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I would like to add a second circuit to my BBQ area. There is conduit running from the panel to the BBQ with three wires, hot, neutral and ground.

Can I just add a second breaker and run a second hot wire and use the existing ground and neutral. Or do I have to pull three wires? I will have a GFI for each circuit.

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The GFCI will specifically cause this to fail (aside from the safety issues with over-utilizing the neutral) since a GFCI measures hot and neutral current, and if they don't match within a certain tolerance, the breaker trips. In this case, you'd have the neutral acting as return for two hots, so the current would never equal and the breaker would always trip –  Steven Jul 17 '12 at 18:46
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@Steven, I suppose thats right for a GFCI breaker, but not so for a GFCI at the outlet, as it only checks its own loads and any "downstream" outlets, right? –  mac Jul 17 '12 at 19:14
    
Correct for a breaker since the entire load is on it –  Steven Jul 17 '12 at 19:14
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There is another potential issue, that nobody has touched on yet. If the original wires are in a sheath (NM sheathed cable for example), you cannot pull a separate ungrounded (hot) conductor through the conduit. –  Tester101 Jul 17 '12 at 20:29

2 Answers 2

If you want two breakers, you can't share a neutral and ground. The shared neutral and ground could be exposed to too much current, because they don't have breakers for protection, only the hot does. An example:

Assuming you've got an existing circuit with a 15 amp breaker and 14 gauge wire, you add a second 15 amp breaker, running a new 14 ga hot, and sharing the neutral and ground from the existing circuit.

You have one load plugged into the "old" outlet, drawing 10 amps. 10 amps comes out through your old breaker and your old hot, the old 15 amp breaker sees 10 amps and doesn't trip, and the 10 amps goes back through your old neutral.

Now you plug in another load to your "new" outlet, drawing 12 amps. 12 amps comes out through your new breaker and your new hot. The new 15 amp breaker sees 12 amps and doesn't trip. 12 amps goes back through your old neutral. Now you've got a problem.

Your old 14 gauge neutral is now carrying 10 amps from the old circuit + 12 amps from the new circuit = 22 amps: thats more than the rated capacty. You haven't tripped a breaker, but you've overloaded your wiring. This is a very dangerous situation.

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This is only a problem in an incorrectly wired multiwire branch circuit, where the ungrounded conductors are both connected to the same leg in the service panel. If there is one hot from one leg and another hot from the other leg, the neutral in your example will only see 2Amps. –  Tester101 Jul 17 '12 at 20:18

If the circuits are out of phase (from hot to hot you get 240v), then you can run a separate line without overloading the neutral. This is frequently done in kitchens to give two circuits using 3 wire (+ground) romex. But if the two circuits are in phase, then mac is right, you can overload the neutral and create a dangerous situation (fire, short, electrocution).

For the GFCI, you cannot share the neutral on the load side of the GFCI. Otherwise, current will be detected on the neutral that didn't pass via the hot and it will trip immediately. This means the GFCI cannot be in the breaker, but would need to be at individual outlets and only wired to the line side anywhere you have the shared neutral.

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my own house has this setup, and it confused the hell out of me until now! I saw a two-pole breaker labeled "kitchen", and not a single 240V appliance or circuit in sight. Is this commonly done on a two-pole? It would seem to me to make more sense to do it on two single-poles. –  mac Jul 17 '12 at 18:59
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@mac It's a good idea and may even be code to do it with a double pole to avoid a live wire in an outlet that you thought you turned off. –  BMitch Jul 17 '12 at 19:05
    
Thanks for the info, it looks to be safer if I pull the extra wires just in case someone moves the breaker at a later date and does not realize the common is shared. –  David C Jul 17 '12 at 19:09
    
Good point. I guess a downside is that an over current on one phase trips both "circuits," right? Is the advantage solely to save money on copper? I guess you also avoid pulling a second cable, which could be an advantage for a renovation. –  mac Jul 17 '12 at 19:09
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Multiwire branch circuits are required to be on a double pole breaker (with handle ties). This is so half the circuit cannot be shut off, and to avoid overcurrent on the grounded conductor (neutral). As @BMitch points out, if you have a neutral and 2 hots that are out of phase, you don't have to worry about overloading the neutral. For example, if you have a 10Amp load on one hot, and an 8Amp load on the other, the shared neutral will only see 2Amps. –  Tester101 Jul 17 '12 at 20:15

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