multi-wire branch circuit (shared neutral) question

[dcg9381]

I've got a (master) electrician wiring a residence for me currently. My electrical plan had 240V in a closet (30a). The electrician wired it with 10/3 (correct). He made a decision on his own, based on what he thought I had going into the closet (servers) and wired 2 x 20A outlets into the closet.

I asked him about it - he told me that he thought I'd prefer 20A x 2 circuits. I asked him how we wired it and he said he set it up as a "shared neutral".

So on his 10/3, he's got:

1) Single pole breaker, 20A (1)
2) Single pole breaker, 20A (2)
3) Common neutral

Seems to me that it'd be possible to overload that 10ga neutral (20A + 20A = 40A), which is not protected? Note, I haven't stuck my head in the breaker box to determine if there is a way he's managed to protect the neutral.

I can have him make it 240V @ 30A, of course...

 

[theoldwizard1]

Everything is correct. No worry about overload as each leg is 180° out of phase.

 

[Norcal]

If both breakers were on the same leg then a overloaded could occur but what about AFCI requirements since we are talking about a residence?

 

[aandpdan]

He should have used a double pole, two 20 amp breakers with a handle tie, breaker. As above, it most likely should be an AFCI.

 

[dcg9381]

Everything is correct. No worry about overload as each leg is 180° out of phase.

Thanks. That's the part I was missing.

If both breakers were on the same leg then a overloaded could occur but what about AFCI requirements since we are talking about a residence?

Our governing body is county, which (last I looked) was about 10 years out of date on NEC requirements. The breaker panel for this circuit is not in yet, but everything upstairs is AFCI/GFCI.

He should have used a double pole, two 20 amp breakers with a handle tie, breaker. As above, it most likely should be an AFCI.

Aren't all double pole breakers tie handle? Why not two single pole AFCI (no tie handles) - the netural is shared, but only the hot is protected?

 

[TRWham]

I thought this was supposed to be a 240 V circuit based on OP. If so, nothing is right about this installation.

 

[dogdog]

Thanks. That's the part I was missing.



Our governing body is county, which (last I looked) was about 10 years out of date on NEC requirements. The breaker panel for this circuit is not in yet, but everything upstairs is AFCI/GFCI.




Aren't all double pole breakers tie handle? Why not two single pole AFCI (no tie handles) - the netural is shared, but only the hot is protected?

#not all douoble pole breakers are tie..


Aside from overloading it's safety...

because if you overload one branch and tripped one breaker.. the other one still active and conducting electricity... as in a Circuit for the other leg... it will "electromacute" you. safety ?


say you have 30AMP overload L1 breaker for L1 popped. L2 breaker you decided not to tie it with the other one. It didn't pop, decided to poke around with a live neutral... fun things happens to make you smarter.

better to wire out two neutrals.... if you want 2x 20AMP 120V circuits.

 

[dcg9381]

I thought this was supposed to be a 240 V circuit based on OP. If so, nothing is right about this installation.

That's how I spec'd it. We can easily return it to 240V - that's no problem. Dedicated 2 x 120V circuits work too and make the equipment in there a little less expensive. The shared neutral threw me, but I didn't consider phase.

 

[dcg9381]

#not all douoble pole breakers are tie..

say you have 30AMP overload L1 breaker for L1 popped. L2 breaker you decided not to tie it with the other one. It didn't pop, decided to poke around with a live neutral... fun things happens to make you smarter.

better to wire out two neutrals.... if you want 2x 20AMP 120V circuits.

Live neutral - got it.. Makes sense.

I agree, better to do this as two separate circuits, we've got 10/3 in there... I can either do it as 240/30A or 2 x 120V/20A where either side pops both breakers.

 

[TRWham]

That's how I spec'd it. We can easily return it to 240V - that's no problem. Dedicated 2 x 120V circuits work too and make the equipment in there a little less expensive. The shared neutral threw me, but I didn't consider phase.

It’ll work either way. It’s just that for 240 you won’t need the neutral and it’s oversized for 20 A. And you will need to breaker for 20 A to use the 20 A receptacles.

 

[exranger06]

#not all douoble pole breakers are tie..


Aside from overloading it's safety...

because if you overload one branch and tripped one breaker.. the other one still active and conducting electricity... as in a Circuit for the other leg... it will "electromacute" you. safety ?


say you have 30AMP overload L1 breaker for L1 popped. L2 breaker you decided not to tie it with the other one. It didn't pop, decided to poke around with a live neutral... fun things happens to make you smarter.

better to wire out two neutrals.... if you want 2x 20AMP 120V circuits.

