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While drilling through drywall in bathroom to install a handicap bar to studs, I later discovered that the microwave oven breaker had tripped. Apparently one of the drill holes had missed a stud and went through PVC pipe in the wall (this was determined later with a tiny scope camera inserted through the drill hole). Although the microwave has been working fine after resetting the breaker, not sure if I should open up the wall to examine or wait and do so only if the microwave breaker trips again. I'm assuming that the breaker would trip before any potential fire might occur due to compromised wire.

I must say that I am very puzzled that there might be wiring to the microwave using electrical PVC conduit instead of cable! I live in California and the house was built in late 1980's. I thought California code called for cable for all electrical wiring within the walls!
The paragraph in italics has been revised. I've attached a photo of the conduit and it doesn't appear to be electrical pvc conduit after all. It seems to be much smaller diameter, 1/2" or so, and possibly not rigid. Hope this helps!

enter image description here

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    What was the pipe for? How did you repair the hole if you didn't open the wall up more? Whatever type of pipe it is, it shouldn't have a hole in it, and this should probably be addressed.
    – JPhi1618
    Nov 2 '21 at 16:00
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    "I'm assuming that the breaker would trip before any potential fire might occur due to compromised wire." I believe that is a dangerous assumption. The damaged wire could easily ignite well before over-currenting the circuit breaker, if it ever does. Nov 2 '21 at 16:12
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    @Triplefault True, however OP could mitigate some of the danger by replacing the breaker with an AFCI breaker.
    – Glen Yates
    Nov 2 '21 at 16:15
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    @Harper-ReinstateMonica Undamaged metal conduit won't hurt new wires being pulled through, but metal conduit that's been accidentally drilled into might end up with burrs/splinters inside.
    – nobody
    Nov 3 '21 at 0:27
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    "I'm assuming that the breaker would trip before any potential fire might occur due to compromised wire." This is a dangerous assumption. A damaged wire does not require an overload in order to overheat. Nov 3 '21 at 1:09
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Code never prohibits the use of conduit. Even if it isn't required, some installers might want it for future pulls or just protection. Non-metallic cable is the minimum standard. Many municipalities require metal conduit for fire protection.

If you punctured conduit, and if you damaged the wires enough to short them, this is a potential safety concern and they should be replaced. A damaged conductor creates a point of high resistance, which can result in overheating even under relatively low current. Even if the wire conductor wasn't damaged, the insulating sheath was. That can result in arcing under high loads.

I'm not too concerned about a small hole in the conduit, though an inspector would probably flag it. Seal it with silicone or something if you can. If you use something like epoxy, do it before you pull new wires.

The fact that you have conduit is good news. It means that you should be able to pull replacement wires using the originals. If you did have cable, which would be stapled to the framing, you'd be opening up the wall, at a minimum.

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    I might suggest cleaning out any burrs from the conduit before pulling new wire. That's a pretty ragged hole and the OP doesn't want to risk damaging the new wire. Or catching the wire on it while pulling the new and disconnecting from the pull wire/rope/whatever. Nov 3 '21 at 15:23
  • Out of curiosity, what does load have to do with the potential for arcing? I thought that was generally just a function of the potential difference and the medium (i.e. the breakdown voltage of the medium being arced across?)
    – reirab
    Nov 3 '21 at 21:14
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    Generally, it is. But load spikes and other noise can trigger arcing that might not have otherwise occurred just by virtue of the disturbance. I once witnessed a dryer circuit cable arc to its strain relief coming out of the panel when it was first loaded. My wife was in the room. She levitated, as I recall.
    – isherwood
    Nov 3 '21 at 21:25
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Answer based on new images, previous answers were assuming conduit.

Based on the picture that was edited in later, you have nicked an electric cable directly rather than drilling into conduit. I don't know how common this grey-sheathed cable is, but I do have some in my house that was built in the 90s, and its a round 14/2 cable in my case. I have to assume they made 12/2 or /3 varieties as well. It's NOT outdoor rated Romex that is colored grey now, it's obviously an interior rated cable from before they somewhat standardized colors of the jacket.

When your drill bit entered the cable, it would have stripped the internal wire and contacted two wires at the same time, tripping the breaker. Once the drill bit was removed, the wires are no longer in contact so the microwave seems to work fine, but you still have bare conductors in the cable that are very close together. This has a potential for arcing, and arcing causes intense heat, but doesn't draw enough power to trip a standard breaker. And of course, intense heat is a major contributor to fire.

