Two circuits sharing neutral?

[hchinaski]

I'm looking at putting GFCI outlets in my detached garage, and found something that surprised me when I opened the double gang outlet box that sits on top of the conduit that leads back to the breaker panel in the house.

There are two hot wires, (each controlled by a breaker in the main panel in the house), but only one neutral wire.

Is this legit or did someone cheap out on running a second neutral wire?

Thanks for any advice you can give!

 

[hemiredneck]

Not an electrician, but I've seen it lots of times in buildings with EMT conduit and THHN...I asked about it myself and they referred to it as 'networking'

 

[ishiboo]

Google MWBC.

It's legal, however using two individual (non-tied) breakers is not, if it ever was. The neutral shares the imbalance between the two, so if there are two 20A circuits, the neutral will have 0A if both are loaded to 20A, 10A if one is loaded to 20A and one is loaded to 10A, etc.

Installing GFCIs will not be easy.

 

[Barnabas]

This works only if the two circuits are on different phases. That means there is 240 volts between the two hot wires. Using a double-pole breaker insures this is done correctly. ishiboo is correct.

 

[malibu101]

GFI's can be installed on a MWBC (multi wire branch circuit) and will operate properly
HOWEVER
the GFI outlets can not be fed-through.

You need a GFI outlet at each location.

Barnabas's above post is on the mark too.

 

[Charles (in GA)]

For clarity, what is being described is known as a Multi-Wire Branch Circuit (MWBC) or sometimes called an "Edison Circuit".

It used to be quite legal to have two separate breakers that the handles are NOT tied together on, and (if I recall without looking at the code) there is at least one circumstance where that is still allowed. That being said....... it is a very poor practice to power a MWBC any way but with a double pole (handle tied) circuit breaker. This insures that (1) the two hots are on opposing "sides" of the bus, and (2) that when the breaker trips, or is purposely tripped, it shuts off both sides of the circuit so you don't accidentally end up working on a hot circuit.

Charles

 

[Zeke]

The question then might be is there a double pole GFCI breaker?

 

[hchinaski]

I'm thinking that this is yet one more reason to update the 1982 Zinsco panel in the house to something bigger and more modern.

Right now, one of the branches is feeding the lighting and the garage door opener, with the other feeding the wall outlets. This makes sense to me: trip the breaker using a tool and the lights stay on; but if you're saying I need to have the handles tied together, then that goes out the window.

When I get back out there tomorrow I'll measure the voltage across the two hot wires and see what I get.

Thanks to all of you for the expertise!

 

[Junkman]

The question then might be is there a double pole GFCI breaker?

Yes... I bought a couple in the past that were Square D brand. Picture upon request.

As to two hots and one neutral, that is the way that the fans in my living room were wired. It is a 12 gauge romex with a black, red, white, and a bare copper ground. This was done 30 years ago by a licensed electrician, and signed off by the electrical inspector.

 

[malibu101]

I'm thinking that this is yet one more reason to update the 1982 Zinsco panel in the house to something bigger and more modern.

Right now, one of the branches is feeding the lighting and the garage door opener, with the other feeding the wall outlets. This makes sense to me: trip the breaker using a tool and the lights stay on; but if you're saying I need to have the handles tied together, then that goes out the window.

When I get back out there tomorrow I'll measure the voltage across the two hot wires and see what I get.

Thanks to all of you for the expertise!

Let me be the first to say it.
When you say you have a Zinsco panel the first thing is to replace it.

NOT to scare you but they are notorious for having failures where the breaker stabs into the buss. Google it.

And while you are updating from the Zinsco going 200A is the only wise move.

 

[malibu101]

When I get back out there tomorrow I'll measure the voltage across the two hot wires and see what I get.

Thanks to all of you for the expertise!

IF you don't have 240V across the 2 hot wires you have a problem.
You could overload the neutral.

 

[Gooch]

Yes... I bought a couple in the past that were Square D brand. Picture upon request.

