Just another caution - never trust your wiring (or maybe electrician)

Thought I should mention this in case it helps someone else avoid injury. Also curious about whether the wiring on a switchbox in my home has been botched or not.

I was installing a new GE light switch yesterday, and did the normal process to go out and turn off the breaker for the circuit that includes that switchbox. Then I returned to the switch box and verified via trying each switch repeatedly that the power was out.

Removed switch cover, removed the switch I was replacing, and started to install the new switch. Was part-way through when suddenly I got a huge shock. Totally freaked me out. I don’t remember exactly which wires were connected at that point to the new switch (should probably have written that down.)

I immediately stepped away and went out to the panel to confirm I had really turned off the correct circuit. I confirmed it was off (and as noted the lights weren’t working after I had switched it off). However, I noticed another switch directly below the one I turned off had been tripped. Huh? I turned it on again and returned to the switch box w/my trusty voltage tester, and sure enough, with the second circuit switch on again the black wire to the switch I was installing was hot, the other black wires in the box were not. I turned off the second circuit as well and the rest of the installation went normally.

So it appears that the switch box I was working in was wired to two different circuits/switches in my panel, and the labeling that the electrician had done when he installed my new panel some years ago did not communicate that at all. (I had actually reconfirmed the labeling a few years back, but since the lights didn’t work when that switch was out, it looked OK at the time.)

The second switch that tripped when I got shocked was labeled as controlling power for another area in the family room. It does control that, but somehow is also tied into the switch box for the light switches I was working on as well.

I’m no electrician, but it seems like this is probably not compliant with code, and even if it is it is very dangerous if not carefully labeled.

And after the fact I realized I had broken my own safety rule when working on electrical stuff - always re-confirm w/tester that the wires I’m working on are all dead. So that’s on me.

But I’m unhappy w/this odd setup my electrician left me with, and wondering if other electricians here can confirm if this setup is as bad as I think it is, or if it’s uncommon but sometimes done, and the main issue is the lack of correct labeling on the panel.

So learn from my mistake and never assume lights not working means no power to the box.

And thanks for any feedback.

This is what is known as a multi-wire branch circuit, assuming that’s how it was wired. Perfectly legal to do, but I believe code now requires them to use a multi-pole breaker so one breaker kills all of the branches. Sounds like you don’t have that.

There are no labelling requirements for MWBS installs that I know of, but it’s always a good idea.

Never trust anyone, that’s my motto. I always assume the last person did it wrong and/or dangerously. This includes myself when I go back to change something I previously did. I meticulously label everything, but I still probe everything with a sniffer before touching anything.

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Thanks, Robin.

It was definitely not a neutral - the neutral bundle was in the switch box wire-nutted up and didn’t have any exposed wires. I confirmed it was the black line wire that was still hot when I checked w/my tester.

I have never turned off the master switch when working on lights - didn’t know that was something I should do. So even when just the local circuit is off to the switch (and no other circuits are connected to it), the neutrals can still shock me? Wasn’t aware of that, and having done a lot of these switches I’ve never run into any issues when connecting the new switch to the neutrals as a part of the process.

I’m pretty weak on how circuits work w/neutrals, clearly.

Thanks for the additional info on this. I definitely don’t have a multi-pole breaker as far as I can tell. My brother is a general contractor and does electrical so when he comes over to install a fan in my hall bath I’m going to have him look at it and let me know if he can install a multi-pole breaker. BTW - does that just mean you put a metal bracket over the two switches so that they have to be disabled/enabled in tandem, or similar physical change at the switches themselves?

Regarding trust no one, even yourself, have to agree. :slight_smile:

Multi wire Branch circuits are supposed to be on a ganged breaker to make sure that each of the two hot legs are on opposite sides of the AC and if one trips the other will also be turned off. I’m not saying they always are but they are supposed to be. I know I put one in a friend’s house to get two circuits up into his attic using one 12-3 romex and I had to sit and rearrange his entire electrical box so that I could do it properly ( there were two open spots in his box but they weren’t together at the time).

With that said based on the description I don’t think this was a mwbc, I think it was simply a hot taken off of the nearest circuit. I mean the only way to confirm that would be to cut your main power and take the cover off your breaker box and see if you have a wire going to both Breakers with one neutral, it would probably be a 12-3 Romex if it’s relatively new.

Personally I’ve never turned off the main breaker doing any of my switches but on the flip side to that I test everything against ground. I’ve done a couple switches live, not smart switches but standard dumb switches, when I’ve had to. As long as you know what you’re doing it’s fine but I wouldn’t recommend anyone else do it.