There's nothing wrong with having one leg of a MWBC tripped while the other is still hot. If you took 2 single pole breakers and handle-tied them together, you could STILL have only one leg trip and the other stays hot (a breaker can trip without the handle physically moving and shutting off the other breaker). The reason for the handle tie is if you were to work on the circuit and you shut the breaker off (meaning you have to manually flip the breaker), it forces you to shut off both legs. This ensures the neutral is not carrying current if/when you disconnect it from something.

If this circuit is going to be AFCI protected though, you HAVE to use a double pole breaker. This is because AFCI breakers need a neutral connected to them. So if you tried to use 2 single pole AFCI breakers, you'd need a neutral connected to each one. You can't take one shared neutral and split it off to 2 breakers; you won't have the right amount of current on the neutral and you'll "confuse" the breaker, and it simply won't work. With a double pole breaker, you connect both hot legs to it, and the one shared neutral to it. The breaker sees how much current is on BOTH hot legs AND the neutral, sees that all of the currents add up properly, and works fine.

 

[wyliesdiesels]

Everything is correct. No worry about overload as each leg is 180° out of phase.

how do you know he used a double pole breaker? if he used 2 single poles and theyre landed on the same phase, then its not correct and the neutral could very well be overloaded.

 

[wyliesdiesels]

Aren't all double pole breakers tie handle? Why not two single pole AFCI (no tie handles) - the neutral is shared, but only the hot is protected?

no. sq d QO double pole breakers have a single handle

A neutral NEVER has an OCPD protecting it unless the OCPD simultaneously opens the ungrounded/hot conductors at the same time. in a residential panel, this is not the case, so the neutral just goes straight to the neutral bus.

 

[TRWham]

no. sq d QO double pole breakers have a single handle

A neutral NEVER has an OCPD protecting it unless the OCPD simultaneously opens the ungrounded/hot conductors at the same time. in a residential panel, this is not the case, so the neutral just goes straight to the neutral bus.

It's true the neutral is not interrupted by the breaker in a residence, but it cannot carry any current that did not also go through the hot leg, so, unless there is a short to another circuit or lightning or some other calamity, it is effectively protected from overcurrent by the same breaker that protects the hot.

 

[dcg9381]

Wylie is right (as usual) - in the case here, there is "another circuit" in play. If I turned off the breaker to one outlet and grabbed the neutral on that outlet, I'm likely to get shocked by the other breaker being active. This is why these guys are saying that both breakers need to be tied together over-current protect at the same time (tie handle) and out of phase.

I was a dummie and didn't consider "phase" on the over current protection.. I came from a DC world.

So I need to verify:
1) 2 breakers, 20A (as the outlet limit is 20A)
2) breakers need to be out of phase in the box
3) Need to be tied together so that if one overprotects, they both go off.


Appreciate the help here.

 

[dogdog]

There's nothing wrong with having one leg of a MWBC tripped while the other is still hot. If you took 2 single pole breakers and handle-tied them together, you could STILL have only one leg trip and the other stays hot (a breaker can trip without the handle physically moving and shutting off the other breaker). The reason for the handle tie is if you were to work on the circuit and you shut the breaker off (meaning you have to manually flip the breaker), it forces you to shut off both legs. This ensures the neutral is not carrying current if/when you disconnect it from something.

If this circuit is going to be AFCI protected though, you HAVE to use a double pole breaker. This is because AFCI breakers need a neutral connected to them. So if you tried to use 2 single pole AFCI breakers, you'd need a neutral connected to each one. You can't take one shared neutral and split it off to 2 breakers; you won't have the right amount of current on the neutral and you'll "confuse" the breaker, and it simply won't work. With a double pole breaker, you connect both hot legs to it, and the one shared neutral to it. The breaker sees how much current is on BOTH hot legs AND the neutral, sees that all of the currents add up properly, and works fine.

I am not sure about this statement...

 

[pattenp]

I am not sure about this statement...

The statement is correct. Two single pole breakers with a handle tie provides independent trip of the two circuits. Whereas a double pole breaker is common trip so both circuits are disconnected on a fault.
The double pole or singles with a handle tie on a mwbc is a code requirement for servicing.

