Multiwire circuits - opinions? experiences?

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Old 07-22-04, 11:03 AM
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Multiwire circuits - opinions? experiences?

In another <a href="http://forum.doityourself.com/showthread.php?t=170057">thread</a> moderator <a href="http://forum.doityourself.com/member.php?u=11297">John Nelson</a> said:
Originally Posted by John Nelson
When GFCI is required, it is not advisable to use a multiwire circuit. In fact, I don't recommend multiwire circuits at any other time either. The slim advantages seldom outweight the disadvantages.
I was planning to use multiwire "shared neutral" circuits in my new house under certain specific conditions (and prohibit it otherwise), and would to get opinions on this. The way I would be doing it is that it would be used to feed double duplex receptacles in a two-gang box (for 4 total places to plug in), be under control of a single handle 2-pole breaker, with that circuit used for no other purposes (though it might feed a 2nd box like it in the same room).

The reason I want to do it this way is because I want to have the extra capacity at that point (40 amps total at 120 volts instead of just 20 amps), while doing so under a single breaker (because it's in one box). There being no such thing as a breaker than can provide 40 amps total to 2 circuits while protecting them at 20 amps each, while shutting off both if either causes a trip, unless it does so on opposite poles, this seems to me like the only way to go. Of course my other option when sticking to my "one box, one breaker handle" rule is to have a lot more boxes. One of the locations I want to do this is in the kitchen where lots of extra boxes is going to be less attractive. And in the kitchen, I'm already going to be using a GFCI breaker instead of a GFCI receptacle because I don't want those kinds of receptacles there for a couple reasons.

Note that I will not be doing this kind of circuit where the circuit spans more than one room (which I can't do in the kitchen, anyway, since the code requires not sharing those receptacles with other rooms). There would be no "single duplex" outlet boxes on such a circuit. Also, to identify such a circuit, the receptacles would be oriented with the neutral prongs in the center and the hots apart (ground pin up on the left and down on the right), while any "double duplex" that is simply on one ordinary 120 volt circuit would be done the other way with the hot prongs in the center (ground pin up on the right and down on the left).

I do know about <a href="http://www.mysouthwire.com/Southwire/cda/content/article/0,1443,36_38,00.html">"double neutral"</a> cable. I might use such cable on these multiwire circuits to block the risk of a broken neutral. But are there any other disadvantages besides loss of neutral connection?
 
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Old 07-22-04, 11:32 AM
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There being no such thing as a breaker than can provide 40 amps total to 2 circuits while protecting them at 20 amps each, while shutting off both if either causes a trip
Sure there is. Except for a GFCI breaker, that exact same double-pole breaker you are planning to use now for the multiwire circuit can be used for two circuits, each with their own neutral! And you'll still have your "one box, one breaker handle" rule. Then if you want GFCI protection, use a pair of GFCI receptacles. The only time you'd really have to have a multiwire circuit is if you for some reason want to use the GFCI breaker rather than the GFCI receptacles. But a GFCI breaker is (1) more expensive, (2) requires you to walk farther to reset it, and, perhaps most significant of all, (3) when it trips, you don't know whether you have an overload or a ground fault (makes troubleshooting more difficult).

So you can still provide this same power in one box with two independent circuits rather than one multiwire circuit. Furthermore, in my opinion, two single-gang boxes separated by a few feet are usually more convenient than one double-gang box with two receptacles.

I do know about "double neutral" cable. I might use such cable on these multiwire circuits to block the risk of a broken neutral.
I see little point to such cable, and the sales of it support this. 12/2/2 cable costs more than twice 12/2 cable, so who wants it? Note that if you use 12/2/2, it is no longer a multiwire circuit.

My list of the advantages of a multiwire circuit:
  1. Run one cable instead of two.
  2. Less voltage drop (only significant when the run is long).
My list of the disadvantages of a multiwire circuit:
  1. More difficult and/or costly to provide GFCI.
  2. Extra requirement for neutral pigtails.
  3. Requires double-pole breaker which usually costs more than two single-pole breakers.
  4. Requires 12/3 cable, which in many lengths is more than twice the cost of 12/2 anyway.
  5. Reduces panel flexibility in that you can't later put these circuits on tandem breakers if you run out of space.
  6. Reduces panel flexibility in your ability to move breakers around later.
  7. Increases the likelihood that future modifications of the panel (either moving breakers around or going to tandem breakers) will create a fire hazard.
  8. More likely for the DIYer to screw up the installation.
  9. More likely for the DIYer to misunderstand what he's looking at later.
 
