Shared Neutral

[Sparky in Pa]

Hello everyone,
I don't know if this has ever been discussed or not, but here I go. When using a shared neutral, say for instance when wiring a dishwasher and disposal on a 3 wire, how should the circuit be breakered? I have always used a double pole breaker for this application, where the place I work now, uses single pole breakers. I was always told that, reguardless of how the circuit is wired at the "device" end, it should always be breakered with a DP breaker because even if the one device is turned off, say in a lighting circuit or a receptacle circuit(it's a shame I can't draw a picture to help explain it better), there is a potential voltage on the shared neutral. Is there also a section of the code that explains this? Any info would be greatly appreciated.

 

[jtester]

Re: Shared Neutral

I believe that you need to use handle ties or a multipole breaker only when the devices are on the same yoke, 210.4(B). Otherwise you don't need, and usually shouldn't use multipole breakers. They would just add nuisance outages to the problem.

Jim T

 

[jeff43222]

Re: Shared Neutral

It shouldn't matter that there would be current on the shared neutral if one breaker is off while the other is on. As long as the neutral is sized appropriately and the two circuits sharing the neutral are on opposite busses, the neutral wire should be able to handle a single load or both loads at the same time.

You want to use a DP breaker if the two circuits are going to a split-wired receptacle so as to make sure the entire receptacle is dead in case someone goes to work on it. You don't want someone turning off one circuit and thinking the duplex is dead when it isn't.

 

[al hildenbrand]

Re: Shared Neutral

And don't forget 300.13(B)

300.13 Mechanical and Electrical Continuity ? Conductors
(B) Device Removal In multiwire branch circuits, the continuity of a grounded conductor shall not depend on device connections such as lampholders, receptacles, and so forth, where the removal of such devices would interrupt the continuity.

 

[allenwayne]

Re: Shared Neutral

210.4B is a bit confusing in as it states a muli wire branch circuit supplying more than ONE DEVICE OR EQUIPTMENT on the same yoke shall La da da use a 2 pole.T
That more than one device is confusing,I interpet that part as to require a 2 pole or if on same yoke also 2 pole required.IMHO

 

[infinity]

Re: Shared Neutral

That more than one device is confusing,I interpet that part as to require a 2 pole or if on same yoke also 2 pole required

The requirement is for the multiwire supply to be on the same yoke. Without being on the same yoke the simultaneous disconnect is not required.

 

[apuat]

Re: Shared Neutral

Given a multi wire branch circuit with shared neutral and single pole breakers, with one breaker off and the other circuit under a load, would the grounded conductor not present a hazard to someone working on the other circuit? I know "proper procedure" is to verify the circuit being worked on is off, but it seems to me that it ought to be on a double pole breaker, even if code does not call for it, but for safety's sake.

I hope my expressed my viewpoint clearly enough, sometimes I have issues with words.

 

[LarryFine]

Re: Shared Neutral

Originally posted by apuat:
Given a multi wire branch circuit with shared neutral and single pole breakers, with one breaker off and the other circuit under a load, would the grounded conductor not present a hazard to someone working on the other circuit?

Under proper conditions, the neutral "should" be at 0 volts to ground, but there is alsways some voltage drop. The real danger is opening the neutral while one circuit is still energized, believing they made it safe by turning off one breaker.

I know "proper procedure" is to verify the circuit being worked on is off, but it seems to me that it ought to be on a double pole breaker, even if code does not call for it, but for safety's sake.

A common neutral will read dead to ground until it is opened, after which the load-side wire will be hot through whatever loads are on the still-energized circuit.


In my opinion, a handle-tied breaker pair (or trio) or two- (or three-) pole breaker is a must whenever line-to-neutral loads are fed from a multi-wire circuit, even if the neutral is connected as per the afore-mentioned 300.13(B).

 

[George Stolz]

Re: Shared Neutral

Originally posted by LarryFine:

I know "proper procedure" is to verify the circuit being worked on is off, but it seems to me that it ought to be on a double pole breaker, even if code does not call for it, but for safety's sake.

A common neutral will read dead to ground until it is opened, after which the load-side wire will be hot through whatever loads are on the still-energized circuit.


In my opinion, a handle-tied breaker pair (or trio) or two- (or three-) pole breaker is a must whenever line-to-neutral loads are fed from a multi-wire circuit, even if the neutral is connected as per the afore-mentioned 300.13(B).

To each his own. I pull multiwire circuits for the dishwasher/disposal in a house. If one or the other has a problem, then someone will want to shut off the breaker until it can be serviced. Why unnecessarily require the disposal to be shut down for the period that they can't use the dishwasher?

The very nature of many multiwire circuits is that it is the MWBC portion of the circuit is generally to the first box. From there, one leg goes one way and the other goes another way, using two-wire. Once you're out of the first box, the neutral current from only it's leg is found there.