When we first moved into our (old) house, I replaced all the older almond toggle switches w/white decora paddles, and did the majority of them w/the power on. Luckily I didn’t Darwin myself out of the gene pool. The misplaced self-confidence of youth. :slight_smile:

I always hit the breaker now - either I’m older and more chicken, or older and smarter. Prefer to believe the latter. :smiley:

Yes… and no. I was in IBEW for a couple years, and working on hot wire is common. It’s OK if you know it’s hot, and use proper cautions.

But I got zapped a couple times in ‘private’ scenarios where it ought not have happened, and it’s because (have to get slightly political here) there are places where licensing and inspection are both lax. Once was down in Clearwater Florida. I was doing some work for my in-laws on a relatively new build in a condo complex. I shut down a breaker panel, began working… and BAM. Got knocked off my step stool. It turned out to be a sub-panel, and whoever did it wired it backwards. The hot and neutral were reversed, so every neutral (and every ‘ground’ wire) were ALL HOT.

I conveyed this to the manager of the complex. He checked it out. Turns out many of the sub-panels in the complex were wired wrong.

So if you are doing this kind of work, yes turn off the mains… but TEST ANYWAY. Do not presume that merely taking down a panel means you’re safe.

Multi circuit breakers you talk about are for 220 volt use. Two 110 volt breakers, opposite phases for 220 volt appliances, that are “pinned” together so that both breakers “pop” if there is an overload. You do not want a 220 volt appliance running on single phase. There is a breaker called a “twin” which is illegal here in Chicago / Cook county. It takes the one circuit and splits into two circuit (same phase). Used when there are no more empty slots left.

There are no empty slots left for a good reason, you are at capacity and should not be using the twin breakers. Testing the circuits are the only sure way. Killing the main breaker is a little bit of an over kill and causes way too many inconveniences.

Be safe.

Code requires this labeling only industrial (power from multiple sources)
You should not have power coming from multiple breakers in the same switchbox (but it’s allowed if it powers 2 different circuits). The other thing is do you have 2 independent neutrals coming to it? If they’re put together, it’s against the code for if one goes bad now current closes through one wire only but from 2 loads and could exceed current rating.
See what loads the 2 switches control and maybe you can use one power source, abandoning in place the other, cap the wires and disconnect the breaker.
When I bought the house, the first thing I did was replace ALL wiring and panel. Put a 42 breaker panel and separated lights from outlets in every room. Plus separate breakers for fridge, oven, microwave, dishwasher, washer and dryer.
Not saying that you do the same, but at least label everything both at the load center as well as inside switchboxes.
And make sure the 2 breakers have a tie bar that disconnects both at the same time.

I did very similar to you when I re-did my current place. The downside, when I later added a generator and transfer switch I needed to combine some of those separate, but low wattage circuits…

Several similar, but separate computers, tv’s, refrigeration and freezers. Not a biggie with an unfinished basement, but pulled more wire and more junction boxes.

  • Everything is labeled, including all junction boxes,. I have only ONE box with dual feeds, it’s labeled under the switchplate it was too difficult to add another box but was a high wattage kitchen application. It is tied in the breaker box and labeled there as well.

This is why I turn off the main breaker and just deal with fixing the time in the microwave. I’m overly cautious since I have been electricuted (sp?)before.

My house was re-wired after Hurricane Katrina by a licensed electrician and inspected by the parish for code compliance. All parties involved should be hung by bare copper wiring. The crap I find is inexcusable, and I am a civil engineer, not an electrician. I am constantly finding things and fixing them to be in compliance with NEC.

I have at least 4 boxes, where the common line is engaged even if the local circuit is off due to improper wire splicing in the attic. Most of this is concealed in insulation or under ducts. I have wiring run in my ducts and I have a bird nest where 7 wires are all tapped together under my HVAC drip pan. I have to remove the 5-ton HVAC unit tie fix this one. In my main panel, all the common and grounds are terminated together and not always on the bus bar. Many circuits are double lugged and I have 2 circuits that often trip because of this.

While I am very comfortable fixing spot issues and changing switches/outlets, replacing fixtures, etc. However, my long-term fix will be to put a new lighting sub-panel and move the circuits over correctly - and I do plan to hire an electrician for this task. The new openings on the main breaker will be used for new air conditioner equipment I plan to install in the next two years.

I was NOT the owner of the home at the time of this or people would be in court.

I also have several 3-way setups in my house that bridge circuits. For example, the switching comes off one circuit, but the common termination is in an outlet box on a different circuit. I have now learned where all these are and trying to correct them as well. This may or may not be an NEC issue, but it should be cleaner than it is.

