Shared neutrals

[Ian Gills]

I was hoping to install some AFCIs on my panel and found two shared neutrals covering four circuits/breakers.

Any opinions on whether it is worth running new wire on these circuits to have their own neutral? Are there any benefits other than being able to use AFCI breakers?

One shared neutral is for all receptacles and the basement lights (two circuits).

The other shared neutral is for the kitchen receptacle and the disposal (two circuits).

All of the wiring is accessible (unfinished basement).

Oh, and there was one other problem. In trying to disconnect the neutral for my lighting circuit, the lug will not move and I have stripped the screw. Am I allowed just to cut the neutral near the lug, strip it and place it in another lug? Again I hope to attach an AFCI to this circuit, that's why the issue arose.

[Frank P. Newman]

No way is it ok, as the shared neutral is carrying sum of the currents that each of the breakers is providing. So, if for example, those are 20a breakers with #12 conductors for the hot as well as the shared neutral, the neutral could see almost as much as 40 amps before tripping either breaker if each breaker was carrying almost 20 amps. Code aside, that's way too much for a #12 conductor.

I have seen such a situation where multiple feeders supplied a (older) commercial lighting system panel, but the neutral was sized so that it could safely carry the combined load between the two panels which were immediately adjacent to each other.

There are a lot of other questions about how the conductors have been installed, etc., but I expect you will get a lot of help from others on the topic.

Bottom line - even if the neutral is properly sized, I would definitely fix it, given you relatively easy access to do so

[Jim Port]

Originally Posted by fnewman

No way is it ok, as the shared neutral is carrying sum of the currents that each of the breakers is providing. So, if for example, those are 20a breakers with #12 conductors for the hot as well as the shared neutral, the neutral could see almost as much as 40 amps before tripping either breaker if each breaker was carrying almost 20 amps. Code aside, that's way too much for a #12 conductor.

That is why it is important that the 2 hot conductors are on opposite legs of a single phase panel. On a 3 phase panel you would have 1 neutral for the 3 hot phase conductors. Each hot would be from a different hot phase.

To the OP, yes, if the screw in the buss has stripped it can be cut off and moved to another terminal. Do this only with both breakers that feed this circuit being turned off.

The 08 Code addresses this need to identify MWBC's by having all the conductors grouped with a tie wrap.

[William J. Decker]

Originally Posted by fnewman

No way is it ok, as the shared neutral is carrying sum of the currents that each of the breakers is providing. So, if for example, those are 20a breakers with #12 conductors for the hot as well as the shared neutral, the neutral could see almost as much as 40 amps before tripping either breaker if each breaker was carrying almost 20 amps. Code aside, that's way too much for a #12 conductor.

Actually, of the two hots from from different legs, the neutral carries the difference on the amp load, not the sum. The amps sum ONLY of both hots are from the same leg (which no good electrican would ever do.

I have seen such a situation where multiple feeders supplied a (older) commercial lighting system panel, but the neutral was sized so that it could safely carry the combined load between the two panels which were immediately adjacent to each other.

There are a lot of other questions about how the conductors have been installed, etc., but I expect you will get a lot of help from others on the topic.

Bottom line - even if the neutral is properly sized, I would definitely fix it, given you relatively easy access to do so

You can use a shared neutral (with the hots from two different legs) for AFCI is you use a double pole AFCI circuit breaker (now available). Then you don't have to pull a seperate neutral for each circuit.

[Frank P. Newman]

Yes, I understand that the worst case may not apply - that's why I said could. If only all wiring were done by competent electricians we would never have to worry.

[Paul W. Abernathy]

Yes....yet again Eaton does make a (2) pole "shared" grounded conductor AFCI breaker. But I am not sure if they are the only one that makes one.

Many problems with Multi-Wire circuits......never a good idea in my mind but the code allows for them.....if done properly.

I wont go into why they are bad....enough people have posted that...but it is yet again another reason WHY the NEC nor the Manufactures have ever allowed neutrals " grounded" conductors to share a termination on the neutral " grounded " buss....if by chance it is removed.....WHAMO...you take a 120V multiwire system and turn it into a 240V series circuit.......let the damage begin.....

SO we are aware of the issues of improper installation and what it can do on the " grounded " conductor.....just lets not forget what it can do to connected equipment as well...if lifted....

[rromoser]

FYI A comment on multi-wire branch such as a 12/3 romex w/ ground. If one circuit on "A" phase had a 16 amp load and Phase "B" had a 14 amp load on it, the shared neutral only carries the unbalanced portion. In this case 2 amps is all that is flowing on the neutral. But if these 2 circuits were on the same phase the full 30 amp load is on the shared neutral.

[Paul W. Abernathy]

Yep.....been teachin (and preachin) it for many years...lol

[George P. Wells, CMI]

Originally Posted by pabernathy

Many problems with Multi-Wire circuits......never a good idea in my mind but the code allows for them.....if done properly.