 

[wyliesdiesels]

I have actually seen some double pole riveted breakers with the handle tie that had no connecting internal components to initiate a trip on both poles despite the breaker being labeled common trip

 

[Terry D]

I have actually seen some double pole riveted breakers with the handle tie that had no connecting internal components to initiate a trip on both poles despite the breaker being labeled common trip

Siemens 2 pole breakers are like 2 single pole glued together. There is no connection between the two poles other than the tie handle

 

[dogdog]

The statement is correct. Two single pole breakers with a handle tie provides independent trip of the two circuits. Whereas a double pole breaker is common trip so both circuits are disconnected on a fault.
The double pole or singles with a handle tie on a mwbc is a code requirement for servicing.

The ones I see are what wyliesdiesels mentioned... they are riveted or cross bar mechanically together. and if one trips. the other will get turned off mechanically by the linkage. I rarely sees the other ones around my neck of the woods.. Used that for both 220V ( ) and MWBC but the city's upgraded electrical doesn't allow MWBC any more. Pretty sure I have read some one in the NEC requirements. they needed to mechanically trip the other 1/2... Not sure if there are other breakers definition of "tie" together means...

 

[Metal-Marc]

2x20 amps outlets in a closet for servers. Are you mining bitcoins or what?

 

[Firebrick43]

Wylie is right (as usual) - in the case here, there is "another circuit" in play. If I turned off the breaker to one outlet and grabbed the neutral on that outlet, I'm likely to get shocked by the other breaker being active. This is why these guys are saying that both breakers need to be tied together over-current protect at the same time (tie handle) and out of phase.

I was a dummie and didn't consider "phase" on the over current protection.. I came from a DC world.

So I need to verify:
1) 2 breakers, 20A (as the outlet limit is 20A)
2) breakers need to be out of phase in the box
3) Need to be tied together so that if one overprotects, they both go off.


Appreciate the help here.

What????. All neutrals are tied together, and in fact eventually all tied to ground. You shouldn't be able to get shocked because there is zero potential. Only way you could get shocked is to unhook the neutral from the hot outlet and then hold the neutral screw terminal on the hot outlet.

They used pigtails for the neutral to each outlet right?

 

[dogdog]

What????. All neutrals are tied together, and in fact eventually all tied to ground. You shouldn't be able to get shocked because there is zero potential. Only way you could get shocked is to unhook the neutral from the hot outlet and then hold the neutral screw terminal on the hot outlet.

They used pigtails for the neutral to each outlet right?

I think you have been misinformed by information. All neutral are tied at the panel but MWBC share that particulars neutral until it reaches the panel. It’s call a circuit for some reason.

 

[Firebrick43]

I think you have been misinformed by information. All neutral are tied at the panel but MWBC share that particulars neutral until it reaches the panel. It’s call a circuit for some reason.

It doesn't matter, you will not get shocked by a neutral that is connected correctly to the neutral bar of the panel, even if it has current going through it, as you have no potential in comparison. Maybe to make it easier. Wire is the path to ground, and a better one than you could ever be therefore any and all current goes along that conductor and not through you. You can only be shocked (we are talking house wiring) when you touch a live hot and your body is the path to ground.

Another test without fear, Take your multimeter and put one lead to the neutral screw of a hot circuit even with an appliance hooked up and on, and touch the other to any thing else including the ground wire. It reads zero. No potential.

 

[dogdog]

On this mwbc setup. If one circuit breaker is off the other one is not. Some one turned on the appliance on the one that is not. The neutral is the return path for electricity. If you touch it while it is energized and your body provided a easies path to ground. You get shocked.

 

[dogdog]

Try it sometimes, it might be fun and shocking

 

[Firebrick43]

On this mwbc setup. If one circuit breaker is off the other one is not. Some one turned on the appliance on the one that is not. The neutral is the return path for electricity. If you touch it while it is energized and your body provided a easies path to ground. You get shocked.

Your body can NOT be the easier path to ground if they neutral is hook up to the neutral bus bar. The copper neutral wire in all instances will be the better path to ground, even if you were standing in a pool of saltwater or licking the ground wire.

Don't trust me. Google "why you don't get shocked touching a neutral" There are videos and tutorials galore.

