PDA

View Full Version : Shared neutral



beekerc
10-27-2008, 01:54 PM
definitely a question for the pro's here.

In the process of moving some circuits from the main panel breaks to the generator transfer switch breakers, i discovered a 14/3 wire in the panel. the black wire was connected to one breaker. the red wire was connected to another breaker. white and ground were connected to the bus.

1) is this code legal? it looks like original construction wire so i would assume that it was inspected back in 1968, so if it's not compliant for new construction it's probably grandfathered

unless....

2) is it safe? I've already labeled the breakers for "common neutral" so i'll know to shut down both breakers if i want to work on either circuit. beyond that, i really can't think of any other risk.

am i missing anything that i need to be concerned about?

Speedy Petey
10-27-2008, 02:03 PM
Perfectly normal, legal and safe.
You did good by labeling them so you remember.

maintenanceguy
10-27-2008, 02:14 PM
Today, they'd be required to be on a 2 pole breaker instead of two single pole breakers.

And today, two separate buss bars are required, one for the neutrals and one for the grounds. In a main distribution panel, the neutral and grounding bus bars are bonded to the panel. In all sub panels, only the grounding bus bar is bonded.

But two circuits are allowed to share a neutral. It's called a "multi-wire circuit".

Speedy Petey
10-27-2008, 02:27 PM
Today, they'd be required to be on a 2 pole breaker instead of two single pole breakers.

And today, two separate buss bars are required, one for the neutrals and one for the grounds. .......... In all sub panels, only the grounding bus bar is bonded.This is ONLY under the 2008 NEC with no amendments.
Not nearly all areas are under 2008 yet.

Bill Arden
10-29-2008, 02:27 PM
It's safe as long as the two breakers are adjacent.

FYI: I did a dual outlet run using 12-3wg and then ended up having to buy a 20 amp 240 volt GFI breaker. I could not use GFI outlets at the start of the string since it shared the neutral.

Speedy Petey
10-29-2008, 02:35 PM
I could not use GFI outlets at the start of the string since it shared the neutral.This is not at all true.
At the first box you split the circuit to two 2-wire circuits and have a GFI receptacle at each.
I do this all the time in kitchens.

Bill Arden
10-29-2008, 03:02 PM
FYI: I could not use GFI outlets at the start of the string since it shared the neutral.
This is not at all true.
At the first box you split the circuit to two 2-wire circuits and have a GFI receptacle at each.
I do this all the time in kitchens.

But in this case the neutral was fed to a string of outlet pairs. I would have had to run two 12-2 wires between each 2 gang box to keep the two neutral wires separate for the rest of the string.

Combining the neutral after two GFI outlets will cause them to trip at times due to unbalances in the neutral current.

I am just pointing this out in case the original poster decides to upgrade it to GFI in the future.

Chris75
10-29-2008, 04:57 PM
But in this case the neutral was fed to a string of outlet pairs. I would have had to run two 12-2 wires between each 2 gang box to keep the two neutral wires separate for the rest of the string.

Combining the neutral after two GFI outlets will cause them to trip at times due to unbalances in the neutral current.

I am just pointing this out in case the original poster decides to upgrade it to GFI in the future.

Who wires like the way you did? :D

Speedy Petey
10-29-2008, 06:31 PM
But in this case the neutral was fed to a string of outlet pairs. I would have had to run two 12-2 wires between each 2 gang box to keep the two neutral wires separate for the rest of the string.
OH. I see. I misunderstood. I thought you meant you just used 12/3 for the home run to the first box.

I agree with Chris. This is a very unorthodox way of wiring receptacles.
And just to let you know, you would have had to use a two-pole breaker even if you did not need GFI protection. You have two circuits on the same device yoke, that requires a handle tied breaker.

jar546
10-29-2008, 06:40 PM
Put them on a double pole breaker no matter what code edition in enforced in your area. it is just a safer and professional practice.

Speedy Petey
10-29-2008, 07:11 PM
it is just a safer and professional practice.I am not looking to get into a pissing match, but this is purely personal opinion.

Sure, it is "safer" for an untrained lay person. On the other hand, anyone with the knowledge or experience to justifiably be in an electrical panel would know immediately.

