Residental mains wiring questions (USA)



Sure. I would about always use separate neutrals or multiwire (Edison) branch circuits. With a multiwire you need a common disconnect, which could be a handle tie or 2-pole breaker.
Group (tape, tie wrap, ...) the wires for each circuit at each end of the conduit.
If you use either GFCI or AFCI breakers you have to have separate neutrals on that circuit connecting to the breaker.
Not obvious whether AFCIs will be required (ask the inspector), but with wiring as old as ungrounded you might want them anyway. For new wiring they are required for 15/20A 120V circuits that, in general, are not required to be GFCI protected (210.12).
-- bud--
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Yes, I was responding to the common neutral idea. Bear in mind if you are running separate neutrals each one adds to the wire count for derating so to avoid it you are in 3 pipes for your installation from what I see. That is the right way to do it although I am not really sure what the violation would be if you did use a common neutral. Another option is to double up your circuits in a multiwire (edison, shared neutral or whatever they call it in your area). That would require double pole breakers but you reduce the neutrals by half and cut the voltage drop in that section of wire. The down side is if one trips you lose 2.
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On Jul 1, 10:55 am, snipped-for-privacy@aol.com wrote:

I was surprised there seems to be no definition of a multiwire branch circuit - not in 100 or 210.
Suppose you run a common neutral as the OP originally proposed - say A- B-A-B-N.
Is that a multiwire branch circuit? Seems to be. Does it have to have a simultaneous disconnect of all hot wires (210.4- B)? Would it then have to use a 4-pole breaker or a listed 4-breaker handle tie? Does that then mean you can't do a common neutral as proposed?
I never thought about it until this thread.
-- bud--
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Bud Does the current version of 210.4B still read "(B) Dwelling Units. In dwelling units, a multiwire branch circuit supplying more than one device or equipment on the same yoke shall be provided with a means to disconnect simultaneously all ungrounded conductors at the panelboard where the branch circuit originated."? I only have the 2002 handbook here at home. -- Tom Horne
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Tom Horne wrote:

That was moved to 210.7-B - didn't look up when - and now applies everywhere, not just dwelling units. (Looks like you can still do multiple circuits to a common yoke if they are separate branch circuits with separate neutrals.)
In the 2008 NEC the requirement was extended to *all* multiwire branch circuits. "210.4 Multiwire branch circuits. (B) Disconnecting means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at a point where the branch circuit originates."
The rationale was the hazard of 'hot neutrals' if you just disconnect the branch circuit you are working on.
(IMHO this essentially kills the use of multiwire branch circuits, which as you have likely seen, were extremely common - like a 3-phase 4-wire set.)
--
bud--

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wrote:

Why would it stop people from using multiwire circuits? You just need handle ties or common trip breakers. Note the 2008 also requires these conductors to be grouped where they enter the panel enclosure
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On Jul 1, 4:22 pm, snipped-for-privacy@aol.com wrote:

Entirely true.
But I would guess a whole lot of installations won't want to kill 3 circuits to work on one of them. What came to mind when I first read that the code had changed was a hospital where the vast majority of 120 and 277V circuits were 3-phase 4-wire sets. If they were building it now, I don't see much possibility they would use a multiwire for most of them.
Also taking a toll is the requirement for residential that most of the circuits need AFCI protection. Unless you use an expensive 2 pole AFCI (if they exist) you can't use common neutrals.

I liked that change.
-- bud--
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wrote:

Article 100 Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

Interesting catch ... Kudos. A half dozen inspectors missed that in a conversation we had about the same thing. I agree.
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On Jul 1, 4:14 pm, snipped-for-privacy@aol.com wrote:

Damn - I looked in 100 for "multiwire branch circuit". I need the NEC on a searchable CD (which the NFPA would be happy to sell me).
-- bud--
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wrote:

184 bucks for my handbook ;-(
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Bob E. wrote:

See if you can use Ground Fault Interrupters in the feeds to these circuits. Put an 1110 box or something similar on the new panel board and use a 'dead front' GFCI in it.They're cheaper than GFCI breakers. If allowed, it will save you having to run ground wires.
Then, feed the old circuit from there. I used to do that on rewires to ungrounded circuits. It saved trying to fish in ground wires. Your local code may be different. That wiring on the roof scares me. Any way to run the stuff under the floor? Crawl space, basement?

There are three things here. One, the temperature around the pipe. Two, the number of wires in the pipe and three, the allowed pipe fill percentage. Find a web page that has the Electrical Code Tables and look up table 310-15xxxx or somewhere around there.
Your derating *may* be as high as 50 percent, I'm guessing. So, a 15 amp wire, derated by 50 percent is now 15 / 0.5 = 30 amps. Then divide by the temperature derating percentage, if applicable. Your 14 AWG has become at least a 10 AWG.
Going up in pipe size does NOT allow you cut back on the wire size.
See where this going? Once you get all the wires sized, add up the total of the cross section areas. Divide that by the allowed pipe fill, probably 40 percent (0.4 division).
That gives you the MINIMUM allowed pipe size. If it doesn't equal a regular size, you have to go UP to the next one. If this adds up to a three or four inch pipe, you will have your hands full and wallet empty.
Perhaps consider a new sub panel with only a feed to it. The costs, even with ground fault breakers, would be less than the above stuff and take many hours less to do.
Good luck, regardless.