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Old 07-22-04, 12:24 PM
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What I meant in my reference to a two circuit breaker was one that would attach two circuits to the <i>same hot phase</i>. A normal 2-pole breaker attaches to 2 different "phases" (I'm misusing "phase" here to distinguish it from "pole" because "pole" can also mean number of conductor paths ... but this is still all about a single phase power system). I suppose I could take one of those 20+20 single space tandem breakers and attach the two handles together, but I don't know it would reliably switch the other one off it an overload trip happens (since the handle may move partially or not at all, depending on model and manufacturer).

And obviously I can't just run it all from a 40 amp breaker. That's why I was thinking it through as using an ordinary 2-pole 20 amp breaker on opposite "phases".
 
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Old 07-22-04, 12:45 PM
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I must confess that I don't quite understand. Are you still planning a multiwire circuit, and, if so, why?
 
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Old 07-22-04, 12:53 PM
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I'd like to focus for this post on just the kitchen receptacles (and why I am planning it the way I am). For my house design, the kitchen is probably the most significant room in terms of appearance, for me. A lot will go into it to get the "look" that I want. And every GFCI receptacle I've ever seen conflicts with that look. This is why I was looking at doing GFCI protection in the panel instead. But a possible option, if I can do this by code, is to go ahead and use GFCI receptacles. The catch is that they have to be in another room. Normally the code doesn't allow sharing receptacles in another room with those in a kitchen (or at least not the minimum two required by the code). But there is some code that waives things like GFCI requirements in certain areas for receptacles that are "not conveniently located". So what I'm thinking is having a bank of 2 to 4 GFCI receptacles in a semi-obscure location outside of the kitchen. It would be sufficiently convenient to go reset them as needed, but they would be located in such a way they wouldn't be useful for plugging things into. One possibility is to have them beside the breaker panel itself. A short piece of cable from the panel to the boxes with the GFCIs, then on from the load side of them to the kitchen. Would that conflict with the code regarding the sharing of the kitchen receptacles?

BTW, I will probably be putting a subpanel in a mini-closet right next to the kitchen, anyway. A mini-closet for this purpose would be a shallow closet just deep enough to hold the surface mounted panel, with a door wide enough to meet the access space requirement. The idea is to make it be a closet not at all convenient for storing anything significant, and certainly not for storing anything that can imped access to the panel (a full closet has this "power" to attract junk that fills it up, and hence is inappropriate for placing an electrical panel).
 
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Old 07-22-04, 12:55 PM
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Originally Posted by John Nelson
Requires double-pole breaker which usually costs more than two single-pole breakers.
The list prices in the Square D catalog for GFI breakers (prices do not vary by amperage) are ... for Homeline: $141 for 1-pole and $275 for 2-pole ... for QO: $155 for 1-pole ($321 for 22k AIR) and $275 for 2-pole.

The list prices in the Cutler-Hammer catalog for GFI breaker (prices do not vary by amperage) are ... for CH: $168 for 1-pole and $298 for 2-pole ... for BR: $159 for 1-pole and $280 for 2-pole.

Also AFCI breakers in 2-pole (Cutler-Hammer makes them but Square-D does not) have a similar slight savings of 2-pole over 2x 1-pole. I am considering AFCI in a few places beyond the bedrooms.

But that is list pricing. It could very well be that street and box store pricing discounts the 1-pole devices a whole lot more than the 2-pole devices. Do you have any typical discounted retail pricing?
 
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Old 07-22-04, 01:05 PM
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Originally Posted by John Nelson
I must confess that I don't quite understand. Are you still planning a multiwire circuit, and, if so, why?
I have been planning it before the post that started this thread. I'm not making any decision changes just yet. I'm exploring my options with the feedback in this thread and will eventually make a decision. But the idea of just using two cables of 12/2 looks attractive. I can't really think of a safety reason where 12/2/2 beats 2x 12/2. And since this will be new construction, there would not be much labor advantage to 12/2/2 over 2x 12/2. So I'm probably going to end up going the way of 2x 12/2. The GFCI issue is still up in the air for me, but either 12/2/2 or 2x 12/2 leaves open options for that decision.

Would I have to derate 2x 12/2 going through a hole in 1 or 2 layers of 2x4? What if I use 2x 10/2?
 
