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Old 01-08-2007, 09:32 AM   #1
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One neutral for two circuits??


Can someone please refresh my memory on this one, I want to make sure I have it right. I would like to add (2) twenty amp circuits to my basement for shop tools. It is permissible to run a 12-3 with ground (black, red, white, ground) to run both circuits? If the circuits are on different phases, will one neutral suffice for both circuits?

Tron

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Old 01-08-2007, 10:47 AM   #2
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One neutral for two circuits??


TronCarter,
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If the circuits are on different phases, will one neutral suffice for both circuits?
Yes, one neutral will suffice as in this configuration the neutral conductor only carries the difference current.

Other than this type of dedicated wiring, never borrow or share a neutral.

...Christopher

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Old 01-08-2007, 12:32 PM   #3
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One neutral for two circuits??


I personally won't jump to this complication for the sake of saving some costs on the wires... I would build 2 separate circuit on two set of wires....
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Old 01-08-2007, 12:54 PM   #4
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One neutral for two circuits??


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TronCarter,
Yes, one neutral will suffice as in this configuration the neutral conductor only carries the difference current.
So, if I have a 15 amp load on one circuit, and a 15 amp load on the other circuit (30 amps total), it is still ok? If I understand correctly, it is because when one phase it at a peak, the other is at a trough (valley). However, two circuits on the same phase would be peaking at the same time?
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Old 01-08-2007, 01:00 PM   #5
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One neutral for two circuits??


Builders use this code approved circuit configuration for dishwashers and garbage disposers in thousands if not millions of homes.

...Christopher
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Old 01-08-2007, 01:09 PM   #6
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One neutral for two circuits??


TronCarter,

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So, if I have a 15 amp load on one circuit, and a 15 amp load on the other circuit (30 amps total), it is still ok?
In your previously defined circuit configuration, the neutral current in the case of two 15A loads is 0 (zero) Amps. The two loads are electrically in series powered from 240 Volts. The neutral conductor defines a 120 Volt center point.

It is always ok provided the line wires connect to opposite sides (L1 & L2) of the electrical service.

...Christopher
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Old 01-08-2007, 06:02 PM   #7
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One neutral for two circuits??


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I personally won't jump to this complication for the sake of saving some costs on the wires... I would build 2 separate circuit on two set of wires....
I'm not sure why so many folks think this is "complicated" or unsafe.
Like Christopher said, multi-wires are used in almost every home built. There is nothing complicated about it, just some rules to follow.

Also, there are more reasons it is done than the knee-jerk reaction "The electrician wanted to save money."
Multi-wire circuits save wire, save room in the panel, lessen voltage drop, lessen cable fill in bored holes, and more....
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Old 01-08-2007, 09:03 PM   #8
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One neutral for two circuits??


TRon -- I too dislike multi-wire circuits, regardless of how wonderfully cheap they may be. Article 210.4 NEC covers the rules but doesn't elaborate on the dangers. The NEC handbook covers the dangers in gory detail.

For example, if you loose continuity in the neutral of a multi-wire, all hell breaks loose. Depending on their position with respect to the break in the neutral, some loads will find themselves in series in a 240 V circuit. Then when you turn on the toaster the light bulbs dim, and if you turn off the toaster and something else that's normally on (the TV) the light bulbs explode. Motors are burned out this way regularly. Draw yourself some pictures and you can see how screwy this can get.

So if you're determined to do it this way, you need to avoid loose neutrals. One thing that will help is to always pig-tail them. Ie., cut the neutral after it enters the j-box. Cut a short wire (pig-tail) that goes to the neutral screw on the outlet, and then wire nut both of those together along with and neutrals that exit the j-box on their way elsewhere. This way, if you remove the outlet for some reason (or if the wire just happens to fall off the screw), at least the rest of the circuit won't have things exploding. I would think that if you want to split the upper half and lower half of the receptacle between the two circuits, you could remove the connection tab between top and bottom on the hot side only, but leave it on the neutral (silver) side.

Also, if you really want to run these sort of circuits, I would use a ganged breaker-pair so that you can't disable only one leg. If you don't do that, and leave say the black leg hot while you're working on the red leg, you're going to have current flowing in the neutral while you work on it and will have to deal with some nasty white arcs. The ganged breaker is actually required in some situations (ie if you want to run line-to-line 240 load off the circuit).

