Don't trust breaker panel lables!

[Arizona CJ]

This probably goes without saying, but I'm posting it to warn people of the fatal mistake I almost made.

Backstory: I'm moving a washing machine plug as part of a remodel. On the main supply panel, the builder had labeled one 20 amp circuit "washer". I have the wall open, so I could see the supply line dropping down from the attic, going into the plug receptacle. I could also see a wire going out to a receptacle that serves the fridge on the other side of the wall. I used a plug in night light as an interim tester (I'm still alive because I didn't rely on it) and checked both outlets, and the light lit up. I then killed the breaker marked "washer" and checked again; the light didn't work on either outlet.

I *assumed* the "washer" circuit supplied those two outlets. I then pulled the outlet for the washer a bit out of its box (carefully, as if it was live) and checked live-to-ground. I found 120v with the breaker off. Had I not done the second check, I'd be a crispy critter.

Turns out, here's how it was wired. The romex coming down from the attic is 4 core. 2 live wires in it, on two different circuits. One, NOT THE ONE MARKED WASHER, supplies the washing machine outlet. The other supplies the fridge outlet and that's the one marked "washer". The washer outlet was backstabbed (12 gauge wire right into the back, not the screw port) and the neutral was a bit loose (hence my plug in light didn't work the second time).

Had I not checked twice, that combination of a mislabeled breaker plus the intermittent connection would have had me handling live wires thinking they were off. I consider myself lucky to be alive.

So, don't ever trust a breaker label to be what it says, and always double check that the power is actually off, in every circuit within a receptacle box (don't assume there's just one circuit feeding it).

 

[Stuff]

Good point. An LED plug-in circuit tester would have caught that with a loose neutral (but not if it had a loose hot). I try to remember to also use a non-contact tester.

If you want to be nice to the next victim see if you can re-arrange the breakers to be next to one another and add a handle-tie.

 

[Sponsor]

 

[Arizona CJ]

Good point. An LED plug-in circuit tester would have caught that with a loose neutral (but not if it had a loose hot). I try to remember to also use a non-contact tester.

If you want to be nice to the next victim see if you can re-arrange the breakers to be next to one another and add a handle-tie.

Great idea, but I wasn't willing to open up the main breaker panel so what I did was rearrange the labeling, so washer now goes to washer, plus I added a note on the labels that it's the same romex and the same box (the breakers aren't adjoining, unfortunately). I will also be going through the other circuits one by one to see what else might be mislabeled.

I absolutely will invest in an LED circuit tester and non contact tester (and still do the multimeter check). Thanks!

 

[Stuff]

Most digital multimeters will show phantom voltages so recommend having an analog meter to double check.

Just curious. 4 wire 12/2/2 is not very common. Did they actually use that (white,black,red,white w/red stripe) or did they cheat and use 12/4 (white,black,red,blue)?

 

[Arizona CJ]

Most digital multimeters will show phantom voltages so recommend having an analog meter to double check.

Just curious. 4 wire 12/2/2 is not very common. Did they actually use that (white,black,red,white w/red stripe) or did they cheat and use 12/4 (white,black,red,blue)?

I'll get an analog multimeter. I've noticed that the digital leaves much to be desired sometimes.

What they used was 12 gauge; red, black, white, and copper. The red was one live, the black the other, with the white as a common neutral (pigtailed together in the washer's receptacle). To be honest I found the common neutral rather odd. I've never seen that before on circuits that aren't on linked breakers, but this was done by a professional. (errr, probably the same guy who mislabled it.). It's bothering me because if I understand correctly, with this configuration that common neutral could end up carrying up to 40 amps, which seems to me to be a horrendously bad idea (my understanding is it's 20, max, for 12 gauge.).

If I've got to run new wire to the breaker panel, I'm sunk; there's no access for about 40 feet of that run, so I'd be ripping apart a lot of walls and floors.

 

[Reach4]

I've never seen that before on circuits that aren't on linked breakers, but this was done by a professional. (errr, probably the same guy who mislabled it.).

This was the norm at one time.
You can find info by looking for discussion of MWBC.