But yes, the common lines do carry a small voltage and is not “shut off” when you shut off the hot circuit breaker.

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Why did you buy a house in this condition? I would have walked away.

It was not visible on the surface and not revealed in the inspection. Much of it has been discovered in doing other work, such as running new LAN wires throughout the house 5 years ago, or pulling the HVAC coils for service and fixing leaks in duct work.

Granted, the common and ground wires being lugged together, but the main breaker is also the service disconnect so that is ‘technically acceptable’, albeit shitty and not best practice. Since I want to have a dedicated service disconnect outside the home, this would not be acceptable. The reason for that is to tie in a generator transfer switch…again, something I will leave to the professionals.

The wire run down the duct work was not realized until I was investigating installed a 3-way run for the hall light switch. Once I saw it was in the duct, I shelved that project and will deal with it when the duct work gets replaced.

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One thing I wish I had done was to adde an external main disconnect. I even replaced the meter box as well as wiring and never even thought of it at the time.

I added a generator and transfer switch (later) - all in the basement and no real need for a disconnect when I did it.

HOWEVER - if one knew they were replacing their main - and adding a generator, one of the main/transfer switches is nice.

Mine had many similar issues but I knew it going in. 3 different fuse boxes, one looked like Niagara when it rained. Water came in the meter box, traveled inside the outer wire covering and drained in the tiny main box. I was all/mostly behind anything hot but was sure ugly.

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Neutral (or common as you call it) provides current return path from the load. If you have that low voltage between neutral and ground, then grounding is no good. With breaker on no matter if load is off or on, should be NO voltage.

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“as you call it”, nice red herring. This forum is for DIY’ers in home control, not code officials for NEC.

In theory, sure, they should be the same, I suppose. In reality, no, your statement is incorrect.

You have voltage loss from your main disconnect to your load along the hot line. You also have voltage loss from your load back to disconnect via the neutral (or common, regardless of what I call it). If your common and ground (true 0v) are tied at the breaker, then you can NOT have 0v on the neutral at your load. This is only possible at 0 amperage (no load).

I just took measurements in my kitchen and I have 118v from hot to ground, 117 from hot to neutral and 1v from neutral to ground.

Having voltage on the neutral does NOT mean the ground is no good - that statement is wrong. What it means is the ground is working exactly as designed…to provide a path to Earth in the event of a fault on either hot or neutral line.

Now if the voltage on neutral was "supposed’ to be 0, why go through the expense of insulating? Why be afraid to “touch it”. Because the current can kill you, and it can only existing if the voltage is > 0.


They each have their own bar to be tied to.

That’s good. Could be from contact resistance meter tips to conductor.

I said between neutral and ground.

Only on a fault on the neutral. A fault (breakage) on the hot yields no voltage or current.

Because neutral is a current carring conductor and has to be isolated from ground until load center (breaker box)

Per the code, they can be lugged together ONLY at the first disconnect.
In older houses that have no grounding, lugging them together and installing grounding outlets was done so that you can use 3-prong plugs and cheat a lazy inspector that does not look behind faceplates.

I’m not code official. Just electrical/electronics engineer with a lot of experience (what about you) and was trying to help and explain things.
Promise I won’t do it again for your posts.

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I think we are confusing terminology but saying the same thing. I was only responding to your post that voltage on the neutral was 0 - it is not.

Not an electrician and do not pretend to be one. I am an engineer, civil by license and also practice mechanical I work wth electrical engineers and electricians a lot in my line of work. I was only responding to your NO voltage comment. You can only have NO voltage differential when the there is no load present.

This might help you understand
Link http://www.diychatroom.com/attachments/f18/1961d1199122156-detached-garage-sub-panel-grounding-q-3-wire-feeder-detached.jpg


link http://indexxit.com/wp-content/uploads/2017/11/drop-dead-gorgeous-how-install-subpanel-main-lug-neutral-and-ground-breaker-box-inside-connected-on-same-bus-wire-reversed-difference-in-panel-tied-together-bonding-bar-wires-bonded.jpg

That means nothing in reference to commenting electrical work. If you really want to know, do some reading. And lots of it.

Voltage and voltage differential are used interchangeably.
Two wires bonded together have the same potential. Difference between potentials is called voltage. Once bonded together and grounded properly, voltage between neutral and ground is 0 or very close to it. If it’s high you have a floating ground.
Load has nothing to do with voltage value. You measure 120V AC at a light switch whether or not your light bulb is lit in reference to both neutral or ground.

I am not confusing anything. Common is only used in DC systems. Neutral is used in AC systems.
This is my last reply to this thread.