Neutral: To Share, or Not to Share

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Old 09-26-06, 06:27 PM
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Neutral: To Share, or Not to Share

I've heard there are certain times you can't share a neutral. Can you give me a brief example of when you can, and when you can't. Thanks in advance.
 
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Old 09-26-06, 07:00 PM
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opposing legs (or phases if you wish) generally can share a neutral. 2 hots to 1 neut. (with single phase power, such as in a house)

You do not put two of the same leg on one neutral. Although it will work, the current from the hots will be additive rather than subtractive so using 2 20 amp circuits, you could have 40 amps or more coming back on the neutral. Not good.

Did you have something particular in mind.


Sharing neutral makes a multi wire branch circuit. I utilzie them in my work often. I often work with 3 phase electric so I can share 3 hots to neut. Reason for doing this? Today I saved about 600 feet of wire by shareing the neut instead of running 3, I used 1. That 600 feet (savings) translates to around $90 of wire.
 
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Old 09-26-06, 09:18 PM
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First, thanks for the info. Next, was just curious about sharing neutrals, so I wouldn't accidently do it when I wasn't supposed to. If I used a multi-wire branch circuit, using, for example, 12/3w/g Romex for the first part of the circuit, before using a junction box to split off into two 12/2w/g cables, going to separate receptacles,my house is SOOOOOO small, that I wouldn't see any substatial savings by doing this. Now, however, the benefit of using a multi-wire circuit, as I see it, would be that you wouldn't have so many cables running through your top plate, or bottom plate, AND, you wouldn't have so many wires, and thus, connections in the main breaker box. Does that sound correct, or reasonable?
 
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Old 09-27-06, 04:35 AM
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You are correct, with a multi-wire circuit you have less cables entering/leaving the main breaker box and two less wires to connect per multi-wire circuit( one ground and one neutral as opposed to two of each).

However, these benefits pale in comparison to the potential danger if someone incorrectly connects the two hot wires to the same leg of the incoming 240 volts instead of to the opposite legs.

I don't recommend a multi-wire circuit in a residence unless there is a specific need for one, or at least a very good reason for using one.
 
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Old 09-27-06, 06:43 AM
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All of my new wiring is multi-wire. The benefits, as already identified, was less wire, less cost, and less space; I could run more wires/circuits thru the conduit using multi-wire then regular two-wire.

The hazard is putting the two hots on the same phase, or not turning off both breakers (both hots) on the shared circuit if you have to work on one or the other.

I'm in the process of preparing a very detailed list of what circuits are shared and what phases they are on, so anyone who opened the panel door will know exactly how it's set up....my memory isn't the greatest, and the next owner will have no clue what is shared and what isn't, so this list will take care of that.
 
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Old 09-27-06, 06:55 AM
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fuente, if the next owner has no idea what a multi-wire circuit is, they won't know to turn off both breakers at the same time. An electrician will understand, but the average homeowner will not.

Not only do you need to identify each breaker pair, you need to post a brief explanation of why it is necessary to turn off more than one breaker when working on the circuit.
 
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Old 09-27-06, 07:51 AM
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you're right....and that is exactly what will be posted on the inside of the panel.
 
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Old 09-27-06, 08:11 AM
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In general, multi-wire circuits are incompatible with both GFCI and (moreso) AFCI protection. You cannot have a shared neutral on the LOAD side of either a GFCI or AFCI device. Since AFCI protection is now required for all circuits which serve bedroom outlets, that means you cannot use multi-wire for any circuit which serves a light, receptacle or smoke detector in a bedroom.

There are some additional rules to be aware of with multiwire circuits that make them more troublesome than they're worth in most situations. For example, you cannot use the feedthrough capability of a receptacle for a multi-wire circuit neutral; you must wirenut and pigtail. Also, a multi-wire circuit which has both legs terminating on any one yoke must use a double pole breaker. You also must ensure that the legs of the MWC are opposite, which can be difficult to maintain if you ever have to move breakers around in the panel. Like racraft said, residential multi-wire circuits are generally a bad idea unless you have a specific need for one.
 
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Old 09-27-06, 08:33 AM
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Could you please let us know why you can't use the feedthru on the receptacles, and also what the consequence of this is? Also, if you wire a downstream GFI receptacle from the LOAD side of another GFI receptacle, what will happen?

Thanks.
 
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Old 09-27-06, 08:54 AM
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For a multiwire circuit you are not allowed to feed the neutral through a device. Instead the neutral must be pigtailed. This is because the neutral is very important. If the neutral gets disconnected then you could (depending on the circumstances) end up with 240 volts on your 120 volt devices, which would generally cause them to fail and possibly burn up.

You also asked about feeding a GFCI from the load side of a GFCI. This is not related to the discussion at hand, but I will answer it. The problem with feeding a GFCI from the load side of a GFCI is that a ground fault after the second GFCI will trip either the first GFCI, the second GFCI, or both of them. You might not know that there are two GFCIs and might not check them both. It's just confusing.

I think what you meant to ask was in regards to GFCIs on a multiwire circuit. If you want to install a GFCI receptacle on a multiwire circuit, then you cannot use the LOAD terminals on the GFCI to continue the multi-wire circuit. This is because the GFCI will see a current imbalance under normal operation after the GFCI and trip.

What you can do is split the multiwire circuit into two regular circuits, and use two GFCI, one for each of these circuits. Or you can use a 240 volt GFCI breaker. Or you can use a GFCI receptacle at each location and only use the LINE side of the GFCIs. That can add up quickly. for a kitchen where each counter top receptacle must have GFCI protection.
 
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Old 09-27-06, 09:02 AM
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got it bob. Thanks. I've got a multi-wire circuit, where one hot is dedicated to a bathroom. The second GFI receptacle is wired from the LOAD side of the first. So I'll just wire the second using a pigtail.

Thanks again.
 
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Old 09-27-06, 09:15 AM
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> Could you please let us know why you can't use the feedthru
> on the receptacles, and also what the consequence of this is?

Bob covered the consequences, here's the code that says you can't feedthrough:

300.13(B) Device Removal. In multiwire branch circuits, the grounded conductor [neutral] shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.
 
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Old 09-27-06, 09:18 AM
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thanks Ben !
 
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Old 09-27-06, 12:29 PM
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Cutler Hammer manufacturers 2 pole AFCIs and also markets a device that is dual rated, that is, rated as both a GFCI and AFCI. While all the AFCIs I have checked out have a GFCI as part of their circutry it isn't rated for GFCI protection because it trips at around 20-25 MA rather than the 5 MA a GFCI rating requires.

While I respect the opinions of the moderator I strongly disagree that multiwire circuits should be avoided in residential work. Where they can be used they save a substantial amount of wire - 4 wires in one cable versus 6 in 2. With copper prices out of sight and aluminum going up rapidly as a result all resonable savings need to be examined.
 
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Old 09-27-06, 01:10 PM
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I agree about the multi wire circuits in residential - Technically, anything electrical in my jurisdiction must have a permit, any new work that is - whether the homeowner understands any of the wiring in his/her home is really not our concern when working on a bid price.

not to mention fire and whether of not the ins. co will pay if faulty wiring is the source

Faulty wiring by an elecrician that results in death by fire or electrocution will carry a prison sentence
 
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