It's bothering me because if I understand correctly, with this configuration that common neutral could end up carrying up to 40 amps, which seems to me to be a horrendously bad idea (my understanding is it's 20, max, for 12 gauge.).

It's just the opposite. With the common neutral, the current in the neutral will be no bigger than in either hot. It will normally be less, and would be zero if both legs had the same value of load. It is very slightly more power-efficient.

 

[Stuff]

Key with MWBC is that each breaker is on a separate leg of the service. If on the same leg then yes, the common neutral sees the combined amperage of both circuits. When on different legs it then sees the difference in amperage. To ensure the circuits are on different legs use handle ties which forces them to be on adjacent breakers for most panels.

 

[DonL]

I'll get an analog multimeter. I've noticed that the digital leaves much to be desired sometimes.

What they used was 12 gauge; red, black, white, and copper. The red was one live, the black the other, with the white as a common neutral (pigtailed together in the washer's receptacle). To be honest I found the common neutral rather odd. I've never seen that before on circuits that aren't on linked breakers, but this was done by a professional. (errr, probably the same guy who mislabled it.). It's bothering me because if I understand correctly, with this configuration that common neutral could end up carrying up to 40 amps, which seems to me to be a horrendously bad idea (my understanding is it's 20, max, for 12 gauge.).

If I've got to run new wire to the breaker panel, I'm sunk; there's no access for about 40 feet of that run, so I'd be ripping apart a lot of walls and floors.

One of these are handy to have, You can connect probe leads or alligator leads for testing if needed.

It is always a good idea to check polarity on every outlet in a building. One of these testers with the GFCI test button is the berries.

I have never been dead because of household voltages, But I have had them wake me up many times.

Be careful when playing with electricity.

Good Day.

 

[Arizona CJ]

Key with MWBC is that each breaker is on a separate leg of the service. If on the same leg then yes, the common neutral sees the combined amperage of both circuits. When on different legs it then sees the difference in amperage. To ensure the circuits are on different legs use handle ties which forces them to be on adjacent breakers for most panels.

Hrmm. If they are on separate legs, I agree; it's fine. But if they are on the same leg (phase), it seems I was right, it's a huge mistake. Therefor, I think I need to find out. I can measure the voltage between the two lives, and see if it's around zero (meaning same leg) or around 240 (different legs). Okay, just went and did it; I read a changing fraction of a volt (might be zero, this multimeter isn't very good), which would mean same leg (phase). Do I have that right?

Further evidence; I have only had a supply panel open a couple of times, and if I remember right, they are laid out so that the breakers are staggered vertically between phases (legs). In this case, both breakers are on the left, with one breaker between them, so *if* I remember right on the staggering and this panel is that way, they are on the same phase (leg).

I'm thinking I need to open the panel and put these circuits on different legs in adjoining (vertically) slots?. I think the advice I've been getting to put a handle-tied set of breakers on this is a good idea, especially if I have to open the panel anyway. I do have the room - it's a 200 amp panel, with two free slots. And while I've got it open anyway, it'd be super easy to put in a couple of GFCI breakers by swapping out the ones for two circuits I'd like GFCI on.

I'm okay working inside my panel (assuming I am absolutely certain that what I want to do is okay); this panel does not contain the main breaker. The main breaker for the house is upstream in a separate box. So, *theoretically*, I can kill all power to the panel (I would of course test that!) and thus not be working with live.

*EDIT* I found a marking on the romex that supplies those two circuits; it's 12/3. I was using the wrong terminology when I called it 4 core; apparently the ground isn't counted as a core.

One of these are handy to have, You can connect probe leads or alligator leads for testing if needed.

View attachment 47097

It is always a good idea to check polarity on every outlet in a building. One of these testers with the GFCI test button is the berries.

I have never been dead because of household voltages, But I have had them wake me up many times.

Be careful when playing with electricity.

Good Day.

I'll get one! And I'll for sure get the GFCI test button with it.

If you take 120v and the path is through your chest, it can be a very bad day. I'm glad you're okay. I prefer to assume it's probably fatal if you get shocked at all, keeps me on my toes.