As to two hots and one neutral, that is the way that the fans in my living room were wired. It is a 12 gauge romex with a black, red, white, and a bare copper ground. This was done 30 years ago by a licensed electrician, and signed off by the electrical inspector.

That's not the same as a MWBC. your red and black wires going to the fan are switch legs and are the same circuit, which only requires the one neutral.

 

[malibu101]

Let's not forget another "problem" with a MWBC----
IF the neutral opens, the 120V decives downstream will see 240V and probably let out their magic smoke. Granted wires coming undone should never happen.

Again, MWBC are allowed, safe, save wire, and make sense for some circuit runs but they must be used properly.
Just like everything else electric.

 

[Junkman]

That's not the same as a MWBC. your red and black wires going to the fan are switch legs and are the same circuit, which only requires the one neutral.

The fans are 120 volt, and I would have to check, but I believe that there is separate breaker for the red wire, and one for the black wire. I don't remember seeing a red and black on one breaker, but I guess that it is possible. It was done this way for a reason, but I have no idea what the reason was, other than the fan switch pulses the wire to make it change speed, or turn on the lights. The fans are Casablanca Intella Touch fans... circa 1984.

 

[C96]

It's legal, however using two individual (non-tied) breakers is not, if it ever was.

It absolutely was legal all the way up until the 2008 NEC.

If the OP’s wiring was done prior to that, it’s totally legal.

Problem is, if he starts remodeling, modifying or adding to it, he is then required to update the system to current NEC codes. Not a big deal though, nothing a simple handle tie won’t cure.

Also, no problem installing GFCI receptacles on a multiwire circuit.

As far as the Zinsco panel is concerned, most definitely change it out. They have a huge problem with melting down, it’s a known fact.

 

[ishiboo]

It absolutely was legal all the way up until the 2008 NEC.

If the OP’s wiring was done prior to that, it’s totally legal.

Problem is, if he starts remodeling, modifying or adding to it, he is then required to update the system to current NEC codes. Not a big deal though, nothing a simple handle tie won’t cure.

Also, no problem installing GFCI receptacles on a multiwire circuit.

As far as the Zinsco panel is concerned, most definitely change it out. They have a huge problem with melting down, it’s a known fact.

As I said, "if ever" as I wasn't sure the history. Keeping single breakers on a MWBC is a very bad idea IMO and one of few things I'd update to "current" code.

 

[Norcal]

This works only if the two circuits are on different phases. That means there is 240 volts between the two hot wires. Using a double-pole breaker insures this is done correctly. ishiboo is correct.

Different "phases"? It is residential so doubt there is 3 phase.


To the OP, what size is the conduit? Depending on the size you could pull the other conductors & pull larger conductors then add a subpanel, your only allowed 1 circuit to a outbuilding so the multiwire circuit has to stay or a sub.

If you have a 1982 Zinsco/Sylvania panel, that would be right around when they were being discontinued (good riddance) as the replacements are really costly for ChiCom UL classified breakers, it's a good time to get rid of it.

 

[Mustang51js]

The phases he was talking about was with single phase and using two separate breakers putting them on the same phase. As far as gfi outlets it's normal to use a mwbc and I use it all the time in kitchen counter circuits. You just need to wirenut the neutrals on the line side with a tail to outlet.

 

[Norcal]

The phases he was talking about was with single phase and using two separate breakers putting them on the same phase. As far as gfi outlets it's normal to use a mwbc and I use it all the time in kitchen counter circuits. You just need to wirenut the neutrals on the line side with a tail to outlet.

Since it's single phase, different legs would be more correct. Yes, it's not a problem to tap from the multiwire circuit to add the GFCI receptacles

 

[teamextreme]

Since it's single phase, different legs would be more correct. Yes, it's not a problem to tap from the multiwire circuit to add the GFCI receptacles

Now you're splitting hairs. Yes, it's single phase, but everyone in the industry refers to the 2 hots as different "phases".