Why you don't get shocked touching a neutral

 

[dogdog]

I will let you read that first paragraph from your own link
You can basically test this theory your self, electrician/lineman wear rubber boots for some reason. Dunno

Can you get an electric shock from neutral?
2 Answers. The main points to note is that the neutral and ground(earth) wire at homes are at zero potential. ... So even the current returns through neutral (only from a connected load that completes the current flow circuit) you touching the neutral with a 0V cant get you a shock.Jun 12, 2015

 

[Firebrick43]

I will let you read that first paragraph from your own link

Can you get an electric shock from neutral?
2 Answers. The main points to note is that the neutral and ground(earth) wire at homes are at zero potential. ... So even the current returns through neutral (only from a connected load that completes the current flow circuit) you touching the neutral with a 0V cant get you a shock.Jun 12, 2015

Yep, read it, and your point? So the guys grammar isn't perfect when he says "cant get you a shock" instead of "you can't get a shock." My grammar is imperfect as well sometimes.

But there are many other links in that search list with the theory from professors and engineers not just the excerpt from another anonymous poster on a forum.

 

[dogdog]

I guess the NEC is wrong requiring the breakers to be tied on mwbc.

 

[sparky 1971]

Siemens 2 pole breakers are like 2 single pole glued together. There is no connection between the two poles other than the tie handle

That's what I used to think. Until I had a service call for a breaker that wouldn't reset. Siemens panel, and before getting there, I assumed I was going to have to show the customer to turn the breaker off completely, then on, collect $125 and move on down the road. Sure enough, the breaker wouldn't reset. Ok, bad breaker. When I went to take it out, the breaker either above or below came with it. It was a two pole with the handle tie taken off. I put it back in, and just for the heck of it, turned both off. The breaker in question would then reset. After verifying it wasn't a mwbc, I just replaced the used to be a two pole with two single poles.

 

[Firebrick43]

I guess the NEC is wrong requiring the breakers to be tied on mwbc.

Absolutely not. The NEC is there to protect people from fools and idiots. Many times its written in blood because some fool or idiot did something stupid and killed themselves(normally) or someone else heaven forbid.

My initial post said nothing of the code but was to the uniformed fear that somehow the OP could be shocked by a correctly installed neutral wire.
Don't confuse the "code" with the laws and discussion of physics. They are not always the same.

I followed through with additional post always being sure to mention "properly bonded" and hooked up neutral and about the neutral wires on the receptacles being pigtailed. I should have expanded that the wire nut should never be removed from said pigtail/wire junction but only the receptacle itself.

The NEC protects a fool working on a MWBC (with the tie bar) from unhooking the pigtails at the wire nut or unhooking the neutral from the neutral bar willy nilly. After doing this you could be shocked.

After doing this you still could not be shocked by a neutral, because once unhook it in that fashion its now a hot even if the wire is white. Hence why the NEC has similarly outlawed people just placing a piece to black electrical tape on a white wire to denote a hot.

But the biggest reason for a tie bar, as previously mentioned, if two separate breakers are spaced apart and end up on the same leg/phase you over load the neutral wire and cause a fire. Most of the NEC code comes from the NFPA . Using a double breaker and or tiebar ensures that the two circuits remain on opposite phases. Electrical fires kill people that thought their electrician knew the laws of physics(at least in regards to electricity)

 

[sparky 1971]

Absolutely not. The NEC is there to protect people from fools and idiots. Many times its written in blood because some fool or idiot did something stupid and killed themselves(normally) or someone else heaven forbid.

My initial post said nothing of the code but was to the uniformed fear that somehow the OP could be shocked by a correctly installed neutral wire.
Don't confuse the "code" with the laws and discussion of physics. They are not always the same.

I followed through with additional post always being sure to mention "properly bonded" and hooked up neutral and about the neutral wires on the receptacles being pigtailed. I should have expanded that the wire nut should never be removed from said pigtail/wire junction but only the receptacle itself.

The NEC protects a fool working on a MWBC (with the tie bar) from unhooking the pigtails at the wire nut or unhooking the neutral from the neutral bar willy nilly. After doing this you could be shocked.

After doing this you still could not be shocked by a neutral, because once unhook it in that fashion its now a hot even if the wire is white. Hence why the NEC has similarly outlawed people just placing a piece to black electrical tape on a white wire to denote a hot.