I've only injured myself once working with electricity, and that was at a supply panel. The cause wasn't voltage, it was gravity; the panel face slipped as I was putting it back, and the corner landed on my foot - and it gave me a nasty cut, right through my shoe. It did wake me up, but all things considered, I'd have preferred coffee.

 

[Stuff]

Most panels are set up so that the phases alternate up and down. So you want an even number of full size spaces between breakers on a MWBC (zero is an even number). Their should be a drawing of the bus bar layout on the back of the panel door.

Strange standard that cable in walls you don't count the ground. Cable going to an appliance (cord and plug) you count the ground.

 

[Jadnashua]

FWIW, the ground wire is never supposed to be used as a current carrying conductor, so the number refers to the (non-fault) quantity of power conductors in a cable. The ground wire is there to provide a secondary path to trip the breaker or fuse if there's a problem with the neutral or on one of the conductors of a 240vac circuit.

 

[DonL]

If you take 120v and the path is through your chest, it can be a very bad day. I'm glad you're okay. I prefer to assume it's probably fatal if you get shocked at all, keeps me on my toes.

One reason to play pocket pool.

I work with 3000 v to 35 kv on a normal.

You need to respect electricity.

 

[Arizona CJ]

Most panels are set up so that the phases alternate up and down. So you want an even number of full size spaces between breakers on a MWBC (zero is an even number). Their should be a drawing of the bus bar layout on the back of the panel door.

Strange standard that cable in walls you don't count the ground. Cable going to an appliance (cord and plug) you count the ground.

I just looked, and there is a diagram on the back of the door, which I hadn't previously noticed even though it was right under my nose (oops). And, it's alternating vertically, so that too indicates that this MWBC is miswired (there is one breaker between the two) as they are both on the same phase.

My current plan; throw the upstream main breaker (nearby, but not in the panel), test to make certain the panel is totally inert everywhere, then move these two circuits to a new two pole breaker in two slots (so they'll be on different phases). I'll also keep an eye out for any other miswiring.

One reason to play pocket pool.

I work with 3000 v to 35 kv on a normal.

You need to respect electricity.

Wow. From what little I've heard of those kinds of voltages (High tension lines), it's a massively different world, where even a pinhole in a glove can be lethal.

 

[Bgard]

I think that the latest code book requires a two pole breaker when the neutral is shared in a nm cable like you have described, so that what happened in your case cant happen,

 

[Reach4]

I think that the latest code book requires a two pole breaker when the neutral is shared in a nm cable like you have described, so that what happened in your case cant happen,

The 2 pole breaker is not required. A 2-pole breaker would trip both circuits if there is an overload on one. The newer codes require a handle tie to let them be turned off together.

 

[Stuff]

Even in the 2017 NEC handle ties are still allowed by for the OP's MWBC. The exception is when you have both 120 and 240 loads on the circuit. Then you need a common trip breaker.
That being said, a double pole common trip breaker is almost always easier to find than the correct listed handle tie.

 

[Bgard]

The 2 pole breaker is not required. A 2-pole breaker would trip both circuits if there is an overload on one. The newer codes require a handle tie to let them be turned off together.

yes you are correct, i haven't read the book recently but remembered there was something about using a common neutral in 2 120 volt circuits

 

[Bgard]

in the op's case a common trip breaker could cost him a lot of money, if the washing machine circuit caused an over-currant situation then his refrigerator would also go off possibly ruining a lot of food

 

[Arizona CJ]

I finally got around to fixing it. (I had the breakers off in the meantime). What I did was change the breakers around so that these two lines are now on vertically-adjacent breakers, on opposite legs of the 3 phase supply. I very clearly labeled them in the supply panel. I didn't make them a linked pair, because as pointed out above (thank you!) that could have resulted in tripping the circuit the fridge is on. Also, now they are clearly labeled, I don't see a problem.

However, speaking of electrical problems, I've found a nasty one in my mother's house. The original builder hung cabinets with screws, long ones, one of which missed the stud but went right through a Romex 12-2 20 amp. It grazed black and was touching mental on the neutral. A fraction of an inch either way and either it'd be a short, or a live screw head on the cabinet. Fortunately, it proved very easy to pull new Romex, so it was a minor thing to fix.