Repairing this wire is a must, and will be a bigger job than the other answers assumed because there is no conduit.

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    based on the bulge to the right of the obvious hole, I'd agree with you that this is a cable and not a piece of conduit. I saw that earlier but based on the preconceived notion that it was conduit, I couldn't reconcile it. You've nailed it.
    – FreeMan
    Nov 3 '21 at 15:57
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    @FreeMan pun intended?
    – JDługosz
    Nov 3 '21 at 16:32
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    No, no, @JDługosz, that would be "drilled it". ;)
    – FreeMan
    Nov 3 '21 at 16:56
  • I don't know USA wiring codes. Whether it's a big job or not will depend on those, and whether there's any slack in the cable. If there's as little as an inch, it may be possible to cut the cable exactly at the puncture, put both ends into a junction box, and re-join them. If there's no slack, two junction boxes and a short bit of new cable between them may be almost as easy. If there's no space for two junction boxes, start swearing.
    – nigel222
    Nov 5 '21 at 11:58
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Actually, it is quite the opposite. Most states allow the use of non-metallic cable or conduit, but some only allow conduit. Conduit comes in several varieties, including PVC. PVC conduit is similar to, but not identical to, PVC used for water.

The big concern is that you have a damaged wire. The damage won't fix itself. A short-circuit is the best-case scenario - it trips the breaker and prevents a fire. But once there is damage, you can have arcing between damaged wires (or between a damaged hot wire and a bare, undamaged, ground wire). That will probably be caught by an AFCI (Arc Fault Circuit Interrupter) but most houses more than a few years old don't have AFCI. PVC provides reasonable, but limited (as you found out the hard way) protection against physical damage, but it doesn't protect against arcing the way that metal conduit does.

The good news, as noted in another answer, is that you may be able to replace the old wires with new wires without having to open up the walls. That is the benefit of conduit + wires vs. cables. However, I would still be a bit concerned about the integrity of the conduit - if the hole is large enough then it may need to be patched (replace a segment of the conduit), which would require some drywall work. That could get a bit complicated if there are circuits in the conduit besides the one you need to fix.

If the wire is actually in a cable and not conduit (the picture isn't clear to me) then this is a much bigger deal. The options are either to open up a small part of the wall and splice in a new section with a permitted in-wall splice such as Tyco Romex Splice or to replace a section between a pair of junction boxes. Ordinary splices (e.g., wire nuts) can't be hidden - they must be in junction boxes or other accessible locations.

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The wire is nicked. It is thinner than it's supposed to be, and will overheat and start a fire there unless it is repaired. PVC's fire retardant capabilities are notional at best, so you really want/need to solve this.

Fortunately, this is stupid-easy if the conduit was properly installed. The thing about conduit most people misunderstand is, that you never assemble the conduit around the wires. The wires must be pulled in after the conduit is complete. But this is easier anyway.

Therefore the conduit must be built to be pullable! Every curve will be a gentle "sweep" and every run must go between junction boxes, the panel, or a special access point called a "conduit body". And all of the above must remain accessible forever, they are not allowed to be buried under finish surfaces like drywall.

So this is easy work for an electrician, as long as you know which circuit tripped.

Conduit is a vastly superior wiring method because of its maintainability.

One more trick you should know: Code requires that electrical wires be at least 1-1/2” beneath a finished wall surface, or be guarded by a steel guard plate. That is so you can do projects like yours with confidence, knowing that if you only drill to 1-1/4” and don't force your way through any steel plates, you will not hit wires.

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  • Unfortunately, missing the stud when drilling leaves less room for the confidence noted in the last paragraph. Even light pressure and slow reflexes when pushing through the drywall could, when the bit clears the back of the drywall, force the bit into the conduit placed the proper distance back from the stud face, causing the damage noted by the OP.
    – FreeMan
    Nov 3 '21 at 15:02
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    I know this is off topic, but congrats on 200k Harper! That's quite a milestone.
    – JPhi1618
    Nov 3 '21 at 16:26
  • @JPhi1618 Thanks! Appreciated. Nov 3 '21 at 17:59
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    @FreeMan That can be managed with good drilling technique, e.g. put a stop on the drill bit so it can't go too far. they make collars for that, or just use corks. Nov 3 '21 at 18:01

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