Split Neutral Wiring

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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.
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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.
Brian
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When done properly, the two breakers would be tied together, so either both would be on or off.
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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!
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Noozer wrote:

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.
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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.
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Or GFCIs for that matter, unless you use a double pole GFCI.
nate
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FALSE!
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).
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Doug Miller (alphageek at milmac dot com)
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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"]
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Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
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(Chris Lewis) wrote:

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...
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Doug Miller (alphageek at milmac dot com)
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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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Pardon me if I don't simply take your word for it. Got any calculations to back that up?
I didn't think so...

How much does it take to kill, Wade?
Hint: the trip threshold on a GFCI is 20 mA.
When are you going to figure out that neutral and ground are NOT the same?
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Doug Miller (alphageek at milmac dot com)
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Doug I hope you have all these responses canned in word or something. Seems we just had this thread about a month ago. <G>
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Steve Barker




"Doug Miller" < snipped-for-privacy@milmac.com> wrote in message
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Doug Miller wrote:

In the USA the trip threshold on a class A GFCI is five milliamperes. On a class B GFCI such as the ones built into AFCIs the trip level is thirty milliamperes. Class B GFCIs are only intended to protect equipment. -- Tom Horne
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On Apr 13, 6:36 pm, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I discovered a shared neutral in my old house by working in a box that _should_ have been safe. Found the shared circuit is a yardlight on a 3 way switch in the back yard. I still haven't figured out how to correct it without running new wires underground. They only used 12/2 WG for the wiring.
Harry K
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I'll repeat what I said. In those circumstances, the voltage between neutral and ground is at most a volt or two, dependent on IR voltage drop on the neutral due to the load. It doesn't matter how sweaty you are, you're not going to feel it. How sweaty do you have to be to feel 3V off two AA batteries?
The minimum "feel voltage" is at least 10V, and usually considerably higher. If the neutral was connected to ground yet was still showing 10V or more relative to the ground, it'd probably be glowing red hot.
Now, if you skewered yourself in the heart muscle with the ground and neutral, you'd feel it. But people tend to avoid that.
The hazard with shared neutrals is contacting a disconnected-from-the-main neutral without having all the hots shut off.
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Chris Lewis,

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(Chris Lewis) wrote:

Assuming all the connections in the neutral wire between that point and the service entrance are good.... no imperfectly-formed splices, no high-resistance connections due to corrosion, or improper Al-Cu splices... nothing of that sort...
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Doug Miller (alphageek at milmac dot com)
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wrote:

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?
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On Sat, 14 Apr 2007 01:33:12 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I'm sorry Doug, but my neutrals are tied to ground in the breaker box. Same was true of the last 2 houses I owned.
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wrote:

The service entrance is the *only* place where they *are* the same.
In branch circuits, the ground conductor does *not* carry current in normal operation. It carries current only if there has been a failure somewhere: either the hot or the neutral has faulted to ground somehow.
By contrast, the neutral *is* a current-carrying conductor under normal operation. And that's why you can't presume that it's safe to touch.
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Doug Miller (alphageek at milmac dot com)
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