I thought I was pretty good when it comes to wiring. I thought I knew enough to be safe and have good quality work when I do it on my own. Then I learned about shared neutral lines for different hot phases. I am shocked I never heard of this. No book I ever read mentioned shared neutrals for household wiring. Given that a shared neutral is a possibility, I don't understand how anybody can do electrical work without shutting off power to the entire house. You can test for hot wires and verify at least one circuit is off, but then somebody comes along and turns on a light and suddenly the neutral is live. Not to mention the fact that if somebody came along who didn't know about split neutrals who knows what they may have mistakenly done in the past.
So what is the best way to proceed when trying to determine if a box has any live wires or wires which could become charged if somebody turns on a light upstream? Are shared neutrals avoided where possible? An electrician is about to rewire just about my entire house. Should I request no shared neutrals or is this a silly request? Thanks.
If you share the neutrals within the circuit to a breaker, then when that breaker is off no one is turning anything on, right? I wouldn't share neutrals between different breakers.
That is not really a concern; you can't get a shock off a neutral unless it is unconnected at the breaker box and then you have much bigger problems. They reduce voltage drop, save a little material, reduce congestion in the breaker box and maybe save a little labor. The big downside as I see it is that they can be confusing to people who don't know what they are. My house had one with both hots on the same leg. Didn't matter since not much was plugged into them, but it could have been a disaster. Also, I doubt you can use AFCI breakers, if that matters to you.
Good point. I did read something about a double pole breaker. That IS what I should have in my box. So either I don't have one or the neutrals that I was disconnecting actually weren't part of my intended circuit at all. There were a lot of wires in that box, perhaps another group was actually the neutral for my circuit. The wiring is 50 years old and it's hard to tell what color these wires were supposed to be!
The neutral is a current-carrying conductor under normal operation. Contrary to common belief, electricity does not follow "the path of least resistance." Rather, electricity follows all possible paths, and when you touch the neutral, you create with your body a second, parallel path to ground for the current flowing in the neutral. Granted, that's probably (though not necessarily) a fairly high-resistance path, which makes it *unlikely* that you will be shocked.
But definitely *not* impossible.
Your statement is false twice, actually: suppose the neutral is connected just fine *at* the breaker box, but is disconnected somewhere between there, and where you're working. In that case, you're putting yourself in *series* with the neutral current, not in parallel with it, and that makes a shock from touching it *likely* (if the circuit is energized).
This is more of a "independent feeds into a single box" than a shared neutral issue per-se.
In a properly wired shared neutral circuit, getting zapped by the neutral can't happen - the neutral cannot be more than a volt or two away from ground.
If you disconnect the neutral, then someone fires up a lamp on the other side, yes, you can get zapped.
But:
1) Electrical code requires that neutrals on shared neutral circuits are pigtailed, so that it's possible to disconnect devices without breaking the neutral to downstreams.
2) There's a very strong hint that the neutral might be shared if you see 4 conductor wire (eg: 14/3 or 12/3 plus ground). Electrical code (at least ours) requires that, at least until the hots go off in different directions, that it's in a common cable. Since you can't parallel cable in normal residential wiring (eg: rejoin neutrals after splitting cable), at least theoretically, you'll always see /3 cable where the neutral is actually shared. Past the split point, it don't matter.
3) Because of (2), and electrical code requirements for common trip ("same strap" _requires_ common trip in the NEC, all shared neutrals in the CEC require common breaker trip), you'll almost always see a single cable connected to adjacent common-trip breakers You can't deenergize one hot without de-energizing the other.
Actually, until our (CEC) kitchen counter requirements changed a few years ago, at least two shared neutral circuits were mandatory in every house.
I wouldn't worry about it. Most of the concerns about neutral stupidities regarding what you've read is for industrial wiring botches, which is largely inapplicable to housing.
The electrician probably won't be tempted to use it much anyway.
What you _should_ do is insist that the electrician use tie-barred/common trip breakers everywhere he'd consider using common neutral (or two feeds into the same box - even a switch box), regardless of whether the NEC requires it for that specific instance.
The only times I've ever been zapped is where the electrician violated the CEC rule about having non-common-trip feeds into the same box. They weren't shared neutrals.
If the neutral isn't broken between you and the panel, the _maximum_ ground-neutral voltage you'll see in the OP's scenario is a volt or two. You won't notice it anymore than you'll notice a shock from a D cell.
If the neutral is cut between you and the panel, for all intents and purposes it's a hot if anything is switched on with the corresponding "real" hot.
The dangers with common neutral arise when the neutral is broken without killing both hots. That's impossible in most code-compliant situations (unless you do your rewiring on hot circuits).
[In the CEC, it's _all_ code-compliant residential situations. The NEC has an exception for circuits "not on the same strap"]
Gee Doug, with all the millions of dryers and ovens that have the chassis attached to the neutrals and are touched daily by wet hands, how many shocks do you think people get? What the hell is wrong with you?
Shutting off power to the entire house is not a guarantee that the neutral won't be hot. Under rare circumstances if your neighbor lost his neutral connection it is possible that his return current would travel through the earth to your neutral connection and then back to the transformer.
You can test for hot wires and verify at least one
Always treat the wires as though they are hot. Every once in a while I get zapped by a neutral. Even though the ground and neutral are bonded together at the main you don't know what the condition of the neutral going back to the transformer is. You also don't know how good the neutral connections are at the panel. There is always the possibility of having a difference in potential between the ground and neutral at the load. In an old house it is a good idea to be extra cautious as you don't know what changes have been made over the years.
Some states are requiring arc fault circuit interupter circuit breakers for bedroom wiring. You cannot use a shared neutral on these circuits.
It is not a bad idea to discusss with him what his plans are. Afterall you are paying for it, you have a right to ask.
Coming off of the power company's transformer to a single family residence are two hot conductors and a neutral conductor.
The neutral conductor is the return path for the current to go back to the transformer from the two hot conductors.
The neutral conductor is bonded to earth via the water pipe and ground rods.
If you were to disconnect the neutral conductor the current will need to find another path back to the transformer.
It can go through the earth via the ground rods and water pipe directly to the transformer ground rod and grounding conductor.
However, depending on the location of the transformer and the neighbor's house (Or the quality of the transformer ground) the current might find a better path back to the transformer by going through the neighbor's water pipe and ground rods into the neighbor's electrical panel and then continuing through the neighbor's neutral conductor back to the transformer.
I have shared neutrals in my box, done by the original builder on separate breakers. I know that doesn't make it right. BTW, the house is 35 years old. Also, in my local village hall building, they have 3 phase shared neutrals (208 Y connected) and all are separate breakers. So I don't think there is a requirement for common trip.
That depends entirely on how well you are (or aren't) grounded. Under normal circumstances, there won't be enough current to do any damage. One sweaty hand on a grounded junction box, and the other sweaty hand touching an active neutral, though...
and you might have 5% as much conductivity as the neutral and see 6v. Not likely but possible. Harmless, but possible. Stupid, but possible. Doug, but absurd. Oh, that is redundant.
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