I think I know her sister...

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a 15 amp wire (there is no such thing, you are referring to a 14 AWG wire), derated by 50% would be: 15 * 0.5, or 7.5 amps max. Not 30 amps!

Again, totally flawed math. The 14 AWG would be (roughly) the equal to 18 AWG. Not 10 AWG!
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Nope. The "ampacity" is derated 50% 14ga wire is rated at 25a (in the 90c column) half of that is 12.5 so it can't be used at 50% derating. That bumps you up to 12ga, 30a in the 90c, half if that is 15 and off you go. Same is true in your 20a circuit. You bump up to 10ga, 40a in the 90c, half is 20 and off you go.
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PeterD wrote:

I guess I should have been exact to the minutest extreme, but I don't feel the original poster was mislead.
If you have a wire that is allowed to carry a specific current and you have to keep feeding the same load, you don't derate the existing wire. You find the next size up, that, when derated, allows you to maintain the present load.
If your new wire is capable of 30 amperes, when derated because of pipe fill, will become a 15 amp capacity wire, hence my phrase "Your 14 AWG has become at least a 10 AWG." That means your new conductor in the pipe has to be a 10 guage. Then it connects to the existing 14 guage in the junction box.
A capacity of 15 ampere flow, when derated by 50 percent means that 15/.5 = a 30 amp needed capacity in the NEW wire before it is derated.

Again, we're NOT derating the existing wire. We are derating a HIGHER guage wire, in this case a 10 AWG, to give us the same ampacity that we had before stuffing these wires in a conduit.
You have to go with the existing current requirements divided by the derating factor, then use THAT current capacity to select your new wire size.
I'm sorry if I have said this in a confusing manner.
mike
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Just remember we are using the 90c column of 310.15 not 240.4(D) so a 12 guage wire is OK for 30a when you are derating.
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Wire ampacity is not derated due to conduit fill. The derating for operating temperature is usually minor. (pretty much for small conductors #6 AWG and smaller)

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JosephKK wrote:

The number of wires in the pipe determine what percentage of their rated current they can carry. You are referring to the percentage of cross sectional area being used in the pipe. That, does NOT have any bearing on the case.
I should have been more careful in my wording.
mike
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Huh? Your outlet wires go over the roof? Uh, no you don't take seperate runs for the ground wire, all need to go together. You mention 'passing inspection'. That won't...

Yes, they do.

You can get and use conduit as large as you want.

Consider that the main breaker must be within a given distance of where the service entry wire comes into the building. A long run of SE to a 'remote' box will likely also raise some questions with the inspector.

I'd strongly recommend a session with the building inspector in your area, describe what you want to do, and ask him/her if that will pass. If they say "no", then look for other alternatives. If they say it is "OK", ask if they have any suggestions or things to watch for.
Their advice, without any doubt, will be worth more than all the advice you will ever get on the Internet.
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Cite that please
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Zinsco is one of the wonders of the electrical industry.
You may be able to make a cover made for the old box if that would make the installation easier - a question for the inspector. That is what I would try to do if the enclosure is in good condition.

Existing ungrounded wiring can be grounded by adding a ground wire which does not have to be run with the power wires. It is in 250.130, which also details where the added ground wire is to be connected at a 'source'. I believe the ground wires does not necessarily have to connect through boxes on the way back to the grounding 'source' , but the connections probably have to remain accessible.
Over the roof? Doesn't sound like a good idea, but minimal information has been given. I might try running a #4 bare copper ground wire, which is relatively immune from abuse, or a ground wire in PVC. Could use one ground wire for all the receptacle boxes you are adding grounds to. Not obvious now the wires over the roof connect to the receptacle boxes.
Another thing to clear with the inspector.

For existing ungrounded circuits see 250.130.

I would split into multiple conduits so the derating is reasonable.
Note that if you are using #12 THHN wire, the table ampacity (310.16) is 30A. If derated to 70% the allowable ampacity is 21A. (The wire can not be used at over 20A.)
For #14 THHN the table ampacity is 25A. If derated to 70% the allowable ampacity is 17.5A (and can only be used at 15A).
If some of this is over the roof the wire would, I believe, be a wet rating and THHN would use the THWN rating that all that wire (that I have seen) also has. THWN wire has different table ampacities.
Derating is in 310.15.
If wiring is going above the roof you will likely have to derate it for the higher temperature it will be at.

I certainly agree. I see lots of questions.
-- bud--
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