ECN Forum Forums Code Related Discussion NEC & other Code issues Multiwire circuit neutrals

I think that we discussed this one previously.

Article 225 _Outside_ Branch Circuits and Feeders clearly permits 'shared neutral' circuits for lighting ( 225.7(B) )

Common neutral circuits are also described for _feeders_.

Article 210 does not mention 'shared neutral' circuits. 210.4 defines and permits multiwire circuits, and as described a multiwire circuit is a specific type of shared neutral circuit. But as far as I can tell nothing in article 210 either prohibits or permits shared neutral circuits.

-Jon

George I have never seen it or considered it.

Beside the fact I can think of very few ways to make it economically attractive I just don't like it for it's 'weirdness' factor.

Bob

You might be able to call that shared neutral circuit something but it isn't a "multiwire circuit".
I posed a similar question a while ago when I asked if you could have a remote junction box with a number of branch circuits and a common neutral, sized by 250.66 to be on the safe side. (As in a service upgrade where the conductors didn't reach the new panel)
Answers were mixed.

I had a proposal on the issue of common neutrals that was rejected. My proposal was to delete the wording shown below in bold.

215.4 Feeders with Common Neutral
(A) Feeders with Common Neutral Two or three sets of 3-wire feeders or two sets of 4-wire or 5-wire feeders shall be permitted to utilize a common neutral.
(B) In Metal Raceway or Enclosure Where installed in a metal raceway or other metal enclosure, all conductors of all feeders using a common neutral shall be enclosed within the same raceway or other enclosure as required in 300.20.

4. Statement of Problem and Substantiation for Proposal:

There is no code violation in using a common neutral so the specific provision to use one is not required

I had a number of proposals rejected that tend to overturn the idea that if the code doesn't say you can't, then you can.
Don



[This message has been edited by resqcapt19 (edited 05-03-2006).]

In a single phase panel you can share 2 circuits, and 3 phase 3 circuits as long as the power is on separate phases. What ever the difference is in amps between the 2 phases on a single phase, that difference will be what is on the neutral

My question refers to a 120/240 VAC Single phase panel. The kitchen circuits would be the 2 small appliance circuits that require GFCI protection above the countertops and then a feed with the 3rd ungrounded conductor to the cord connected garbage disposal. I'm just wondering where you draw the line on sharing the neutral since the receptacles would be getting use by small appliances (with motors) and the garbage disposal alone can pull up to 7 to 10 amps. What is the general rule of thumb to make sure if you run multiwire branch ciruits to different parts of a house that you do not risk a fire by overloading the neutral. Lets say one receptacle has a toaster plugged in and it is running at 10 amps and then a blender is running at 2 amps and then the garbage disposal gets turned on (another 10 amps). Well the circuit breakers are not going to trip because you have not exceeded the maximum amperage on each ungrounded conductor. But is the neutral then carrying a total of 22 amps?

Kyelectric, the problem is not what you know will be plugged in, it is what might be plugged in.

Take the two ungrounded conductors on LEG #1 and LEG #2 and load them to 20 amps each, the neutral will carry 0 amps.

Now take and add another conductor to LEG #1 and load it to 20 amps.

All three conductors are now loaded to 20 amps.

One conductor on LEG #1 will cancel with the conductor on LEG #2 which will leave the neutral carrying 20 amps. (the current on the other conductor connected to LEG #1)

Now, if the cicuit feeding the conductor on LEG #2 is turned off you no longer have a MULTI-WIRE BRANCH CIRCUIT, and the grounded conductor will no longer be a neutral, it will simply be a grounded conductor and will be carrying the total of the two ungrounded conductors on LEG #1, which is 40 amps and your # 12 would be overloaded.

Roger

Your grounded conductor will only carry the difference in current off of your ungrounded conductors as Roger just explained. A true neutral will carry no current...I find definite overheating potential in this situation and the only code reference I could give you for my reason for thinking this is a violation is once again article 100 definition of multiwire branch circuit. This definition clearly states that each ungrounded conductor will have the same voltage to the grounded conductor, and that there will be a voltage between each of the ungrounded conductors. If you run two conductors off of the same phase (which is what you are talking of doing in a single phase system) they will not have a voltage between them and that is a violation.

kyelectric,

The _general rule_ for neutrals is that the ampacity of the neutral shall not be less than the _maximum net computed load current between the neutral and _all_ ungrounded conductors connected to any one phase of the circuit.

As applied to 'multi-wire branch circuits', you simply have one, and _only_ one hot conductor per supply leg, and one neutral of the same ampacity as the hots, and call it good. In a multi-wire branch circuit, the maximum current on the neutral is equal to the maximum current on any _single_ hot conductor. This means that in a single phase system, you can run 12/3, put black on one 20A circuit, red on another 20A circuit _on the other supply bus_, giving a _maximum_ of 20A on the neutral.

Note that you are _required_ to supply the hots from different supply legs. Use the exact same cables, and the exact same circuit breakers, but put both breakers on the same supply leg, and now you can see as much as 40A on the neutral.

Further discussion, sort of a summary of the above:

A 'common neutral' circuit is one in which you have any number of ungrounded conductors sharing a single grounded conductor.

A multi-wire branch circuit is a specific case of a common neutral circuit. In a multi-wire branch circuit, you may only have _one_ ungrounded conductor per supply leg; in a single phase system a multi-wire branch circuit would have two ungrounded conductors from opposite supply legs; in a three phase system you could have either two or three ungrounded conductors from two or three of the phases.

Common neutral circuits are mentioned explicitly in article 225 for outside circuits. Multi-wire branch circuits are mentioned explicitly in article 210 for branch circuits. Multi-wire branch circuits are in common use; the more general 'common neutral' circuit is virtually unheard of, and arguably are _not_ code compliant for inside wiring. (There is also a strong argument that they are permitted since they are not prohibited.)

iwire: I think that I see a situation where using a common neutral circuit might make sense: getting around 240.4(D) and using the full thermal ampacity of small conductors. Consider a 120/240V single phase system. Say I have two fixed loads totalling 24A (say a dishwasher and a microwave). I claim that I could supply these loads and a 20A receptacle circuit using 12/4. The two fixed loads would each be placed on 20A breakers on supply leg A, using the red and blue conductors. The receptacle would be on a 20A breaker on supply leg B using the black conductor. The neutral would be the white conductor. The maximum unbalanced current on the neutral would be 24A, all conductors would be protected by 20A breakers. If the fixed loads were 12A each, then you could do this with a bundled 14/2 and 12/2 and 15A breakers...if you want to get _reallllllllly_ ugly yet remain code compliant

-Jon

Winnie,
Thank you so much for your article. It was very helpful. I do have a better understanding now than I did before. So, basically, when I use multiwire branch circuits on a 120/240v system, I need to make sure the two ungrounded conductors are on separte phases so the neutral current will be cancelled. Can you give me examples where you would commonly use multiwire branch circuits in wiring a house? Like for instance the kitchen two small appliance branch circuits. Thanks!