I've never been comfortable with shared neutrals either. I never installed them when I was in the electrical contracting business. As an engineer, I never allowed them. Loss of the grounded conductor is a leading cause of fires in North America. But, they are still legal.

The challenge we have as home inspectors is figuring out when and how to report them. They have been legal for as long as I have been in the trade. As Paul said, there is nothing wrong with them when they are installed properly. The shared neutral will not ever carry any more current than the load on either on the hot lines. It's not the electricians I worry about. It's what the homeowners do long after the electrican leaves - that's what is realy scarry!

Recipe for a fire:

One Homeowner + One Tandem Breaker (replacing two side by side single pole breakers) = One Big Fire

[Speedy Petey]

There are steps being taken to cull future MWBC (multi-wire branch circuit) issues. Such as recent upcoming changes to the NEC mandating handle ties, and bundline of MWBC conductors when conduit is used.

I myself always bundle neutrals when running conduit. I do not typically use handle ties or two-pole breakers though. Man times in old jam packed panels it is impossible to rearrange breakers to fit one in. This is especially prevalent in GE and FPE panels with their odd buss systems.

I for one hope we never see them outlawed. They are too beneficial to an electrical installation, and NOT just monetary wise.
It is a shame that we have to change the way PROFESSIONALS do their jobs in order to safeguard untrained and unqualified folks from making mistakes that can cause loss.

Originally Posted by gwells

Loss of the grounded conductor is a leading cause of fires in North America.

I ask this truly out of curiosity. Where have you heard/seen this? This is a statement I have never heard before, and I hear a lot of statements about fire being "electrical in nature".

[Barry Adair]

Originally Posted by Speedy Petey

I ask this truly out of curiosity. Where have you heard/seen this? This is a statement I have never heard before, and I hear a lot of statements about fire being "electrical in nature".

Fire deaths

• In 2003, there were 388,500 reported home fires in the United States, resulting in 3,145 deaths, 13,650 injuries and $5.9 billion in direct property damage. In the U.S., someone dies from a home fire roughly every three hours.
• In Canada, someone is fatally injured in a residential fire roughly every 32 hours.
• Candles are responsible for a growing percentage of home fires. In 2001, candles were responsible for 6% of the fatalities that occurred as a result of home fires.
• Roughly half of all home fire deaths in the U.S.resulted from fires that were reported between the hours of 10:00 p.m. and 6:00 a.m. But only one-quarter of home fires occur between those hours.
• Although children five and under make up about 7% of the country's population, they accounted for 12% of the home fire deaths, assigning them a risk almost twice the national average.
• Older adults are also at greater risk of dying in a home fire than the population at large. Adults 65 and older face a risk twice the average, while people 85 and older have a risk that is three-and-a-half times more than average.
• Smoking was the leading cause of home fire deaths overall, but in the months of December, January and February, smoking and heating equipment caused similar shares of fire deaths.

Electrical

• Electrical distribution equipment (including wiring, switches, outlets, cords and plugs, fuse and circuit breaker boxes, lighting fixtures and lamps) was the fifth leading cause of home fires and the sixth leading cause of fire deaths in the United States between 1999 and 2002.
• The most common types of electrical distribution equipment involved in home fires are 1) fixed wiring, 2) lamps or lighting, and 3) cords or plugs.

Not that this info relates to the topic

[Joe Farsetta]

Paul,

We've discussed the "phasing" with regard to the shared neutrals, so, heres another one...

Replacing a "normal" sized breaker with the compact ones. Will two compacts share the same phase, as opposed to two "normal" breakers attaching to A and B?

[Speedy Petey]

Originally Posted by badair

Not that this info relates to the topic

I agree. No where do I see "Loss of neutral" as a leading cause of fire. I call that statement just more scare tactics.

[Greg Fretwell]

Loss of neutral would cause death of equipment in most cases. You might get lucky and have the phases balanced but it is more likely that you would have one seeing 60-70% of the 240v. It might also figure in a pretty good number of electrocutions of workers or DIY folks but I doubt anyone really slices statistics that thin.
I am one who is in favor of the handle tied breakers on MW circuits, particularly in dwellings where there is no management and a very good chance of unqualified people working on the equipment ("professional" or homeowner).

[Greg Fretwell]

Joe, Piggyback breakers only hit one phase. They can't be used for a MW circuit unless you hit two separate and adjacent breakers, not two terminals on the same breaker. I have seen pairs of the old style SQ D piggybacks where there were two full sized handles the long way set up this way with handle ties driving 2 MW circuits (4 total 120v circuits). My condo in Treasure Island was done this way as a new installation with the whole place on a 8/16 slot panel. When I had one of those breakers go bad is was a scavenger hunt finding another one. Nothing else was going to work.