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2 circuits sharing neutral

Posted by dks35 (My Page) on
Sun, Jan 6, 13 at 19:57

Hi all,

We had an electrician install 4 separate circuits (as per contract) for our kitchen remodel. Wires were brought from the service panel into a junction box under kitchen floor and from there went their separate ways to feed future (1) built-in microwave; (2) dishwasher; (3) garbage disposal; (4) one of the kitchen counter outlet circuits. Yesterday I noticed that in the service panel only 3 white (neutral) wires are coming in from that junction box while 4 red (hot) wires are connected to circuit breakers. A closer look at the junction box connections revealed that (3) and (4) share neutral taking it back to service panel... I hate surprises like that and will talk to the electrician but wanted to ask you all - is this "kosher"? Is this safe? I understand that garbage disposal is in use intermittently but still GD has 4-6 AMPacity. All new circuits are 12 AWG and 20 AMP breakers. Thanks!


Follow-Up Postings:

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RE: 2 circuits sharing neutral

he used what's called multiwire circuits, or more commonly known as "edisons"
as long as the phase conductors are on different "phases" it's fine. Recent code requires them to be on a double pole breaker though.
I do it as much as possible in houses. It's easier to pull one three wire for two circuits, than 2 separate two wires.


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RE: 2 circuits sharing neutral

Official term is "MultiWire Branch Circuit."

Hexus has it right. The important thing in kitchens is to make sure that you don't share neutrals between circuits on the protected side of a GFCI. Further, MWBS is hard to use in the latest code where AFCI is mandated. I've not yet seen any 2 pole w/load neutral AFCIs like you can find in GFCI.


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RE: 2 circuits sharing neutral

This should not be a "surprise" to you. Especially since you had no idea what this VERY common wiring method is. It's not like he would have felt the least bit compelled to explain it to you.

I will say, there is NO reasons to have the GD on a dedicated 20A circuit. It can be shared with the DW.


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RE: 2 circuits sharing neutral

Due to high prices for that 3-wire NM cable, savings on material do not exist for the MWBC. But in some usage patterns, the MWBC is (slightly) more efficient with less voltage drop.
I encountered an "inspector" who did not red flag my job but delayed (by 8 days) passing it because he claimed that MWBC are acceptable only in commercial and industrial installations. I did not budge and awaited action from his supervisor. My customer was doubting my abilities as a result. The inspector did so many similar goofs that he was eventually fired.


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RE: 2 circuits sharing neutral

Due to high prices for that 3-wire NM cable

We don't use much NM around here so I never look at prices. The orange box has 250 feet: 12-2 NM is $129 and 12-3 is $220. 12-2 MC is $128 and 12-3 is $215. All Southwire. AFC MC is a little cheaper.

You'd think an aluminum jacket would be more expensive than plastic. Kind of weird.


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RE: 2 circuits sharing neutral

It is standard practice. Nothing out of the ordinary. I do question why you are snooping around in his work? Do you not trust your own electrician? If not, get somebody you do trust. It is too important to leave to someone you have to second guess all the time. If you had to hire it out, I can only assume you didn't know how to do it yourself. If so, what did you plan to accomplish by bird dogging his work to begin with?


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RE: 2 circuits sharing neutral

"Wires were brought from the service panel into a junction box under kitchen floor.."
I hope there is access to this junction box. From the basement?


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RE: 2 circuits sharing neutral

@btharmy: "I do question why you are snooping around in his work?"

It's his home, he's paying for the work, he has every right to know what's going on.

"what did you plan to accomplish by bird dogging his work to begin with?" Read on - I'll tell you my experiences:

I empathize with the original poster. While the work may be 'up to code', what's lacking is communication with owner. Some people like to know details, others do not. If there are several ways to do a job, why not see if the owner has a preference?

I'm very fortunate to have the skill and understading to do most all of my own remodeling work. The several times I have used a contractor, I felt they made decisions that were not the best.

About 15 yrs ago I had a 3 car, heated garage built. I reviewed the work as it was completed. One day I noticed that there were three switches, about 30" off the floor - I called the electrician and asked him what they were for. He informed me that code reqired a 'disconnect' for each circuit in the garage. I asked him: "Why did you put them 30" from the ground, where a child / animal could bump the switch? I'd rather have a small breaker box. He has no response - except to say the box would cost more money. Well this is a $40,000 garage - do you think I care about $100? Just mindless.