Professional practice? Tell that to all the commercial and industrial guys who do not use two and three pole breakers for MWBC's, and for good reason. MWBC's are the standard wiring method in those settings.
I have installed hundreds, maybe even thousands of MWBC's. Rarely will I use a multi-pole breaker if it is not required. And I certainly DO NOT consider it "unprofessional".

jar546
10-29-2008, 07:30 PM
In the residential setting, I have witnessed more problems from people tapping into multi-wire circuits, contractors screwing them up and homeowners moving the breakers around changing the voltage potential of the circuit.

In a professional industrial or commercial setting where there are no DIYrs, then of course it is the norm.

It is of my opinion that it is a safer and more professional installation in any case. However, my opinion and what the code requires or does not require are two different things. Everyday I approve things that I don't like but they meet the code so I do my job.

You are entitled to your opinion just like I am.

Chris75
10-29-2008, 07:35 PM
In the residential setting, I have witnessed more problems from people tapping into multi-wire circuits, contractors screwing them up and homeowners moving the breakers around changing the voltage potential of the circuit.

Maybe they should just not touch it?



In a professional industrial or commercial setting where there are no DIYrs, then of course it is the norm.

Just as many so called maintence guys as there are DIYer's.




It is of my opinion that it is a safer and more professional installation in any case. However, my opinion and what the code requires or does not require are two different things. Everyday I approve things that I don't like but they meet the code so I do my job.

You are entitled to your opinion just like I am.

Do you really think you can protect the unqualified?

Rowdy
11-21-2008, 04:00 PM
Hi all you DIY,ers

Here is the problem with a shared neutral: Virtually all current that exits a transformer will return to the transformer. There are only two ways that I know of that this can occur.

Transformers have multiple taps, single phase transformers used for residential power have three taps, L1 and L2 are your normal hots providing current to your service panel and distributes that power through your branch circuits. Once that current is used to power a load it must return to the transformer in order to complete the circuit.

The third is a center tap is used to drain the current from a load that uses only one side of the transformer, either L1 or L2, or both L1 and L2 if there is an unbalanced load, this center tap is grounded at the transformer and again at the service entrance.

If you have a load that requires 240 volts you are using both L1 and L2 they are opposite each other so when one is pushing current the other is draining current, they offset the current on the opposite leg so you don't need to be connected to the center tap to return current to the transformer, the same thing occurs with multi-phase transformers.

Here is the problem with sharing a neutral if you are using two breakers that share only L1 or L2. The center tap is draining current from both circuits and since they are on the same tap from the transformer if you are using 15 amps on both, you are returning 30 amps on the grounded conductor, 15 + 15 = 30amps. Since there is no overcurrent protection on the grounded conductor you are creating a hazard.

Your panel is designed so that you can use a two pole breaker and be assured that you are using both legs, L1 and L2 of the transformer, do it.

beekerc
12-08-2008, 12:10 PM
Your panel is designed so that you can use a two pole breaker and be assured that you are using both legs, L1 and L2 of the transformer, do it.

I think i follow you. now for the dumb question - is a two pole breaker the same as the double breaker that's used for my stove, dryer and heat-pump? because those are 240V. okay, so i'm asking this because i didn't go to electricians school, but when you're dealing with some these types of items, i notice that the gauge of the wire much bigger than if they were separate.
example. a 240v/30A circuit, is effectively a pair of 120v/15A circuits and breakers. but while 14 gauge wire is sufficient to support a single 15A circuit, all of the wires servicing teh 240v/30A circuit are 10 gauge. (i presume that should all the current some how get directed down one wire, it won't overheat/melt/short/etc). so how would putting a two pole breaker on a pair of 120v circuits using a shared neutral not require upgrading the gauge of the hot wire?

BluesMan
12-08-2008, 06:50 PM
yes...a two pole breaker, or double pole breaker is used on the range, electric dryer, and heat pump.

A double pole 30 amp breaker is just that...two 30 amp breakers. Each leg of that circuit is capable of carrying 30 amps, and the wire must be sized accordingly.

JLAG
12-08-2008, 08:02 PM
That breaker should be changed to a DP RIGHT AWAY or the circuit should be disconnected and wire nutted.

If something happens to you, and some joker comes in and moves one of those breakers without checking to see that the neutral is shared on those circuits you have a SERIOUS risk for the people in that structure. Yes, double current on an unprotected neurtal.