Another whole question for you -- is the basement finished? If not, the outlets must be GFI protected. And I can't see any way GFI will work unless the neutral is paired with one and only one hot conductor. If it's a cement floor that gets at all damp, GFI is a very good idea even if not required.

Speedy -- I agree with and understand all you said except voltage drop. How does multi-wire decrease voltage drop? Because there's less current in the neutral? Seems like the wire length is the same in either case, so what's different? Just curious.

I hope this helps (and is reasonably correct).

Phil S.
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Old 01-09-2007, 06:00 AM   #9
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One neutral for two circuits??


Because voltage drop is greatly lessened in 240v circuits compared to 120v. So the more balanced a multi-wire circuit is the less the voltage drop will be.
Where if you had two 2-wire circuits and had a load on each the drop would be much more sever on BOTH circuits.
Understand?

I do understand your points, but even in this example you automatically first go to the "cheap" argument. This IMO is not at the top of the list when considering multi-wires.
Also, all the things you mentioned are almost a non-issue of your installation is quality, done right and to code from the start.
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Old 01-09-2007, 03:37 PM   #10
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One neutral for two circuits??


Gentleman,
Using this: http://www.csgnetwork.com/voltagedropcalc.html voltage drop calculator I found that VD % remains exactly the same on a MWBC given balanced loads.

When calculating VD we need to consider each Hot of the MWBC seperate anyway. The wire would need to be sized for the higher amp load if there is one.

I take the side against using these. I know that the service and feeders are MWBC, but I prefer not to use them for branch circuits. I have seen too many things go *POOF* to be comfortable with them.

Just my opinion. I do not get in heated debates about someone else wanting to use them. They are legal per code, and CMP is the authority that gets to make these decisions. Not me.
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Old 01-10-2007, 07:59 AM   #11
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One neutral for two circuits??


Speedy -- Sorry, but I still don't get it. With a balanced load, all the current is flowing through the two hot legs - zero through the neutral. With a completely unbalanced load, the current flows through one hot and the neutral. Seems to me these are the same length current paths and same current level, just different (same size) wires. So why would the voltage drop be any different? But I have to admit, AC theory often surprises me and I want to learn.

As for the quality issue, I agree in part. If people do the work right (which IMO means better than code -- don't use the back-stabs or whatever they're called on the outlets, pigtail, do wire nuts correctly, etc. etc. ) there won't be problems. But it seemed to me this is a DIY forum so quality isn't a given and helping folks do things in a way that decreases their chances of messing up seemed like a good idea. But, hey, I'm brand-new here so I'll happily defer to others.

Maybe my main reason for hating multi-wire so much is that I'm involved in a lot of renewable energy work and multi-wire obviously won't work with only one inverter. Home-owners here get a rude "shock" when they discover they have to either rewire their concrete house or shell out $2-3k for a second inverter so they can have 240/120 and all their lighting circuits work.

Good discussion - thanks!

Phil S.
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Old 01-12-2007, 08:38 PM   #12
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One neutral for two circuits??


Its the load not the circuit so to speak. A true 240 volt load will see the same amount of voltage drop in volts, but as a percent it is half that of the 120 volt load.
At least thats what I was told. Someone please correct if wrong.
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Old 01-13-2007, 12:59 PM   #13
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One neutral for two circuits??


Here's a reasonable explanation of voltage drop:

http://www.mikeholt.com/mojonewsarch...s~20030326.htm

Briefly, it's just IxR (current times resistance of the wire) - resistance depends on the wire gauge. For single phase the distance is the same reagrdless of whether it's 120, 240, or multi-wire 120: simply source to load back to source. For true multiphase it gets more complicated but that's beyond residential wiring. And for AC we actually need impedance, not resistance, but they're same except for very large gauge wires (or I'd expect if one really fills conduit with multiple circuits).

So- for 240 vs. 110 we have the same distance but half the current so we get half the voltage drop. But just think in terms of IxR and it should stay clear. To reduce the VD we can 1) decrease the distance 2) use heavier gauge wire and/or 3) up the voltage.

HTH

Phil S.
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Old 01-14-2007, 08:21 AM   #14
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One neutral for two circuits??


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Originally Posted by whybminor View Post
So- for 240 vs. 110 we have the same distance but half the current so we get half the voltage drop. But just think in terms of IxR and it should stay clear.
Half the current at half the voltage would be the same VD percent.

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