On another occassion, I had service upgraded to 200 amps. I wanted a 20 amp circuit for the bathroom, and also a hex box for a light fixture. The Einsteins that they were, installed this super shallow box (like 1/2" deep) - there was hardly enough depth for the wire nuts - replaced the box myself. When you're paying $2,500 you have a right to expect reasonable work. Apparently they lacked common sense.

That's your answer.


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RE: 2 circuits sharing neutral

It is a foolish consumer who does not verify to the best of his/her ability the quality of work that they are purchasing. HVAC is not my trade, but I know enough to know that the first 4 contractors were not doing the job correctly on my new house. The 5th guy got it right.
Plus the person first posting here probably learned something additional about electric wiring. Even at my advanced age, I do deliberately learn something new each day.


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RE: 2 circuits sharing neutral

I've lived in quite a few homes and never seen a multiwire branch circuit outside of work. To me, it seems rare in residential.

No one explained to the OP that the circuits that share the neutral must be on different legs on the service so that the max current on the neutral is equal to that on one circuit breaker when one circuit is being used and will actually zero of power use on both circuits is equal.

doug_gb, look at it from the electrical contractor point of view rather then the whole project. The electrical part is relatively small. A couple of hundred might make the difference between winning the bid and not. That said, it seems to me that electricians are unduly hung up on saving a few insignificant cents given what I have read hear and elsewhere and experienced.

I ask a lot of questions of contractors. First, the best professionals seem to be eager to enter into dialog. They like to explain their work and why they do things the way they do. That comment covers everyone from physicians to plumbers. Anyone that won't talk with me won't get the job. It also signals them that I want to learn how things are done. Some customers don't want any explanation. I guess they have to find the line with most customers, but if I ask a lot right out of the gate, it can be a signal that I like details.


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RE: 2 circuits sharing neutral

Hi all,

Thank you very much for the input on my original question. It seems to have generated quite a heated discussion. I understand that many of you are professional electricians - that's why I ask you guys for an opinion.
When we bought this house two years ago we embarked on renovation projects. This is all new to us (first house) including code compliance and stricter rules we have in California. The original owner of the house (a lady in her 80s who passed away) did last major updates in the early 70s. When we mapped (by ourselves!) the electrical circuits in this house to decide how to proceed forward we discovered some unusual things that are definitely not up to current code and may not have been even up to the old code as well. This probably resulted from owener's unwillingness to open up walls and deal with the ensuing mess or bad advise or else - it doesn't matter at this point. Now we are trying to untangle this and because it is quite an expensive project - I need to make informed decisions. Which is why I solicit additional advise on this forum as well.
As for "Snooping around" and "bird dogging" - I do have the right to know why certain things are done this way, one of the posters referred to that. I care about safety and compliance. I am not stupid, know the basics of physics/electricity and fully capable of grasping some of the concepts of this electrical work. Moreover I am really interested in it and eager to learn. And I did spend ~$6K on electrical work up to now. I guess problem was that our electrician is on a short vacation after new year, inspection is due and we did not have the best of communication on this subject. this will be remedied this week when we talk to him.
Regarding "trust your electrician" - like any kind of trust, it needs to be earned. I don't suppose that our electrician is a fraud or anything, in fact all of his work is excellent and we are building a trustworthy relationship, however I don't get intimidated by electrical (unlike plumbing) and would like to clarify all of these things. No one wants to feel like he/she was taken advantage of, not good for trust.

Hope this clarifies my original intentions - not malicious but I am a control freak in many ways, so ... will have to deal with that.

On another note - it looks like opinions on this subject of sharing neutral are split on this forum. I see the point of double pole breaker for shared neutral circuits but the last comment by ionized regarding opposite legs of service panel - have to get my hear around it yet.

Thank you guys!


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RE: 2 circuits sharing neutral

"No one explained to the OP that the circuits that share the neutral must be on different legs on the service"

from my original post

"as long as the phase conductors are on different "phases" it's fine."


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RE: 2 circuits sharing neutral

Hmm, you are correct. I really meant "why the circuits" rather than "that the circuits".

OP, your whole electrical service is based on the shared neutral concept. that is why you have three conductors in your drop. You can easily find some good explanations of that with good graphics on the web. The power company can use less metal that way.

The disadvantage is when you "lose" the neutral between your service panel and the transformer. That can do a lot of damage because you can get wacked-out voltages. Every device connected to L1 is in parallel as normal as are every device on L2. The trouble is that L1 and L2 are in series with each other so the voltage on each leg can float depending on the total relative resistivity in each leg. Note that since the ground and neutral are connected at your panel, you the ground can substitute for the neutral if everything is ideal. We do not, however, like to depend on this.


