Shared Neutral Question


I have a 3 gang switch box with 4 switches in it all running off the same breaker. 2 - 3 ways and 1 double single pole. I want to take one three way and the double single pole off the circuit that is on the original breaker and put them on another breaker. Is it acceptable to just pull romex to the box and only use the "Black" and the ground wire and leave the existing neutrals all hooked together in the box, not using the neutral in the new cable? They are all tied together now in the box. I know it would work, but is it OK to do so? If not why?
Thanks,
LJ
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You are creating an old "Edison circuit" when the neutral is common. It "might still be legal" if the two hots are on the same 220 breaker or two singles tied with a pin if they still make those.
If it were mine I would make sure both breakers were on the same phase and connect it in the proper manner. I hate finding 220 in a switch box.
There are at least three sharp people in the know who hang around here. Better wait for them to respond.
Colbyt
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Old_Boat wrote:

I will call it on a technicality.
All the current carrying conductors (in a circuit) must pass thru the same knockout in a box/panel, etc.
The intent was to minimize eddy currents in metallic boxes.
Is it significant on a 15Amp circuit? Probably not, but there it is.
Is there a rule about all conductors being in the same cable sheath as well? Can't recall.
Jim
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Speedy Jim wrote:

I think so, re: the cables. I think the intent there is not to cause inductance heating in non-current-carrying conductors by having all the currents within the cable sheath sum to zero.
nate
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No, it's not, for multiple reasons.
(1). Code requires that all conductors of any circuit must be in the same cable, conduit, raceway, etc. You'd have your hot and neutral conductors in two separate cables.
(2). When entering (or leaving) a metal box, the hot and neutral conductors of a circuit must pass through the same hole to avoid any possibility of heating the metal frame through hysteresis. There's some chance -- perhaps only slight, perhaps much more than slight -- of heating the box enough to start a fire.
(3). If the hot conductors of the two circuits are on the same leg of the 240V service, you have the potential of overloading the neutral conductor and starting a fire. [This danger can be easily averted by using one double-pole breaker, but you do need to be aware of the need to do that.]
(4). It would not be immediately obvious to the next person who comes along that the neutral wire is shared by two circuits. Suppose that guy wants to replace a light fixture, a pretty common task. He's going to de-energize the circuit that the light fixture is on, but not the other one. And if there's any load on the other one, there's current in the neutral _even_if_ the neutral is still continuous back to the service entrance. It's not good to find current in wires that you thought were dead.
(5). Now suppose the next guy to come along is replacing (or adding) a switch in that box, and opens the neutral there... never knowing that there's a second circuit in the box, an energized circuit -- with a live neutral that now isn't connected to the service panel. He touches it. It grounds through him. You receive an unpleasant phone call from his widow's attorney.
(6). It's confusing. Anything that might make subsequent homeowners, or the electricians they hire, look at it and say "Huh? WTF?" is a Bad Idea. If you do this, five years from now the guy who bought your house is going to be posting here, describing this setup, and asking us to help him figure out what's what. If he's lucky, one of us will remember your post, and ask him if he happened to buy a house from a guy named Old Boat.
(7). It's f**king pointless!! You're running a new cable ANYWAY -- you have the neutral wire there ANYWAY -- why on earth would you want to not use it? How much time do you save by not opening up the existing wire-nutted connection, removing one wire, and re-nutting it? Two minutes? Three? Is it really worth (1) through (6) above to save two or three minutes?
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Regards,
Doug Miller (alphageek at milmac dot com)
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wrote:

OK OK your all right. It was just a theoretical question. There are so many wires in the box now that a couple of more wire nuts would really crowd the box. Originally crimp connectors with one wire coming out the end of the crimp connector for the connection was used. Will do it correctly when the time comes to split the circuit. On another note, I got the idea from all the wiring in this house that was done using 3 wire w/Ground cable that carried two 15 amp 115V circuits using the black for one circuit and the red for the other and the white neutral for both. Don't really like them, but it has worked for 55 years so I guess it must have been an acceptable way to do it back then.
LJ
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Not just "acceptable", and not just "back then" -- that is the right way to do it, and it always has been.
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Regards,
Doug Miller (alphageek at milmac dot com)
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wrote:

I think you've gotten very good answers, so I'm going to tack on my related question.
I'm installing a new burglar alarm that has the option of turning on my hall light and my dining room light when I enter the house, and when I am leaving. For two minutes or so.
I gather from what you've said that I can't just use one hot wire to each, is that right? Would it have to be something like Romex, which is legal here?
The obvious way to do this is to send a hot wire of the same phase from a relay connected to the burglar alarm up to the boxes where the light switches are for each light. (Easier to get to the switches than the lights themselves, because the lights are in the ceiling.)
Should I use 14-2 Romex and connect a second neutral wire to where the first neutral wire is connected? I really will have no way of knowing which neutral the electricity is taking back to the breaker box, except to guess that about half will use each neutral, regardless of whether the light switch is on, or the burglar alarm is powering it.
Should I use a double pole relay to toggle the neutral wire too?** There is no easy way to toggle the neutral that is there now.
**I actually have a 6 pole relay, but I've connected all 6 poles together to use for the hot wire. I would need to use a second relay for the neutral if you tell me it is necessary.
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