Electrical Faux Paux

[PeterTorresani]

I think that I'm in trouble on this one. By trouble I mean a lot of rework. Hopefully, someone has an easy solution.

The wiring layout in my new shop has the oultlets on alternating circuits so that I can use two outlets next to each other and be on differents cicuits. I used 3 wire cable so that I could use the red wire on even circuits and the black wire on odd circuits. Everything fine so far.

The inspector tells me that code in Minnesota states that I need to protect all circuits in a garage with GFI. When I install a GFIC as the first outlet on each circuit I can not get the GFIs to reset.

Thanks to an earlier post, I realize that the reason for this is that the neutral wire is carrying the load of both hot wires. I tested this by switching one of the circuits off and the GFI resets without a problem.

So for all of the electrical experts out there, is my analysis correct? If so do I have a easier solution than removing all of the sheetrock, vapor barrier and insulation so that I can run two separate 2 wire cables?

Thanks in advance for any suggestions

[Jim Dunn]

Peter they inspected it after you sheetrocked it? The only?, solution is a single neutral for each of the circuits, I believe. Sorry, maybe someone in the electrical field could give you a better solution than removing all the drywall.

[Brad Olson]

I'm suprised they approved your rough in like that since it was a garage.

In my opion 12/3 or 14/3 should only be used if you are having a split receptacle not for alternating circuits as it makes future repairs and troubleshooting a real pain as well as potentially unsafe if you have full current from two circuits going down the single shared neutral.

Of particular importance is safety. Down the line it is way to easy to leave half the wire energized by the second breaker and begin work on the wire and circuit and not realize that the third wire has power to it. Also since one of the wires is nutted off you can't directly test the extra wire in the box without uncapping it, again a safety hazard.

At the end of the day you really don't save much by using 12/3 in place of two 12/2 wires and you run into headaches like this

[Chris Rosenberger]

I believe that the electrical code requires that the circuits be tied to a double pole breaker in the panel so both circuits are off at the same time. A simple but costly fix would be to install a double pole GFI breaker.

[Ken Fitzgerald]

Torre,

Some others will jump in and answer your question, but I believe your analysis is correct. It's funny....I roughed in the electical service entry into my new shop and the electrical inspector inspected it and even suggested the 12/3 "edison" circuitry. I work in electronics and am neither an electrician nor an electrical engineer but to me it didn't seem like a GFCI would work correctly for the reason you stated. About that time somebody here at SMC, Chris Padilla, IIRC tried it while remodeling his garage and got the same problem you had. I wired mine 12/2 with 2 circuits down each wall. I saved because of Chis's lesson. You might try PMing Chris...I think he's the one who had the same experience you did.

Good luck!

[Chris Rosenberger]

I changed my mind after I made the post above. I am not sure that the double pole breaker would even work. A GFI trips when there is an imbalance between the hot & neutral. By using only one neutral, there would be an imbalance anytime you would be using both circuits at the same time. You would need to run another neutral for one of the circuits.

[Russ Filtz]

Did you run a separate ground with the 3-wire cable? If not, I think you really did it to the pooch! I think a shared ground is OK, but not a neutral, unless they were on separate phases, like in a 220 breaker. If so, the neutral might not "see" double current, somewhat like a 220 circuit not needing a neutral? Maybe a real electrician could chime in on this though.

[Barry O'Mahony]

Your analysis is correct. using a pair of single-pole GFCI's will not work.

What you've created is called a "multiwire branch circuit" in the NEC. These can be used to supply line-to-line loads (240V), line-to-neutral loads (120V), and combinations thereof.

If you are just supplying line-to-line loads (which is sounds like you are), then you may use separate breakers, unless you are doing a "split receptacle" arrangement where the top socket is fed by one circuit, and the bottom is fed by the other. In this case one must use breakers with tied handles, so that when you open one circuit you open the other. Even if you are supplied two outlets in the same junction box with the 2 circuits, which it what is sound like you are doing, then handle ties are a good idea - that way you can't turn off only some of the power to the j-box before working on it.

If the multiwire circuit supplies both 120V and 240V, you must go one step further, and use a "common trip" 2-pole breaker that interrupts current to both circuits in the event of an overcurrent event. This is to prevent the 120V loads on one circuit from being "backfed" through the 240V load.

So what to do in this case? You must provide GFCI protection to both circuits. You have two choices without re-wiring:

-- put GFCI receptacles at every outlet. "Pigtail" them into the branch wiring, i.e., pick off from the neutral and the appropriate line conductor and wire them to the LINE terminals of each receptacle. The LOAD terminals of all receptacles remain disconnected. Each receptacle provides GFCI protection for only itself.

-- Use a 20A, 2-pole GFCI breaker in the panel. These measure the current flowing through all three conductors, and trip when there is any imbalance. Higher-amp 2-pole GFCI breakers are radily available in home centers for use with hot tubs; you may need to go to an electrical supply house or mail order the 20A ones, though.