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RE: 2 circuits sharing neutral

@dks35: Your 240 service is supplied as two phases - 2 - 120 volt that are 180 degress out of phase. So when you add the two lines together - you get 240 volts. Now in the breaker box you will notice that the breakers attach to 'fingers' - these fingers are each 120 volts - but ajacent 'fingers' are the other phase - thus a double breaker will yield 240 volts.

What's interesting is that since the two 120 lines are out of phase - the current carried through the neutral could be zero - since it's cancelled out due to the phase difference.

The reason they said it should be a double breaker is a safety issue. You don't want the possibility of current traveling in the neutral when you think that the breaker is off.

I myself (as an engineer and computer scientist) think the KISS principle is best. For a couple of bucks more use seperate neutrals and you eliminate possible problems.


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RE: 2 circuits sharing neutral

"Hmm, you are correct. I really meant "why the circuits" rather than "that the circuits"."

sorry, I'm not here (and I wouldn't expect anyone else here) to teach someone electrical theory on a DIY site.


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RE: 2 circuits sharing neutral

doug_gb - thanks, I got that. the safety factor - both breakers need to be off to make sure the shared neutral is not carrying current.


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RE: 2 circuits sharing neutral

Hey, dks35, at least you don't have to worry about these:

Here is a link that might be useful: http://en.wikipedia.org/wiki/Ring_circuit


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RE: 2 circuits sharing neutral

" Your 240 service is supplied as two phases - 2 - 120 volt that are 180 degress out of phase."

It is NOT "two phases."

Two phase power was experimented with long ago.
It is 90 degrees out of phase, and requires 4-wires.

It lost out to 3-pahse (120 degrees) and only 3 wires (delta), though often run as 4-wire Y to avoid circulating currents endemic to imperfectly balanced delta systems.

The standard US system of 120/240 V is a single split-phase system, also referred to as an 'Edison circuit.'

It makes 240 V available for larger loads while limiting the voltage to ground of each LEG (NOT phase) to 120 V.


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RE: 2 circuits sharing neutral

I have seen two commonly-given, and seemingly valid reasons why the people that decide these things don't like to call the common US residential power supply "two-phase". First, the voltage and current in the two legs are in phase, but of opposite polarity. Second, they originate from one winding on the commercial generator.

That leads me to the interesting thought that if I am supplying my house with my own generator, I am left with only one reason not to call it "two-phase" since there are two windings in my personal generator providing 120/240.


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RE: 2 circuits sharing neutral

I am not going to explain electrical theory to every customer I do an install for. If you want to have a neutral for every circuit then tell me that before I begin the work. Otherwise, you are going to get the receptacles you ask for wired the most efficient way possible to the current NEC standards. It is not my responsibility as a contractor to lay out every detail of every job. I wouldn't get anything done if that were the case. It is not "on the job training" for the homeowner when I show up. I am not saying a homeowner can't be curious about the work being done. What i have a problem with is the fault finding, even when everything is code compliant and safe. That is why you select a contractor you trust. An then, now follow me here, trust him to do his job. If you don't trust him, don't hire him.


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RE: 2 circuits sharing neutral

"I am not going to explain electrical theory to every customer I do an install for. If you want to have a neutral for every circuit then tell me that before I begin the work. Otherwise, you are going to get the receptacles you ask for wired the most efficient way possible to the current NEC standards. It is not my responsibility as a contractor to lay out every detail of every job. I wouldn't get anything done if that were the case. It is not "on the job training" for the homeowner when I show up. I am not saying a homeowner can't be curious about the work being done. What i have a problem with is the fault finding, even when everything is code compliant and safe. That is why you select a contractor you trust. An then, now follow me here, trust him to do his job. If you don't trust him, don't hire him."

exactly. I already have apprentices, I don't need "hobby electricians" too. If you want to learn theory and how stuff works, take a class.


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RE: 2 circuits sharing neutral

"I am left with only one reason not to call it "two-phase" since there are two windings in my personal generator providing 120/240. "

It is still not 2-phase.

An induction motor running on 180 degrees of phase difference cannot start rotating, it has zero starting torque unless the phases are 90 degrees out.

You can call it anything you wish, but you are simply wrong.

You also might want to look VERY carefully at your generator wiring.

The first thing is that it uses an alternator.
The magnetic field is rotating.
The output is taken from the stator winding.
It may have been made with two coils for each leg, mechanically separated by 180 degrees, but one end of each coil is hooked together.

The number of phases is not dictated by the criteria you are relying on.