I have seen melt downs from this. People move breakers to get a space in a panel and not check this because the wires are at the back of the panel and are difficult to see.

I have even seen two SP breakers where someone jamed an allen wrench through the holes in the toggles to make them trip as a DP. Crazy world!

Just my two cents.

jadnashua
12-08-2008, 08:37 PM
When you are using a double pole breaker to provide 240 volts, what goes out one leg, goes in the other so you've got more potential and more current, essentially add the two together, and it has to go through those two hot wires (no neutral); therefore, the wire needs to be sized accordingly.

But, if you are using a shared neutral, you have half the voltage (unless the neutral comes loose), and what goes out one leg, returns via the neutral, so you only have 120vac potential, thus lower current (i.e., one half of the double). Since with a double, you are assured of having the two on different legs of the transformer, the current on the neutral would cancel if the load is the same on each leg of the hot, but at worst, can't be more than one leg, which is the single breaker of the pair. So, the current could be up to the single breaker's limit, or zero, if both sides were evenly loaded. If it wasn't a double, but wired with adjacent breakers, if one was moved so it was on the same leg of the transformer, the the current in the neutral would add; i.e., try to carry the current of both breakers, rather than one or nulled out - you'd likely start a fire as the wire heated up.

apparentgenius
12-08-2008, 10:35 PM
Well, this thread is good timing for me. I have a receptacle box in my house that has 2 switches that control living room and kitchen lights on 2 different circuits. I recently changed the switches (wife wanted white instead of tan). The neutrals on these 2 circuits are wire nutted together in the receptacle box. In the panel, these are each on seperate 15a breakers. Is this also a problem?

Thanks,
glen

Speedy Petey
12-09-2008, 05:12 AM
That breaker should be changed to a DP RIGHT AWAY or the circuit should be disconnected and wire nutted.

If something happens to you, and some joker comes in and moves one of those breakers without checking to see that the neutral is shared on those circuits you have a SERIOUS risk for the people in that structure. Yes, double current on an unprotected neurtal.

I have seen melt downs from this. People move breakers to get a space in a panel and not check this because the wires are at the back of the panel and are difficult to see.
Read the bold parts! I am certainly for doing things the right way, but I am also NOT living my life, and doing my day to day work, to protect stupid people from themselves.
All we are doing in this country is "lawing" ourselves into a corner by trying to avoid frivolous lawsuits and protect unqualified and stupid people.
If you remember, Darwin had a theory about that.

If you are not familiar with electrical panels and do not know what a MWBC is STAY the HELL out of the panel!!

I know this is opinion on both sides, but ONCE AGAIN, see post #11.

hj
12-10-2008, 07:14 AM
IT would be too laborious to read all the extensive posting above, but whether it was stated or not, the asker should realize that if the neutal in a combined circuit fails for any reason, all the outlets will default to 240 volts.

beekerc
12-10-2008, 01:01 PM
the asker should realize that if the neutal in a combined circuit fails for any reason, all the outlets will default to 240 volts.

the OP does now. if there was an easy way to find the first junction point and string another wire from the panel, i would. I suspected there might be some issues but did not have specifics. my main point was to determine if this was kosher, even though (i assume) it was inspected and approved at time of construction 40 years ago. i was looking for "it's fine", "it's okay but not recommended", "we don't it any more but it's grandfathered", or "get that out of the wall right now".

Ideally, if i can split the lines, I will, but it may range from impossible to difficult.

thanks to all for the advice and information.

Speedy Petey
12-10-2008, 01:02 PM
HJ, what you say is not true. Any loads become connected in series and the resistance changes the voltage. With no load you simply have 120v to ground with no neutral.
This is all hinging on an open neutral. Wired properly and safely there is very little risk of an open neutral ahead of the multi-wire split.