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Old 07-22-04, 01:07 PM
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Judging by my local Lowes. If you pay more than $40 for a GFCI or AFCI single pole breaker, you're getting scammed.

They don't have 2-pole breakers on their website, probably because they arent very popular.
 
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Old 07-22-04, 03:21 PM
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bink

I agree with John Nelson. I really respect his expertise. I think multiwire circuits are a pain in the Bu*t. I have it in my garage/workshop and if the power to the saw/(whatever) is ezcessive the light go out also. The little extra cost and labor to keep them separate is well woth it. JMO
 
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Old 07-22-04, 04:24 PM
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bink:

That's one of the reasons I posted my question, to get expertise opinions. I was looking for special cases where it might be OK and if my case would fit. But the answer I'm getting is that there are better ways (except, I'm sure, for specific dedicated equipment that needs a multiwire circuit).

BTW, I don't intend to have any light fixture on the same circuit as any receptacle except a switched receptacle designated for lights (this lets me follow my "one breaker throw kills all power in a box" rule with both an overhead light and a switched receptacle on a 2 switch box by the door of a room).
 
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Old 07-22-04, 04:35 PM
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[QUOTE=Skapare]I'd like to focus for this post on just the kitchen receptacles (and why I am planning it the way I am). For my house design, the kitchen is probably the most significant room in terms of appearance, for me. A lot will go into it to get the "look" that I want. And every GFCI receptacle I've ever seen conflicts with that look. This is why I was looking at doing GFCI protection in the panel instead.

Have you considered using plug mold? Not that it will solve your GFI problem outright, but running 2 circuit plugmold along the bottom edge of your upper cabinets makes for a very clean looking install (virtually invisible if you set it up right.)
 
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Old 07-22-04, 04:41 PM
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The solution is a dead front GFCI. It installs like a GFCI receptacle, but it has no place to plug anything in. Thus they can be installed on the kitchen small appliance circuits, but outside the kitchen. This gives you everything you want: the appearance you want for the kitchen, avoiding the high cost of a GFCI breaker, and full code compliance.
 

Last edited by John Nelson; 07-22-04 at 04:51 PM.
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Old 07-22-04, 07:48 PM
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Originally Posted by CSelectric
Have you considered using plug mold? Not that it will solve your GFI problem outright, but running 2 circuit plugmold along the bottom edge of your upper cabinets makes for a very clean looking install (virtually invisible if you set it up right.)
If that is code compliant, I will look into it. Are you referring to the kind that would have the outlet facing downward?
 
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Old 07-25-04, 05:02 PM
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As long as you protect the circuits with a GFI breaker, or faceless GFI (as John suggested) it is legal. Art. 210.52(C)(5) allowes a maximum recept. height of 20" above a countertop. In a typical kitchen, the top cabinet begins 20" above the countertop, thus making a plugmold install under the top cabinet legal, as long as it is GFI protected and meets the 2 circuit requirement. I've done several installs of this type recently and the homeowners have universally loved it. You get a recept every two feet (well more than code requires) and they are generally hidden from view.
 
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Old 07-25-04, 06:26 PM
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bink said:
I have it in my garage/workshop and if the power to the saw/(whatever) is ezcessive the light go out also.
ARE YOU SERIOUS?....I would correct this before the next time I used my saw. Should be on seperate circuits for safety's sake...i.e. - you don't want the lights going out when your hands are around a saw blade (which would still be spinning even without power).

I know that wasn't the purpose of this thread, but I just had to say it.
 
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Old 07-25-04, 07:04 PM
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Are there any references (links) to plugmold devices you can give (manufacturers preferred, as opposed to just online sellers).
 
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Old 07-25-04, 07:34 PM
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Try this link for starters. the page linked to will show you the product in question. You can also jump from that page to a list of distributors. Plugmold is a pretty common item, it should be easy to find at a supply house or home center near you.

http://www.wiremold.com/www/consumer...view/index.asp


I am looking into some alternate brands, to see if anyone makes a plugmold type product with GFI's instead of conventional recpts.
 
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Old 07-25-04, 08:34 PM
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Using the same phase for both circuits with a 3 wire system (one neutral) is illegal and must be on 2 different phases. This is a fire hazard and should not be done.
 