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RE: 2 circuits sharing neutral

"An induction motor running on 180 degrees of phase difference cannot start rotating, it has zero starting torque unless the phases are 90 degrees out."

"The number of phases is not dictated by the criteria you are relying on."

Am I to conclude that the number of phases is determined by the type of motor that can start on it?


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RE: 2 circuits sharing neutral

For the many questions that ionized has, it is unfair to expect exhaustive answers on this forum. Too much work for those who have the answers.
Used volumes of this title are available. Developed about 1953 as training material for the US Navy-- I know of no better material available. 5 volumes-- the combined edition is the better buy and includes all 5 of them. And it affords study at your pace and convenience.
It covers the subjects in surprising depth.

Here is a link that might be useful: Basic electricity


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RE: 2 circuits sharing neutral

LOL, and I thought that bus-driver had No sense of humor!


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RE: 2 circuits sharing neutral

ONE of the criteria for poly-phase systems is that induction motors can be started without start windings, switches, capacitors, etc.

Very large induction motors may still require starting equipment to avoid the huge surge current otherwise required at start-up, but they do not require phase shifting capacitors to have a non-zero starting torque.

Two phase systems are 90 degrees apart, 3-phase systems are 120 degrees apart.
Even higher phase counts have been used, but the number of conductors quickly makes them not worthwhile except in a very few extremely high power applications.
If the source is only a short distance away the extra lines are not as much of a penalty.

Larger poly-phase systems often do not balance out on a neutral.
There are also the typical problems like those found on delta systems with circulating currents from less than perfectly balanced loads that quickly lead to other problems.

Especially saturation of transformer cores.

This post was edited by brickeyee on Fri, Jan 11, 13 at 10:33


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RE: 2 circuits sharing neutral

So the questions posed by ionized are just jokes? Given the potential danger of electricity, other subjects are better for joking.


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RE: 2 circuits sharing neutral

All previous posters on this subject have somehow missed the 500# gorilla in the room. Multiple branch circuits cannot be allowed to ‘share’ a common neutral conductor run without increasing the current carrying capacity of that run (larger size, DO NOT DOUBLE RUN WIRE, gang breakers). In such application power lost to heat more than doubles for a doubling of current, following square law (I-squared-R), dangerously violates code and it will burn down. This is the very reasoning behind licensing of electricians; if you don’t understand this, do not perform wiring and do not shuffle the load center’s runs.
However, ‘balanced neutral/phase’ is valid for load runs and mandatory on the supply-side of the bus; and is taken advantage of, by discriminating technicians. Balanced is green, more efficient, reduces losses, brownout, etc. When a load is balanced between legs (2 just happens to work out to be single phase; and 3, three phase), it is NOT sharing a run, but current is mutually excluded from the common neutral run (i.e., current does not circulate to the neutral run’s lug) to the extent that each leg mirrors the other(s). Hexus, think exclusive OR (XOR, only analog). Each circuit when used alone has exclusive use of that run. Under no circumstances does current exceed the run’s rating, ANYWHERE. When the circuit’s legs have matched loads (in current mirror, reactances in quiescence), think about identical hot plates on each leg, then the neutral run to lug will be cold, out of circuit and may be snipped to no affect. The neutral run BETWEEN hot plates will carry just one circuit’s current for either/all leg(s) (in series for 2-leg circuit; 3-phase ‘motors’ have all neutrals removed--there is no need for them with no current to run). Instead of I-squared-R loss over MULTIPLE neutral runs to lug; the loss is over a short distance ONLY between hot plates. Circuit interrupters (breakers) are now required to be ‘ganged’ (hence branch circuit fuses are not allowed) in order to arrest all branch neutral current. Sleeping rooms (including family/den/living rooms) require a 'split circuit' AFCI. GFCI's will de-energize with PROTECTED neutrals in common.


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RE: 2 circuits sharing neutral

Well six months late and more than a dollar short.

The wire size is NOT required to be increased in a MWBC and there's no reason to do so.

Just about everything you've said is completely wrong drivel.

Balanced on a single branch circuit is going to mean NOTHING to "green" or "brown outs". To a slight extent that comes into play on your entire service, but it's meaningless for a branch circuit.

There's no thing as a "split circuit" AFCI. That's not a real term and as I pointed out back in January, there's no LOAD NEUTRAL AFCI's.

GFCI's, AFCI's, or even REGULAR breakers do not "DEENERGIZE" neturals. Only hot legs.


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RE: 2 circuits sharing neutral

Why can't Mr Fork and Mrs Outlet get along?


