Double circuit wiring - gfci protected?

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Old 03-05-02, 12:30 AM
Nagoy
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Double circuit wiring - gfci protected?

Hi - I'm planning a double circuit (2-pole 20A breaker, 12-3, shared neutral) in my garage to power tools, so I can run say a shop vacc on one circuit, and a sawdust generator of some sort (!) on the other, concurrently, with receptacle circuits alternating around the wall.

Being in a garage, I know such circuits should be gfci-protected. I had originally thought that wiring 2 gfci receptacles (one for black, one for red) at the start of the run would then protect each circuit's following regular receptacles. Then I started thinking, that would mean wiring to the 'load' terminal downstream - and since its a shared neutral, what effect would that have on the gfci protection?

So I ran a test circuit, and lo-and-behold, the red circuit tripped immediately - and only when removed from the scheme (ie capped before reaching any receptacles) did the black circuit test correctly (it tested as hot/ground reversed otherwise).

In this sort of set-up, do you have to wire all-gfci receptacles, each for single location protection then? Or is there a way of wiring using multi-location protection?

Thanks,
Mark

PS Test circuit is now OFF
 
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Old 03-05-02, 04:33 AM
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As you found out, a shared neutral won't work in a GFI circuit. Your options are:
- Put a GFI at each location, each wired to the line side.
or
- Run separate neutrals for each circuit. If your 3-wire goes back to the panel, you can still use part of it, as long as you split it into 2 2-wires before any GFIs. Put a GFI at the begining of each 2-wire, and wire downstream outlets to the load side.
 
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Old 03-05-02, 06:39 AM
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I agree with what abNORMal says.

One more option is to buy a two-pole GFCI breaker. They're not cheap, but it would be the simplest fix.

In general, just say "no" to shared neutral circuits.
 
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Old 03-05-02, 08:49 PM
Nagoy
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Maybe a gfci breaker's cheaper than lots of receptacles...

Not sure which way I'll do it yet, but can you just run a single white wire alongside the NM cable run, for independant neutrals?

Why do garages require gfci's anyway, irrespective?

Thanks!
 
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Old 03-06-02, 04:52 AM
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Not sure you understand - you don't need lots of GFCIs, just two. Run your shared neutral up to the line side of the GFCIs, one GFCI per side of the shared neutral circuit. From the load side of each GFCI run 12-2 to two sets of downstream outlets, one set on each circuit. You have now split your shared neutral circuit. You will be OK now because the neutrals are no longer shared on the GFCI-protected section of each circuit.

You can't run a single white wire outside the NM cable to add another neutral.

The general consensus on this board seems to be that shared neutrals, while code compliant if done right, are far more trouble than they're worth.
 
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Old 03-06-02, 06:34 AM
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Mike, he needs a bunch of GFCI receptacles if he wants to make do with the cable he's already run. abNORMal told him how (i.e., use only the line side of each GFCI).
 
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Old 03-06-02, 07:41 AM
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OK, now I'm confused. I'm assuming he already has the 12-3 run from the breaker box to his garage and does not want to replace it. In the garage, why can't he:

(1) Connect black and white from the 12-3 to the line side on one GFCI

(2) Connect red and white from the 12-3 to the line side of a second GFCI

(3) Connect 12-2 to the load side of the first GFCI and use it to power one set of outlets

(4) Connect 12-2 to the load side of the second GFCI and use it to power a second, independent set of outlets

His present problem is caused by the fact that the current in the shared neutral is different from the current in either hot wire, causing the GFCI to trip. In my scenario neutral is only shared from the breaker box to the GFCIs. From the GFCIs onward each circuit has its own neutral, so the GFCI is happy.
 
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Old 03-06-02, 07:48 AM
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OK, I just read abNORMal's post more carefully and realized that what I am suggesting is his second suggestion:
or
- Run separate neutrals for each circuit. If your 3-wire goes back to the panel, you can still use part of it, as long as you split it into 2 2-wires before any GFIs. Put a GFI at the begining of each 2-wire, and wire downstream outlets to the load side.
Sorry for the redundant answer.
 
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Old 03-06-02, 11:16 PM
Nagoy
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Sorry my own response was confusing, but yes, what John said! I should've just said "thanks" and left it at that

I hadn't quite grasped that the suggestion was to re-wire just from the (existing) gfci's - hence my not-so-well thought out question about running an additional neutral all the way back to the box, which is clearly unnecessary.

I think I was having a bad day?!

In any case, thanks again.
Mark

btw, why are shared neutral's not considered such a great idea? It will be a 'joined' (proper term?!) double breaker that I'm using in the panel.
 
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