The neutral current from leg A doesn't head upstream up the leg B neutrals, it goes to the panel.

Putting the two circuits on a two-pole breaker doesn't do much. The one box you could get nailed in is the one that looks like a MWBC. A multiwire home run is there, it should be a giveaway. IMO.

 

[normbac]

Re: Shared Neutral

have to agree with Larry To easy for an apprentice or ? to make the assumption that since the ocpd is off that he is safe. opening a neutral could be fatal. it is to easy to get between the neutral returning from the second circuit and a egc or the grounded conductor from the line side.
P.S. Especially since homeowners are cluless of this scenario.

 

[peter d]

Re: Shared Neutral

Originally posted by normbac:
have to agree with Larry To easy for an apprentice or ? to make the assumption that since the ocpd is off that he is safe. opening a neutral could be fatal. it is to easy to get between the neutral returning from the second circuit and a egc or the grounded conductor from the line side.
P.S. Especially since homeowners are cluless of this scenario.

Sorry, but this industry has never been based on worrying about what an apprentice or homeowner might not understand. Besides, it's dangerous to open up any grounded conductor, shared or not.

Furthermore, an apprentice should be taught what a multiwire circuit is and the dangers involved in the first week of tranining.

I am tired of this attitude that we need to idiot proof everything.

[ October 08, 2005, 10:29 PM: Message edited by: peter d ]

 

[sheldon_ace]

Re: Shared Neutral

Originally posted by peter d:

Originally posted by normbac:
have to agree with Larry To easy for an apprentice or ? to make the assumption that since the ocpd is off that he is safe. opening a neutral could be fatal. it is to easy to get between the neutral returning from the second circuit and a egc or the grounded conductor from the line side.
P.S. Especially since homeowners are clueless of this scenario.

Sorry, but this industry has never been based on worrying about what an apprentice or homeowner might not understand. Besides, it's dangerous to open up any grounded conductor, shared or not.

Furthermore, an apprentice should be taught what a multiwire circuit is and the dangers involved in the first week of training.

I am tired of this attitude that we need to idiot proof everything.

I totally agree. If someone can't see, and recognize a MWBC than they should take out their hammer and tack themselves on the head because they obviously don't belong in that box. I would never allow an apprentice who hasn't demonstrated proper ability to work on a box that has a shock hazard. As far as a home owner? What's the difference between changing out their own light fixtures for fans, or changing outlets to decora? There are shock hazards all over the house, our job is to make the house safe under working conditions, not maintenance conditions.

For example, a company orders the guts for 3phase panel can and it ends up being 42 SP20A breakers. The electrician decides to share neutrals to lessen amount of conduit and copper. Some of you guys are saying that 3pole breakers should be added because of shock hazards. This is a complete waste. If you don't understand MWBC, and their advantages, then you really need to re-evaluate your knowledge.

 

[iwire]

Re: Shared Neutral

Originally posted by sheldon_ace:
If someone can't see, and recognize a MWBC than they should take out their hammer and tack themselves on the head because they obviously don't belong in that box.

LOL

I also agree, electrical enclosures should only be opened by qualified people.

 

[normbac]

Re: Shared Neutral

for me it is all about safety (and lawyers) The probability that a homeowner will change out his own receptacle is high. I dont pride myself on knowing that he deserves what he gets for playing mr. electrician If I can prevent with ease this accident from happening (and we all know it is going to happen) then I will do it. How many times have you had a senior try to fix his own electric problems only to call you when he cant figure why his three way switch stopped working after he replaced it etc. etc. you feel if he opens that box let him die!

 

[apuat]

Re: Shared Neutral

The very nature of many multiwire circuits is that it is the MWBC portion of the circuit is generally to the first box. From there, one leg goes one way and the other goes another way, using two-wire. Once you're out of the first box, the neutral current from only it's leg is found there.

The neutral current from leg A doesn't head upstream up the leg B neutrals, it goes to the panel.

Putting the two circuits on a two-pole breaker doesn't do much. The one box you could get nailed in is the one that looks like a MWBC. A multiwire home run is there, it should be a giveaway. IMO.

Don't I feel slightly dumb, I had not thought of that part of the argument. On our houses, we normally pull straight to the Disposall switch box, from there we feed out to the dishwasher.

 

[al hildenbrand]

Re: Shared Neutral

normbac,

The probability that a homeowner will change out his own receptacle is high.

How does this present a problem? The neutral terminating on the receptacle is a pigtail, right? 300.13(B) as much as says that the receptacle cannot splice the neutral of a multiwire branch circuit passing through this receptacle outlet.

If the yoke holding this receptacle only has one phase terminating to it, and the multiwire branch circuit overcurrent protective devices (OCPD) are single pole, turning that OCPD de-energizes that yoke and receptacle. Even if the other side of the multiwire is still energized, the homeowner's replacing the receptacle doesn't involve the splice in the multiwire neutral.