This graphic is very simplistic.

http://www.ecmweb.com/images/archive/205ecm18fig1.gif

beekerc
12-10-2008, 11:36 PM
I'm still looking to see if it's possible (and not difficult) to run a new romex from the panel to the first junction and split the circuits - for reference, both sets of runs are buried in kitchen walls behind cabinets. i did, however, stop by my neighborhood McClendon's hardware store (if you're not familiar with the greater seattle area, it's a small local family chain of stores that has been around forever and they have the greatest staff working there) and picked up a pair of breaker tie clips (or whatever they're called) so that the breakers that share a neural, which have always been adjacent to one another, are now physically tied together. the eletrical guy there (and i think i read a reference here on this board) said i could just as easily stuff a short nail in between the breaker handles, but i wanted to go with a proper solution so i got the corresponding manufacturer's clips.

eh, not ideal yet, but at least i have double pole breakers now.

jwelectric
12-11-2008, 07:40 AM
the OP does now. if there was an easy way to find the first junction point and string another wire from the panel, i would. I suspected there might be some issues but did not have specifics. my main point was to determine if this was kosher, even though (i assume) it was inspected and approved at time of construction 40 years ago. It is important to understand that just because something was inspected in no way means that it is correct. Code officials are no different than any other people and make mistakes either due to not knowing the difference or just not seeing a violation




i was looking for "it's fine", It is perfectly fine and there is absolutely no danger in the installation at all as long as it was installed correctly and remains correct.
I make multiwire installations all the time as it is both cost effective and saves on labor


"it's okay but not recommended", Yes it is okay and it is also the only way to wire such items as dryers and ranges in dwelling units.


"we don't it any more but it's grandfathered", Not Grandfathered but a code compliant installation today


or "get that out of the wall right now". Only if the circuit is damaged and need replacing for reasons other than it is a multiwire circuit


Ideally, if i can split the lines, I will, but it may range from impossible to difficult.

thanks to all for the advice and information. I think that impracticable would be a better adjective.
There is absolutely nothing wrong with the installation and let no one instill fears concerning the installation. As in the experiment that I post with the two lights, if you lose the neutral coming from the service into the house the entire house becomes a 240 volt series circuit. This is the most common aliment with service drops.


eh, not ideal yet, but at least i have double pole breakers now. Never use a nail as this is nothing but hack type work. Even as a do-it-yourselfer you want to make a professional installation.
What you have is still two single pole breakers but they are tied together. It is possible that one would trip without opening both. A double pole breaker would open both if only one trips.

beekerc
12-11-2008, 10:38 AM
thanks for a very informative and thorough post jw.


Never use a nail as this is nothing but hack type work. Even as a do-it-yourselfer you want to make a professional installation.
What you have is still two single pole breakers but they are tied together. It is possible that one would trip without opening both. A double pole breaker would open both if only one trips.

i would not use a nail, it simply sounds hackish and amateur, looks bad and would be too easy to defeat, even accidentally, which is why i went with the OEM made bridge clip - granted it's a piece of stamped sheet metal folded the way it needs to be, but it is made by the same mfr as the breaker. as best i can tell a double pole breaker is simply two single breakers with the handles permanently bonded together, so the only difference is the strength of the bonding mechanism attached to the toggles. i can visualize the possibility of one breaker tripping but not taking the other one with it, but if this were a likely scenario, would the manufacturer have designed a bridge clip that would be stiff enough to esure the other breaker would forced to trip?

not to suggest that a company would make or market a dangerous product, but i suppose that from the profits that they make on these clips (probably less than $0.01 worth of metal and manfacturing, sold for more than a buck) the would be able to afford to pay off the (statistically derived) number of lawsuits that arose from malfunction, than to not make them and only sell double pole breakers or design and manufacture a better and safer clip.

so in the opinions of the pro's - are the bridge clips safe and reliable or not recommended.

i know, the argument - $15 for a double pole breaker vs $1.50 for the clip - is the safety of my house and my family worth the extra $13.50? the answer is obvious, but at some point all those little numbers add up to a big number and unless one is in the top tax bracket, one has to apply probabilities and likelihoods (and actuarial methods if so inclined) to determine what is really the smart and cost effective way to go.

jwelectric
12-11-2008, 12:09 PM
with the handles permanently bonded together, so the only difference is the strength of the bonding mechanism attached to the toggles. i can visualize the possibility of one breaker tripping but not taking the other one with it, but if this were a likely scenario, would the manufacturer have designed a bridge clip that would be stiff enough to esure the other breaker would forced to trip?.

A handle tie ties two single pole breakers together. Each of these breakers still has the ability to open independently of each other.
A two pole breaker has an internal mechanism that opens both poles in the event that one opens. Try pulling the handle tie off a two pole breaker and see if they are one breaker or two. The handle tie will go back on with a little work.
The handle tie means that when you turn one off you turn both off just as in a two pole breaker.