 

[Zeke]

Let's not forget another "problem" with a MWBC----
IF the neutral opens, the 120V decives downstream will see 240V and probably let out their magic smoke. Granted wires coming undone should never happen.

Again, MWBC are allowed, safe, save wire, and make sense for some circuit runs but they must be used properly.
Just like everything else electric.

I don't agree with this. If the neutral is open there is no circuit to the device(s). The only way a device could see 240 is to be connected to both sides of the buss.

If I'm wrong please tell me. I do know that there is potential in any box containing both legs because a circuit could be made by crossing 2 hots or crossing a hot to ground.

 

[malibu101]

When the neutral goes away the only return path is through the other "phase". So if there are 2 devices turned on on each "phase" the 2 hots will find each other through the devices.
"Phase" to "phase" equals 240V.

I have seen this happen more than once.

 

[teamextreme]

I don't agree with this. If the neutral is open there is no circuit to the device(s). The only way a device could see 240 is to be connected to both sides of the buss.

If I'm wrong please tell me. I do know that there is potential in any box containing both legs because a circuit could be made by crossing 2 hots or crossing a hot to ground.

Nope. If you drop the neutral in a MWBC you get floating odd voltages between each "phase" That's probably as much a reason for requiring the tied breaker handles a few years back as it was to prevent other voltages present in a box being worked on. It's also why neutrals connections are required to be made independent of the device in a MWBC (ie; you can't use the neutral terminals on a recep, you have to pigtail it). I learned this the hard way and smoked an answering machine once. The voltage feeds back thru any devices plugged into the circuit, so the voltage will vary based on the impedence of the loads. You won't get 240v, it will be split between each circuit, so you may have 170v on one and 70v on the other, or some other combination.

 

[teamextreme]

I found an article that includes a schematic to show what I was trying to describe.
http://ecmweb.com/content/multiwire-branch-circuits-can-be-dangerous

 

[malibu101]

I found an article that includes a schematic to show what I was trying to describe.
http://ecmweb.com/content/multiwire-branch-circuits-can-be-dangerous

Excellent link!
Shows what I was saying, and, shows what I said was wrong.
The devices won't see 240V but they will see enough higher voltage to possibly be harmed.

Thanks!

 

[Zeke]

I didn't think about the pigtails in the branches being tied together. The light just came on. Thanks.

 

[Stuff]

Since it is a detached garage you only are allowed one circuit/feeder. A shared neutral makes it a MWBC, legal, and common. Note you still need a single point disconnect in the garage for both phases.

 

[electrican]

The question then might be is there a double pole GFCI breaker?

yup!

 

[sberry]

I do it on occasion where I have long runs and there is a chance for loads to run opposite to each other and can reduce a wire in a pipe. I did a kitchen counter top the other day where the panel was right underneath, no reason to add a jbox, no wire to save, no V drop issues to gain from and even though there were 2 new duplex only one would be used at a time. Wire right to panel.

In my own house, early 20 something wife, tribe of kids, original 15A wasn't going to cut it, pull one 12/3g cable 70 ft. Another one wouldn't have hurt but it works. Removed a 30 from electric dryer and replaced with a DP20. GFCI recept on one leg on the user end.

 

[Mustang51js]

I use 12-3 all the time for kitchens, I will be using 14/3 a lot more for bedroom outlets in the future when they have the arc fault outlets more available

 

[n8n]

what size wire/amperage breaker is feeding the garage?

I'm thinking put a subpanel in the garage for more flexibility, if possible.

 

[exmaxima1]

When the neutral goes away the only return path is through the other "phase". So if there are 2 devices turned on on each "phase" the 2 hots will find each other through the devices.
"Phase" to "phase" equals 240V.

I have seen this happen more than once.

That's a very common problem with the outside wires to the house---if the neutral gets broken in a storm or from years of weathering, you end with much higher voltages in the house. One of my co-workers burned up his frig, washer, and dryer due to a bad neutral outside his house.