[Lee DeRaud]

If you are just supplying line-to-line loads (which is sounds like you are), then you may use separate breakers, unless you are doing a "split receptacle" arrangement where the top socket is fed by one circuit, and the bottom is fed by the other. In this case one must use breakers with tied handles, so that when you open one circuit you open the other. Even if you are supplied two outlets in the same junction box with the 2 circuits, which it what is sound like you are doing, then handle ties are a good idea - that way you can't turn off only some of the power to the j-box before working on it.

That tweaked my memory, since I had recently mapped all the circuits in my house. In one case, I have a two-plug outlet under the sink (disposal and dishwasher) on two separate circuits. The junction box next to the sink has the switch for the disposal, a switch for the light over the sink, and a two-plug outlet...on three separate circuits.

And come to think of it, there's also a combination switch/plug box in the downstairs bathroom that uses two separate circuits.

The house was built in 1976, no electrical mods since then, so was presumably up to code at the time and certainly would be more trouble to change at this point than it's worth. But it's definitely something I need to remember in the (admittedly unlikely) event I need to work on one of those locations.

[Rick Christopherson]

I am having trouble locating manufacturer’s spec sheets on 2-pole GFCI breakers, otherwise I would be stating this more authoritatively. With that caveat being said, I believe most 2-pole GFI breakers also monitor the current in the neutral, and therefore will work for your application.
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To the best of my knowledge, the 120/240-volt, 2-pole GFI’s look at the total current flowing through all three conductors, and if the result is not zero, then there is a fault. This accounts for any imbalance between phases that is carried through the neutral wire. The method is similar to using a clamp-on ammeter across several conductors, just much more sensitive.
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That being said, all you need to do is install a 2-pole 120/240 volt GFI breaker, and your setup will be fine. The coiled pigtail from the breaker goes to your neutral buss, and the load’s neutral connects to a third lug on the breaker.
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Unfortunately, these breakers are expensive. For about the same cost, you could probably install individual GFI outlets in each box. Keep in mind that any outlet that is dedicated to a single stationary tool (dust collector, bandsaw, etc.) does not require GFI protection as long as you use a single outlet (non-duplex).
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For your bandsaw, dust collector, etc. install a single outlet that the device plugs directly into. The reason why your refrigerator in the kitchen does not require a GFI outlet is because the outlet is dedicated to the one device, and is not accessible for other uses. In a simple sense, it is kind of like a hardwired device.
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[Mike Henderson]

Regarding the GFCI, you will have to put a GFCI outlet at each outlet in order to get protection.

Regarding using three wires to wire two circuits, these should be wired a special way. The power coming into your main box is three wire, two hot wires that are 180 degrees out of phase with each other, and a common (ground) wire. When you wire two circuits using three wires, you must make sure that the two circuits are wired to the two incoming hot wires (one to each wire). If you wire this way, you won't exceed the current carrying capacity of the common wire.

Think of it this way. If you plug a router into one circuit and draw 15 amps, those 15 amps will flow across one of the hot wires and the common wire. Now suppose you plug another 15 amp router into the other circuit and use both routers at the same time. The current in the common wire will be zero because the two currents are 180 degrees out of phase (Actually, they won't be zero because the motors change the phase angles slightly but the current will be very small). So if you do this right, you don't have to use bigger wire - you could use 14 gauge wire for a 15 amp circuit (for example) and you'd be safe (as long as no one ever switched the circuits at the main box).

Where permitted by code, electricians often use this trick to save running another wire. But IMHO it is safer to wire conventionally.

Mike

[Mike Henderson]

Let me add a comment about Rick's suggestion of using a single, dedicated outlet for certain tools and not using GFCI. This approach is approved for refrigerators because GFCI will, at a very low occurrence rate, trip for no known reason. And if they do, the food can spoil.

But the reason you're putting GFCI on your tools in the shop is to save your life. There's good reasons why the code requires them in a garage and in bathrooms. When you buy GFCI outlets in a bulk pack, they're not that expensive - please don't bypass the safety you can get with them.

Mike

[Mike Henderson]

One more thought. If you have your outlets in double boxes (two duplex outlets per box), you only have to use one GFCI outlet in the box. Just wire the second duplex outlet off of the GFCI (so that both duplex outlets are on the same circuit).

Mike

[Bryan Somers]

I was going to say tear it out, but Micheal may be right when he speaks about the phases cancelling each other out. Will check with the electricians tomorrow and post if it is not.

[CPeter James]

How about making one circuit out of the two and only using the black and white wires and leaving the red wire capped. I could learn to live with it if it meant tearing out everything. Do you really NEED two circuits or just think that two separate circuits would be better?

CPeter