Shared neutral

Users who are viewing this thread

Beekerc

IT Consultant / Network Engineer
Messages
94
Reaction score
0
Points
0
Location
Seattle
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?
 

MaintenanceGuy

In the Trades
Messages
106
Reaction score
1
Points
0
Location
Pennsylvania
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

Licensed Electrical Contractor
Messages
1,108
Reaction score
9
Points
38
Location
NY State, USA
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

Computer Programmer
Messages
584
Reaction score
0
Points
0
Location
MN, USA
Website
www.billarden.com
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

Licensed Electrical Contractor
Messages
1,108
Reaction score
9
Points
38
Location
NY State, USA
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

Computer Programmer
Messages
584
Reaction score
0
Points
0
Location
MN, USA
Website
www.billarden.com
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

Electrician
Messages
607
Reaction score
0
Points
0
Location
Litchfield, CT
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

Licensed Electrical Contractor
Messages
1,108
Reaction score
9
Points
38
Location
NY State, USA
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

In the Trades
Messages
424
Reaction score
0
Points
0
Location
USA
Website
www.inspectpa.com
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

Licensed Electrical Contractor
Messages
1,108
Reaction score
9
Points
38
Location
NY State, USA
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

In the Trades
Messages
424
Reaction score
0
Points
0
Location
USA
Website
www.inspectpa.com
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

Electrician
Messages
607
Reaction score
0
Points
0
Location
Litchfield, CT
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

New Member
Messages
8
Reaction score
0
Points
0
Location
Tennessee
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.
 
Last edited:

Beekerc

IT Consultant / Network Engineer
Messages
94
Reaction score
0
Points
0
Location
Seattle
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

New Member
Messages
18
Reaction score
0
Points
0
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

New Member
Messages
1
Reaction score
0
Points
0
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

Retired Defense Industry Engineer xxx
Messages
32,770
Reaction score
1,190
Points
113
Location
New England
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

New Member
Messages
21
Reaction score
0
Points
0
Location
Oregon
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

Licensed Electrical Contractor
Messages
1,108
Reaction score
9
Points
38
Location
NY State, USA
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.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks