shared neutral

halmcMarch 23, 2014

Try as I might I can't get straight what a 'shared neutral' is, and a search of this forum doesn't help me much.

In a remodeling project of a very small vacation home, I would like to install a GFI BREAKER for the several outlets that are in the kitchen, but the GFI instructions caution against using the breaker with 'shared neutral'.

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Part of a MWBC.

Here is a link that might be useful: GFCI on Multiwire Branch circuit

    Bookmark   March 23, 2014 at 6:53AM
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Ron Natalie

Unless you are in Canada (and even then it's beginning to die out), you'll not see MWBC (shared neutrals) in kitchen receptacle circuits typically.

The key is that the protected (load terminal side) of a GFCI can not connect to *ANY* other circuit. The neutral (or hot) for that matter can't connect to anything but more receptacles (or other outlets). If they come together with a different circuit or even the same circuit from some point before the GFCI, it will think it is a fault and trip.

Of course, one option if you do have MWBC is to just not use the load terminals at all, i.e., put a GFCI receptacle at each location and don't use the load terminals to protect other receptacles. That way the only thing not the "protected" side is whatever is plugged in to the receptacle.

    Bookmark   March 23, 2014 at 7:59AM
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A shared neutral is when more then one circuit uses the same neutral wire instead of its own wire for each circuit. If you are using a shared neutral the two breakers need to be on a common trip breaker or also known as a 2 pole breaker. You can only share it on two circuits in a house cause there are two phases if it was commercial you might have 3 phase then you could share it on 3 breakers (1 common trip). In your case most likely you don't have a shared neutral. Easiest would be to use a GFI outlet if your worried about it.

    Bookmark   March 23, 2014 at 9:43AM
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Ron Natalie

Actually, MWBC doesn't require a common trip. The requirement is that when the handle is moved to off it disconnects all the ungrounded conductor. A common-trip or two pole breaker will certainly meet this requirement, but so will two single pole breakers with an approved handle tie, even if in the event of an overload only one side opens.

Older versions of the code only require this common disconnecting means if both sides fed the same device yoke.

I bring this up only because it's possible in a compliant house to come across a MWBC without a simultaneous disconnect.

This post was edited by ronnatalie on Sun, Mar 23, 14 at 13:17

    Bookmark   March 23, 2014 at 1:16PM
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Actually the OP said he wanted to use a GFCI breaker, not receps. It is pretty likely that the kitchen WILL be wired with one or more MWBCs, we still do it today as it is one of the few places left we can still use a MWBC due to all the AFCI requirements. We run a MWBC for the two SABCs and another for the DW/DSP and use the other half for the built in micro or the fridge or dining room if we are feeling generous.
If you see red wires coming out of the romex in the panel you probably have some shared neutrals. In this case you can use a two pole GFCI breaker on two circuits that share a neutral, or just use GFCIs in the kitchen like Nick suggested. Once the MWBC arrives at the kitchen it will split into two separate circuits and you can protect downstream receps from that point.

    Bookmark   March 23, 2014 at 1:42PM
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Ron Natalie

If you use a GFCI beaker make sure it is one with a load neutral if you are going to feed an MBWC (or anything with 120V loads).

    Bookmark   March 23, 2014 at 1:55PM
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I probably should not have mentioned the GFCI cuz what I really wanna understand is -- is what a shared neutral circuit is. Maybe it would help to know what a shared neutral ain't. It's embarrassing not to grasp it, because I was a Navy Electronic electronics tech, and for years thereafter held a commercial first class license with broadcast TV and shipboard endorsements.

Reflex klystrons and waveguides, I get but neutral ground -- not yet . . .

    Bookmark   March 23, 2014 at 2:52PM
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Incidentally, it's also confusing to me to hear 'phase' and 'leg' used interchangeably. It is an extraordinarily unusual home -- and not cheap -- that would have anything other than single phase AC energy furnished to it, no?

    Bookmark   March 23, 2014 at 2:56PM
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Yes you have single phase with two legs. The transformer puts out 240 volts, the center is tapped for the "grounded" conductor. The two legs together give you 240 volts, one leg to the grounded (neutral) conductor gives you 120 volts. So the 3 wires coming into your home are actually a big "shared neutral" circuit. Sharing the neutral between two branch circuits is just an extension of this, LEG-A/NEUTRAL/LEG-B

    Bookmark   March 23, 2014 at 3:05PM
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All a shared neutral circuit is, is two different circuits that use the same neutral. All circuits require a power wire and a neutral wire. Shared neutral circuits send out 2 power wires too two different things like maybe a dishwasher and a garbage disposal but there is only one main neutral that comes out of the panel for the two circuits but both the dishwasher and the disposal are on there own circuit. So in the end you have 2 circuits with 2 power wires on 2 seperate breakers and phases but you only have 1 neutral for both of them.

    Bookmark   March 23, 2014 at 3:11PM
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" It is an extraordinarily unusual home -- and not cheap -- that would have anything other than single phase AC energy furnished to it, no?"

It is not as uncommon as you might think. In my region in the 1950s, when HVAC cooling was in its beginnings, any home with IIRC, 3 tons cooling or over, got 3 ph. Many were subsequently changed to single phase like a former (1950) home of mine. My current home has active 3 ph. My next door neighbor's home was converted to single ph a few months ago after an HVAC revamp. Now I have a transformer to myself! He had what was probably 100A, service whereas mine was changed to 200A some time ago.

It is a type of 3 ph that is supplied to many light industrial and commercial properties and very common in some areas, sometimes called, "high leg delta". In older urban areas, where residential is mixed with light industrial and commercial, I think it abounds.

You'll find lots of diagrams and commentary on the web. It provides both 3 ph and 120/240V single phase on the same transformer system at a relatively low cost to the utility. The disadvantage, from my standpoint, is that I have a 3 ph panel in which I can pretty much use only 2/3 of the slots. 1/3 of the slots have a voltage to ground of 208V. You can use any of the three phases to get 240V, but my understanding is that you are only supposed to use other than the "high", "wild",... leg. That is normally reserved for 3 ph only although the voltage and wave form are fine.

My simple-minded way of looking at it is to view it as two separate systems from an operational standpoint. The first is a single phase 120/240 system. When I need that, I have two hots and a neutral just like the more common residential system in the USA. Don't think about the other (orange wire) leg except to be sure that I am not using it. When I am thinking three phase, I just see the three legs as I normally would see them.

    Bookmark   March 23, 2014 at 4:25PM
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They are certainly derived from one of the three phases by some transformer, but the two legs coming in your house are clearly 180deg out of phase with each other. If they were absolutely in phase, then you'd have 0 volt potential between the two legs.

    Bookmark   March 23, 2014 at 4:30PM
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Yup, two legs of a single phase. A center-tapped transformer does not a second phase make.

As to the 'shared neutral' niick84 lit a match inside my dark head, and I now understand, thanks.

Gonna start another thread re GFCI breakers, cuz I have a different question. Call it pent-up newbie question . . .

    Bookmark   March 23, 2014 at 5:43PM
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The shared neutral is fine as long as the neutral doesn't become loose. As an example, if you're running your fridge and microwave on the shared neutral, and the neutral gets loose,also called a 'floating neutral", the voltage will increase considerably,to RMS, and usually the micro wave,because it has a lower amperage rating, will then get most of the voltage causing it to burn out. If the problem is not corrected, the "new' micro wave will also burn out very quickly. Just a thought.

    Bookmark   April 1, 2014 at 10:56AM
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