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Old 07-25-04, 10:37 PM
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Originally Posted by lburtell
Using the same phase for both circuits with a 3 wire system (one neutral) is illegal and must be on 2 different phases. This is a fire hazard and should not be done.
You can do that if you have a dual-neutral cable, or just run 2 separate cables side by side. The catch is having a single breaker that shuts both off at the same time. I've never seen such a beast (for the same phase), though I suspect it could be made from a single space (two circuit) tandem breaker.

I had planned all along that the two circuits in the same box with be opposite phases so I could have them on a single handle 2-pole breaker. That can be done with shared neutral, dual neutral, or just dual cable. I'm inclined to do the dual cable thing now with those opposite phase circuits. While NEC only requires circuits on the same yoke to be disconnected by a single handle, my own rule for my house construction applies that to a whole box, even for light switches if they have full voltage running through them (this won't apply to switches with low voltage control circuits operating lighting relays or contactors somewhere).
 
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Old 07-26-04, 08:02 PM
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Ok
There is a double pole tandem breaker. This would still make you put the two different feeds on two seperate phases. I am guessing that you want to set up multiwire feeds and use tandem breakers in your panel? I do not understand why else you would want to use the same phase for the three wire system.
Now as far as you statment that you can use a 2 neutral wire cable (12-2-2) and install it on the same phase that is true. But you can not use a 12-3 wire and install them on the same phase with only one neutral being shared. I hope you understand that, it is a fire hazard. In essence you are installing 2 - 20 amp circuits on the 2 black feeds and trying to complete the circuit with one white (one wire with 40 amps possible that is rated for 20 amps) wire comming back to the panel.
 
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Old 07-26-04, 10:29 PM
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Originally Posted by lburtell
Ok
There is a double pole tandem breaker. This would still make you put the two different feeds on two seperate phases. I am guessing that you want to set up multiwire feeds and use tandem breakers in your panel? I do not understand why else you would want to use the same phase for the three wire system.
Now as far as you statment that you can use a 2 neutral wire cable (12-2-2) and install it on the same phase that is true. But you can not use a 12-3 wire and install them on the same phase with only one neutral being shared. I hope you understand that, it is a fire hazard. In essence you are installing 2 - 20 amp circuits on the 2 black feeds and trying to complete the circuit with one white (one wire with 40 amps possible that is rated for 20 amps) wire comming back to the panel.
It is not the case that I want to use the same phase in a three wire (shared neutral) system. Instead, I was pointing out one of the many reasons it is not a good idea. It is something that could be done with the following:<ul><li>one handle dual circuit breaker that stabs the same phase (maybe from just one stab).<li>proper neutral capacity, such as separate runs of 12/2 for each of the circuits</ul>But I have no specific need to do it that way. I would be just as happy to do it with opposite phases. The reality is that to do it where both circuits operate under the same handle, it has to be done on opposite phases. That's fine by me.

The issue at hand is whether to have the GFCI protection for the kitchen receptacles wired this way done by<ul><li>a 2-pole GFCI breaker</ul>or done by<ul><li>a normal 2-pole breaker<li>2 sets of GFCI receptacles leading the string(s).</ul>The later is definitely lower cost. The former is still being considered. It has the advantage of:<ul><li>every way to disconnect the circuit always disconnects everything in the same box</ul>but has the disadvantages:<ul><li>cannot tell if breaker trips due to overload or ground fault<li>higher cost</ul>But this is something I can probably defer until the time of construction, which is still quite some time off, since I don't think I would need to do any plan changes to make that decision.

An alternative I thought of instead of the pair of GFCI receptacles is to use of those "Spa boxes" which consist of a 50 amp GFCI breaker. It would follow the 20 amp normal branch breaker in the panel. Since it would be used only for ground fault protection, not overcurrent protection, I would think the code would allow this (but certainly if you think otherwise, please tell me how it would prohibit it). If only the 20 amp branch breaker trips, I would suspect an overload. if only the GFCI breaker trips, I would suspect a ground fault. If both trip, I would suspect a solid short fault.

GFCI receptacles are certainly less expensive. But the ones I have handled just don't seem to me to be robust enough. I've also experienced failures of GFCI receptacles, although I don't know if that's just limited to older ones, or if the failure mode I've experienced could also occur with GFCI breakers. The GFCI receptacles do not have much movement of contact mechanism, it seems, when pressing the reset button after it pops out. Maybe I need to sacrifice one of these things and tear it apart just to see how well (or not) it is made.
 
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