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RE: 2 circuits sharing neutral

For IMPROPERLY balanced 2-leg MWBCs neutral WOULD be required to carry twice the Amps and hence would need to be of gauge sufficient to cover that multiple (larger size, smaller gauge). There are several two-pole AFCI breakers on the market; Eaton CH215AFIT, CH215AF, BR215AFIT, BR215AF, Siemens Q215AF; and CombinationAFCIs; SquareD/Homeline HOM120CAFIC and GE DET-719 CAFCI. If a properly phased MWBC does not have ganged breakers, the neutral associated with a fault-cleared hot-run can, and most likely does, still carry deadly current from its other associated uncleared hot runs--don't place yourself in what you might think is a dead neutral run, GANG your MWBC breakers. I'm willing to bet that there's a large base of installed single phase out there which is split into an A-phase and a B-phase. I'm using 6-pole "split" AFCIs shipboard for state rooms!

This post was edited by mouselb on Thu, Jul 11, 13 at 1:48


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RE: 2 circuits sharing neutral

Sorry mouselb but you are completely wrong. Even if all the load is on one side, a 15A MWBC will only have 15A on one side and 15A on the neutral (and zero on the other).
A balanced will have 15A on each hot leg and ZERO on the neutral. You can keep blithering on and on about this but there is no way to overload the neutral on a MWBC.

The HOM breaker you list is not a two pole.

The neutral certainly can carry current, but DEADLY is incorrect. There is good reason why the neutral is called the GROUNDED conductor. The issue occurs when the NETURAL is opened for some reason, of course open neutrals can cause problems even without MWBCs.


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RE: 2 circuits sharing neutral

Mouse, PLEASE stop giving advice on construction electrics. You obviously have a different mindset and are not up to speed on how things are done on land.


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RE: 2 circuits sharing neutral

"mouselb", what the heck are you talking about?


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RE: 2 circuits sharing neutral

I don't give advice, I sell it elsewhere. This is not explicitly an advice column. If your looking for free advice, ask someone else. This is a load wiring forum with topic "RE:2 circuits sharing neutral". This occasion is most favorable to stomp out ignorance and pedagogically address dks35's (OP) and others' concern with 'sharing neutral'. Sharing is NOT the word for what's happening when two circuits, on SEPARATE LEGS are wired to a common neutral (MWBC). Not in the sense of SHARING a soda with two bendy straws! It is somewhat like TIME SHARING a resort condo, however when both circuits are FULLY loaded there is no one at the condo and NO sharing going on at all. Maxwell's equations can be approximated with Kirchhoff's Current Rule and the use of simple vectors (phasors) to study the arithmetic behind this phenomenon where two circuits are balanced about/around/across/over neutral. This reduces the amount of conductors needed, the size of conduit required and derates the amount of heat lost to and troubling to conduit--it's quite green. If a MWBC is improperly setup (on just ONE common leg), neutral-SHARING is what you'll have--and your original concern about safety will be most salient. The word 'share' is used all over the industry with respect to MWBC neutral conductors--and lives have been lost by its misapplication.


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RE: 2 circuits sharing neutral

mouseIb: "I don't give advice, I sell it elsewhere. This is not explicitly an advice column. If your looking for free advice, ask someone else. This is a load wiring forum with topic "RE:2 circuits sharing neutral". This occasion is most favorable to stomp out ignorance and pedagogically address dks35's (OP) and others' concern with 'sharing neutral'. Sharing is NOT the word for what's happening when two circuits, on SEPARATE LEGS are wired to a common neutral (MWBC). Not in the sense of SHARING a soda with two bendy straws! It is somewhat like TIME SHARING a resort condo, however when both circuits are FULLY loaded there is no one at the condo and NO sharing going on at all. Maxwell's equations can be approximated with Kirchhoff's Current Rule and the use of simple vectors (phasors) to study the arithmetic behind this phenomenon where two circuits are balanced about/around/across/over neutral. This reduces the amount of conductors needed, the size of conduit required and derates the amount of heat lost to and troubling to conduit--it's quite green. If a MWBC is improperly setup (on just ONE common leg), neutral-SHARING is what you'll have--and your original concern about safety will be most salient. The word 'share' is used all over the industry with respect to MWBC neutral conductors--and lives have been lost by its misapplication."

In other words, Yes, in a single phase 120/240v system, one "hot" conductor from each leg (on adjacent breakers with their handles tied) can share the same neutral (grounded conductor). This conductor can be the same size as the "hot" conductors. This is a multi wire branch circuit.


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