pulling the new cable,and after separating the rec. did you try the wet vac?
Did you swap out the breaker?

The AFCI- Mfg. aspect is far from perfect as of yet.

Probably a bad appliance.

Did you eliminate the possibility of bad equipment ie plug in a radio and see if it trips?

And if you can megger the cable run to find any traces of bad isolation.

I switched the circuit to a breaker that was working, and it still tripped. I tried the wet vac in another AFCI circuit and it worked fine. There is nothing else plugged into the circuit, since no one is living there yet. I don't have a megger. Got to be something in that wall that is snagging the wire as it goes down or something they haven't discovered yet that's in the fireplace that triggers the AFCI's It's a mystery UIAIW (unidentified alien in wall) :0

Sparky:
Just a thought; you say it's a "shallow" wall. I have to ask, what kind of device box? Metal? Shallow? Any chance there's an issue within the box/terminations/device??

Of course, I'm not there to see things first hand, but my guess would be that you could change that wire all day long and it probably won't make a bit of difference.

What brand of AFCI cb are you using?
Does it have the diagnostic LEDs on it? Those could possibly give you some indication as to what's going on.

I guess you've already checked to be sure you don't have a grounding conductor touching the neutral anywhere or a neutral from another circuit spliced together with the neutral of that circuit.

if all else fails cut in a metal box and use MC. But first a few diagnostic steps, remove the receptacle outlet from the box and try the circuit,remove the receptacle from the wire,disconnect the wires at the junction box. If you grt to the last step and now are holding take your meter and do an Ohm check beteen hot-neutral,hot-ground,neutral-ground this would show if any short exists.

I guess as a last resort you could always invoke the words of Ronald Reagan, "Mr. General Contractor... tear down this wall".

Either that or just tell them the plumber said he needs to run a 1/2-inch Pex line up the wall, then for some reason they won't hesitate to remove entire sections of wallboard if deemed necessary.

If you really think it is something damaging your cable, fish MC.

Plastic box, Cutler Hammer Breaker, no room overhead without cutting ceiling joist in to for MC cable, ohm check ok with no readings between neutral and ground, or hot to ground or hot to neutral. Once I take the wire loose from the circuit the breaker holds. The breaker I switched to test it was a 15 amp AFCI and done the same way. No way the contractors going to tear down the wall When I fished the new wire down the wall, it tore some of the insulation off the "pull" wire, but I didn't see any "nicks" in the conductors. The inspector told me a few months ago, they may change the rule for AFCI's again that would only include the bedrooms again. I may have to let it fall under that

Does the AFCI trip when anything else is plugged into that receptacle, or just the wet/dry vac? Does it trip when the wet/dry vac is plugged in, or turned on?

If the cable was nicked and seeing a parallel short, it should be arcing as soon as the circuit is energized; plugging something in wouldn't make a difference. I don't think the cable is your problem. AFCIs aren't supposed to trip on series shorts, either, but it wouldn't hurt to double-check the wirenuts and other splices.

First, though, I'd replace this receptacle, regardless of how it tests. My gut feeling is there's crap loose in this receptacle and it's shorting out when you plug your wet/dry vac in.

Steve:

You said.."If the cable was nicked and seeing a parallel short, it should be arcing as soon as the circuit is energized; plugging something in wouldn't make a difference. I don't think the cable is your problem. AFCIs aren't supposed to trip on series shorts, either, but it wouldn't hurt to double-check the wirenuts and other splices."

I'm under the understanding that a combo AFCI sees series and parallel faults, hence the 'combo' designation. I don't profess to be an AFCI maven as AFCI are 'new' here in NJ as of 10/6/2009, and resi is still slow. I garnered my info from various mfg sites & info, and from a lot of guys here that have AFCI experience.

sparkync,

That is a real heard scratcher of an issue!!!

Keep us informed on the progress.

I cannot think of anything, not already covered so far, which could be tried.

Does the AFCI Breaker trip if another type of load is plugged in to the Receptacle in question? (other than the Wet Vacuum Sweeper).

Scott

To create a series fault, you'd have to not only nick the wire, but actually break it. I think you'd have noticed this during installation.

Have you checked the tightness of the breaker and neutral terminals in the panel? And other connection points? An untightened terminal with nothing but the spring tension of the wire can cause arcing that might trip it.

Did you try a GFCI breaker as a test?
That will decide whether this is an arc fault or a ground fault.

I never even noticed this comment until now.
Are you saying that the last NM cable out of this device box is also supplying a gas log fireplace?
If so, does the fireplace have a variable speed universal blower motor mounted below it?
Or possibly an electronic ignition module?
If so, then that may be the problem. Either that or the wiring in the gas log itself could have a problem.