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vseven 04-10-2010 05:35 PM

Drilled through 12/3 and breaker didn't trip
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So I was wiring a new circuit I'm adding to my basement and making holes in the joists. I checked it from one side, drilled through just fine. I then went through the other side to clean up the hole and somehow went through at a bad enough angle to clip a 12/3 that was above the hole. Drill bit arc'd, made some little divits in the bit. I opened the breaker box to see what tripped and nothing did...every breaker was fine. So I turned off the mains, opened the box, and traced back the wire. I found the black and red both going to a ganged 20a breaker. Problem is the black (top) is for the dishwasher and the red (bottom) is for the disposal.

Is this improperly wired? Should these be on separate breakers / non-ganged? Is this why the breaker didn't trip?


jbfan 04-10-2010 05:47 PM

It is/was wired correctly.
It is not uncommon to use 12/3 to feed the DW and disposal.
After the 2008 code cycle they are required to be on a 2 pole breaker.

Sometime a short will burn through without tripping a breaker.

I would replace the breaker anyway.

vseven 04-10-2010 05:59 PM

They used 12/3 to feed a lot of things....I was adding lights to a existing circuit and was going to swap in a AFCI to find that the lights and the smoke detectors were wired from the panel with 12/3 but 30 feet from it they split the circuits (they are also on separate 20a breakers). A lot of circuits are done the same way, separate 20a 1/2 breakers using 12/3 that splits once it's away from the panel. Very confusing to track stuff down, at least for me it is.

So if the disposal trips or the dishwasher trips it will take out both (or I guess I should say it should take out both...)? Since it's not a 220v circuit couldn't I just change out the ganged breaker for two separate 20a 1" breakers or would that not be correct?

joed 04-10-2010 06:06 PM

It is called a multi wire branch circuit (MWBC) and they are common. The neutral is shared between the two hots.

jbfan 04-10-2010 06:08 PM

In 2008 code required all mwbc to use a ganged 2 pole breaker.
If you have 12/3 on 1/2" breakers, you may have later issues.
If you want to change to afci breakers, you need a 2 pole to cover the mwbc, which may or may not be made for the breaker panel you have.

vseven 04-10-2010 06:15 PM

Yeah I understand the concept of the MWBC and the shared neutral, the way things were done in this house I'm sure it was to save some money on wiring. I just don't understand why I have some that are on separate 20a breakers and some like this one on a ganged 2x20a breaker. I think I understand them being ganged as if one trips it will (should) trip the other since they share a neutral.

I looked into changing out to the AFCI and actually rerunning some of the MWBC's with 12/2 but it's just not worth it. All I was doing was adding 3 lights to a existing 7 light circuit.

I'm going to rewire a 12/3 from the breaker to the short and splice it in a gang box. Luckily it was 6' away from the breaker box so not much work.

Do I have to replace the 2x20a ganged breaker or should it be ok? Should I replace the separate breakers for the other MWBC's with ganged breakers? House was built in 1999 so I'm assuming they followed the code at that time.

jbfan 04-10-2010 06:21 PM

I think I would replace the 2 pole breaker. Talking $8.
Looking at the picture, it looks like maybe some smoke escaped.
That may not be the case, cause my eyes are not what they used to be!

AllanJ 04-10-2010 09:15 PM

If the short did not last a long enough time or was not that dead of a short, the breaker might not trip.

An arc represents current jumping through air, and there is some resistance at that point which limits the number of amperes somewhat.

I don't think the breaker needs to be replaced.

nap 04-10-2010 09:35 PM

3 things

1. it does look like that breaker flashed. You can see the carbon on the face of it

2. some breakers do not look like they are tripped even though they are. Many times somebody has looked in a panel and so no obvious tripped breakers but when they would go through and actually reset the breakers, they would discover a tripped breaker.

3. I don't call them GE Nevertrips for no reason:laughing:laughing:

I remember working on a job where I had welded the conductor to a steel box. I swear I took one step back, turned and took one or two steps towards the panel before the breaker tripped (the wire chatters inside the conduit. Quite noticeable testimony the breaker had not yet tripped)

whoops, 4 things:

most, if not all breaker manufacturers recommend replacing a breaker if it has tripped on a short circuit. I believe it is mainly for liability purposes but there is science to support that you should actually do it though.

vseven 04-10-2010 09:40 PM

k...I'll pick up another 2x20a ganged breaker tomorrow...gotta buy more 12/2 and some work boxes anyway. Rather be safe then sorry. I'll not worry about the other non ganged MWBC's for now either although I still might consider rewiring the MWBC that's doing the basement lights on one circuit and the smoke/CO detectors on the other. Thanks guys,


joed 04-10-2010 10:18 PM

There is no reason to replace the MWBCs with single circuits. They are code compliant and safe circuits that are used all the time.

vseven 04-11-2010 08:09 AM

No joed, I meant the other way around....the bunch of single breakers I have that feed MWBCs.

vseven 04-11-2010 11:03 AM

So I turned off the mains, spliced the new wire into the old in a workbox, and replaced the breaker this morning. I turned on the mains and about two seconds later I heard a pop. No breakers tripped. So I searched through the house and found a bathroom GFCI was tripped. I tried resetting it and it chattered for 2 or 3 seconds but held in. I thought that was odd so I pulled out my outlet tester and sure enough the GFCI was wired backward (hot/neut reversed) and I'm assuming from the chatter bad. So in some way my stupidity drilling through the wire might have prevented a future shock hazard. :) I replaced the GFCI and all is well, disposal and dishwasher also are working.


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