GFCI tripping after plugging in over-the-range microwave

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Old 12-02-14, 12:34 AM
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GFCI tripping after plugging in over-the-range microwave

Been a reader and a diyer on here for awhile now and the community has always been good about figuring out the solutions to problems, so now I've got one that is likely not unique, but the only other related mention of this kind of issue didn't have a confirmation on what the problem ended up being.

A couple days ago, I bought an above the range microwave and installed it. For the moment, I have it in 'recirculation' mode, though there is a duct coming in from the roof already that I will eventually use. For now, it isn't making contact with the microwave though and I've only just cut out the pieces of the cabinet in prep for that.

The microwave works, but every time I plug it into the outlet, it trips the GFCI in the kitchen which several outlets in the kitchen appear to be tied to. I ran a heavy duty extension cable and it happens on all of the outlets in the kitchen. I then took down the microwave and set it atop a counter and plugged it in and tested it; no issues regardless of the outlet so that lead me to the mounting bracket.

I grabbed a piece of metal and touched the mounting bracket to the metal part of the microwave and the GFCI trips. I have the bracket mounted on two butterfly anchors through drywall and two lag screws through studs and after some experimentation, the problem went away after I removed one lag screw in particular, so I'm thinking I'm completing a circuit somewhere. I chipped away some drywall around the hole to verify I wasn't hitting any electrical cable and I'm hitting solid stud, so I'm running out of ideas.

To give some idea on this environment, this is a newly constructed home where the builder abandoned the jobsite before completion, hence information will probably be scarce. The wall in question is shared with a garage that has been drywalled and the washer and dryer both sit on the other side of the wall, though I don't appear to have hit anything.

Any thoughts on what I'm hitting that would cause this kind of behavior?

Is this something an electrician will need to fix or should I just fill the hole and use a different one?
 
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Old 12-02-14, 04:00 AM
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Welcome to the forums! Sadly the OTR microwave cannot run on the small appliance circuits. It must have its own circuit separate from them. I am not sure what you hit with the lag screw, but it certainly had to do with the grounding system of the small appliance circuit, whether you nicked the casing of a cable or moved a staple leg enough to cause it to compress on a cable running up the stud. Difficult to tell without exploratory surgery.
 
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Old 12-02-14, 06:39 AM
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Be careful touching any metal to mounting plate, or touching plate, it could be 120Volts to ground.

I would cut out entire section of drywall behind unit, 17 x 30 or whatever the size is. After finding the problem replace drywall with 1/2" plywood.

The plywood isn't required to hang unit, but will allow you to use shorter screws when mounting oven.

You must have screwed into a wire, or one of the toggle bolts squeezed a cable and cut through the sheathing and wire insulation.
 
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Old 12-02-14, 06:40 AM
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I'd be cutting out that sheetrock to see what's really going on, not just trying to peek though a small hole.
If there is a wire that's been damaged it needs to be addressed ASAP, it's now a fire hazard.
 
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Old 12-02-14, 11:41 AM
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Yep, already turned off the breaker for both the gfci and the microwave. I only had it on long enough to test my suspicions. I did the small hole initially to ensure I didnt partly enter the stud and screwed it into the edge, but I'll probably cut out more sheetrock to get a better look tonight at the damage.
 
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Old 12-02-14, 12:22 PM
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While you have the hole open be sure to run the cable for the dedicated circuit you will need for MW. Usually the receptacle goes in the cabinet above the MW.
 
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Old 12-02-14, 04:40 PM
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There's already one there. It's probably on its own circuit, but the breaker isn't labeled and I'm not gonna flip the switches and experiment anymore with the potential for a fire hazard. Unless I find something when I open up the wall later today, I'll probably call an electrician. I'll post an update once I have news. Thanks again.
 
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Old 12-02-14, 06:09 PM
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Yes, it's always nice to get updates and it's appreciated.
Just a few points:
If M/W was on a dedicated circuit, it would not trip counter GFCI's.
When you open up wall you will most likely see obvious damage to a cable. My best guess is a screw through a cable. With the dryer behind M/W, it may even be a 240V cable, which is easier to hit with a screw than a typical #12 or 14 cable.
 
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Old 12-03-14, 01:44 AM
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Alright, so I opened up the wall more. I'm hitting the bottom of a yellow oval romex cable running horizontal through the stud. If I understand color coding correctly, that's gonna be a 110v/20A receptacle, correct? There's no give to the cable though, so I can't slide it back and forth through the hole to get a look at the damage. Still, the likelihood of hitting a wire through a stud...I really do have some rotten luck. Probably time to call an electrician. Guess I'll have them install a dedicated circuit while they're out here. Any idea on time/cost?
 
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Old 12-03-14, 06:15 AM
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Can't you tell where the cable runs to?
Generally speaking (not always true), if a cable runs horizontally through studs, it's not going too far.
 
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Old 12-03-14, 06:26 AM
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that's gonna be a 110v/20A receptacle, correct?
No, a 120 volt 20 amp circuit. To repair you would install a junction box about 8" above the damage, cut the cable at the damage and move the remaining cable above the cut to the new junction box. Then abandon in place the cable below the damage and install new cable from the new junction box to the box where the abandoned cable terminated. Splice box would be covered with a blank plate and must remain assessable.

Price for an electrician varies by area but probably the cost for a service call and one hour work (assuming minimum charge is one hour.) for repairing the cable. Running the new cable depends on too many factors to remotely estimate. Both can be DIY jobs.
 
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Old 12-03-14, 06:57 AM
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Theres cabinetry in the way, so no. Probably the GFCI though. Its the closet thing.
 
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Old 12-08-14, 03:10 PM
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Had the electrician come out; costed about 180. Knocked out an 8 inch piece of drywall and a couple inches of wood. When I drilled in the screw, it damaged the sheathing on the hot wire, providing a connection to the unsheathed ground wire. He did confirm that I had a dedicated circuit and breaker switch for the microwave. Anyway, they patched it. Thanks everyone for your insight.

Admittedly, this was probably a DIY project, but they're real funny out here about DIY electrical due to the wildfires, so I figured it was best not to take any chances.
 
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