But the biggest reason for a tie bar, as previously mentioned, if two separate breakers are spaced apart and end up on the same leg/phase you over load the neutral wire and cause a fire. Most of the NEC code comes from the NFPA . Using a double breaker and or tiebar ensures that the two circuits remain on opposite phases. Electrical fires kill people that thought their electrician knew the laws of physics(at least in regards to electricity)

Nice post, but the reason for the handle ties or multi pole breaker is so that a circuit is turned off for service, all of the hot wires must be turned off. That way, as you stated, someone won't break the joint on the neutral and potentially "become the neutral" for the still energized circuit(s), or, with a three phase system, send 208 or 240 through a bunch of 120 equipment or 480 through 277. It's been a long time ago, but I have become the neutral, killed the circuit not realizing it's a mwbc, opened the joints at a box and got nailed on the neutral. I also remember about 25 years ago a foreman telling a story about how he sent 480 through about 30 brand new 277 florescent fixtures by doing the same thing.

It should also be noted that handle ties don't necessarily make them common trip. With ties, A breaker can trip and the other(s) will be fine. Especially with Square D. However, resetting will probably require turning both or all three to off simultaneously then on.

 

[MrSurly]

At the heart of this argument is a profound misunderstanding of the behavior of electricity. “Path of least resistance” is an outright falsity. Electricity travels *EVERY AVAILABLE PATH* at all times.


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[Firebrick43]

At the heart of this argument is a profound misunderstanding of the behavior of electricity. “Path of least resistance” is an outright falsity. Electricity travels *EVERY AVAILABLE PATH* at all times.


Sent from my iPhone using Tapatalk

When you get to 480 ish or even above, you statement is sort of or can be at times correct, but not 240 and below. Its easily tested with repeatable results that low voltage electricity in our homes will flow to neutral or ground along the wire unless that path is broken. Only when those paths are interrupted or bypassed will it find another path.

I would be happy to entertain your idea however. Have a source with repeatable proof?

 

[Metal-Marc]

I would be happy to entertain your idea however. Have a source with repeatable proof?

ohms law

The path with the smallest impedance will carry the highest current. The path with the highest impedance will carry the smallest current, etc.

 

[Terry D]

Who ever thinks you can not get shocked by a neutral is sadly mistaken. If that neutral has current on it and your body gets put in that circuit, you will feel it. Go up to a panel, grab the hot lug on the lines coming in, as long as you are isolated from ground, you will feel nothing, the current has no path. Now touch a some kind of ground, you will get knocked on your ***. This is how birds can sit on the bare wires and not die, no path to ground. Now do the same thing with the neutral, grab it, you feel nothing. Grab a ground with the other hand, still nothing. Why, because it is a the same potential as ground. And that doesnt happen on it own, its bonded in so many places before it gets in your house. Now undo the neutral and hold on to it and grab the lug. You have just put your body in the path of current, and as long as there is current on that neutral, you will get shocked. The same thing can happen on a ground. This is why you do not bond the neutral to ground in a sub panel, it puts current on the ground, and if someone undid that ground in the panel that it is fed from, you have a potential shock hazard. The color of the wire means nothing, they can all have current on them. So please do not try anything I just explained, These are just examples.

 

[theoldwizard1]

say you have 30AMP overload L1 breaker for L1 popped. L2 breaker you decided not to tie it with the other one. It didn't pop, decided to poke around with a live neutral... fun things happens to make you smarter.

I agree, better to do this as two separate circuits, we've got 10/3 in there... I can either do it as 240/30A or 2 x 120V/20A where either side pops both breakers.

You can not prevent STUPIDITY ! Anyone working inside of a receptacle box that does not use a non-contacting voltage tester deserves what they get !

 

[Terry D]

That's what I used to think. Until I had a service call for a breaker that wouldn't reset. Siemens panel, and before getting there, I assumed I was going to have to show the customer to turn the breaker off completely, then on, collect $125 and move on down the road. Sure enough, the breaker wouldn't reset. Ok, bad breaker. When I went to take it out, the breaker either above or below came with it. It was a two pole with the handle tie taken off. I put it back in, and just for the heck of it, turned both off. The breaker in question would then reset. After verifying it wasn't a mwbc, I just replaced the used to be a two pole with two single poles.

Yep your right, i looked at it again.

Sent from my SM-G960U using The Garage Journal mobile app

 

[dcg9381]

At the heart of this argument is a profound misunderstanding of the behavior of electricity. “Path of least resistance” is an outright falsity. Electricity travels *EVERY AVAILABLE PATH* at all times.

Electrically, this is right. Current scales to the path of least resistance, but will absolutely be present (to a lesser degree) in a high resistance path. I remember being slightly zapped as a kid grounding a home-made radio to a 1980s home's plumbing system..