different circuits in same multi-gang switch box?

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Old 10-19-08, 09:48 PM
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different circuits in same multi-gang switch box?

I know NEC puts significant restrictions on having different circuits suppling outlets in same box, but I haven't found any mention of doing this with switch boxes. I will have a four gang box near door (room light, light outside door, and two separate covered porch circuits). the porch circuits are already present, but I don't think these would pass current code. they have two separate breakers and use 14-2 wire to the current double gang box. I THINK there is just 14-3 leaving the box to the lights. each switch works every other light (switch A works lights 1,3,5,7 and B works 2,4,6,8)
 
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Old 10-19-08, 10:30 PM
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Code says "multi-wire" branch circuits are required to have "tie-bar" breakers so that if you turn off one circuit, you turn them all off (for safety). (210.4 B)

But I think switches wired "normally", yet on different circuits and in the same box do not fall under this requirement...

HOWEVER in situations like this, I like to install tie-bar breakers anyway and place a note in the electric panel explaining that the switches in that box are on separate circuits, are tie-bared for safety, and that these are 120V circuits.
 
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Old 10-19-08, 11:19 PM
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thanks. I knew that MWBC had to have common throw. this is little different though in that it is not using 14-3 from breaker with shared neutral. Yet they are not wired "normally" as you wrote because after switches the lights are wired using 14-3. I assume they combined neutrals at the switch double gang box, but won't know for sure what they did till I open it up. If they did this, Then I guess the two breakers MUST be on different poles or couldn't the neutral be overloaded?

side note-when dealing with two 120v circuits is putting in double pole breaker the same as putting tie bar between two single pole breakers?
 
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Old 10-19-08, 11:31 PM
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Yes if you have a shared neutral, both hots would need to be on different "legs" to "balance" out the load.

If they were on the same leg and the neutral was the same size as the hots, then it could become overloaded.

I've been told that double pole breakers have an internal mechanism to link them together in addition to the tie-bar. I don't know if this is true or not?
 
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Old 10-20-08, 06:24 AM
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All of the grounding wires from all circuits in the same box will need to be connected together.
 
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Old 10-20-08, 06:42 AM
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Originally Posted by Bill190 View Post
I've been told that double pole breakers have an internal mechanism to link them together in addition to the tie-bar. I don't know if this is true or not?
This is not true. If you break off the handle tie of a double pole (or triple for that matter) it will operate as single pole breakers. The only exception is Square D QO which only has one handle for the muiti pole breaker.

Your switches are not supplied by multiple circuits on the same yoke. No tie is needed. You could make a note of what circuits are in that box on the back of the cover plate.
 
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Old 10-20-08, 07:02 AM
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I assume they combined neutrals at the switch double gang box, but won't know for sure what they did till I open it up. If they did this, Then I guess the two breakers MUST be on different poles or couldn't the neutral be overloaded?
Neutrals from separate circuits should never be connected together.

As already stated, there are no general prohibitions about wires from separate circuits being used in the same electrical box. Just keep them separate (except for the grounding wires).

There is, however, a prohibition of including low-voltage and line-voltage wires in the same box without a divider.
 
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Old 10-20-08, 11:22 AM
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if I can't leave neutrals connected, couldn't you just eliminate one of them? You then have essentially a 3 wire multibranch circuit (hot and neutral from one 14-2 and hot from another 14-2 ) with the hots going to different switches is same box and then other side of switches connected to the hots of 14-3 wire, along with carrying the one neutral. would need to add tie between the breakers. although this gets complicated if have to use AFCI for outdoor lights on porch? This may all be moot because the wires to these circuits may be the NM with small ground that I mention in other post. therefore may need to re-do them anyway. but you all have answered the question about having multiple circuits in one box.

thanks
 
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Old 10-20-08, 11:51 AM
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if I can't leave neutrals connected, couldn't you just eliminate one of them?
No.

Although this can theoretically work if the hots come from different legs of the service, there are simple ways that this can turn into a hazard in the future. If somebody moves one of the breakers in the panel, you could instantly create a fire hazard. Furthermore, if the neutral ever becomes disconnected in any one of the boxes on the circuit, it will also create an instant hazard (creating a 240-volt circuit out of a 120-volt circuit).
 
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Old 10-20-08, 12:08 PM
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yeh I'm thinking I just need to replace it all. I guess 3 wire circuits have lost favor anyway from feedback i got on this site.

thanks
 
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Old 10-20-08, 05:47 PM
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There is nothing wrong with using 3 wire (black,red,neutral) as long as it is wired properly and the two hots are being fed from different phases in the panel. It's when you have both circuits on the same phase with two different breakers that you run into improperly shared neutrals similar issues.
 
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Old 01-10-09, 01:17 PM
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I just registered and was looking for other an answer to a similar question, which I will post separately. I was reading this thread and see that if two you are indicating that different circuit legs are sharing one neutral in a box, both breakers need to be linked to each other to prevent overloading the neutral. Am I getting this correctly? I seem to recall that this was a common practice in the past. My thoughts are that if the box was wired for 240 volt application, this would be OK. But if it was used to supply two separate 120 volt circuits sharing the one neutral, there is a good possibility that the neutral could be overloaded. What if each 120 volt load was drawing 12 amps each on a 2-pole circuit as described above? Would there would be 24 amps flowing in the neutral or as I think about it, would the current be zero amps, because of the two legs being 180 out of phase?
Thanks for letting me in to your discussion.
 
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Old 01-10-09, 01:24 PM
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if it is double pole breaker and occupying adjacent poles (out of phase to each other) then if each hot was carrying 12amps the neutral would be carrying 0 amps.
 
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Old 01-12-09, 05:14 AM
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Two 120V circuits sharing the same neutral would be called a Multi Wire Branch Circuit (MWBC) and the breakers would need to be on opposing legs.

The reason you would need a tie-bar breaker is for safety. Not good to turn off just one breaker and fiddle with the wires on such a circuit. If you disconnected the neutral, the hot from the other breaker could come around through an appliance and zap you!

Not to mention accidentally touching the hot from the other breaker or working on the wrong hot wire in the same box.
 
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