Two lighting circuits through the same 3 gang box

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Old 05-21-13, 06:54 AM
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Two lighting circuits through the same 3 gang box

I am rewiring my upstairs. I plan for two of the three lighting circuits to go through the same three gang box. My original plan was for two 12/2 from two 120 volt breakers to go into the box. If somneone later works on the box, they should turn off both breakers to de-energise the whole box. A possible new plan would be to run one 12/3 from one 240 volt breaker to go into the box. This way the box would be de-energized by throwing only one breaker. I figure the one neutral in the 12/3 would be ok to serve both circuits since they are on different phases. Is this second plan reasonable? Is the second plan better than the first plan? Is the second plan OK by the 2011 code? Is there a better plan?
 
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Old 05-21-13, 07:19 AM
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Either plan will work and is to-code.

For your original plan, there is no code-issue with running two circuits into a box. Neutrals need to be kept separated, grounds all tied together. Some people recommend writing a warning on the inside of the switchplate cover "TWO Circuits, turn off breakers #11 & #14", but it's not required.

Your second plan of a multi-wire branch circuit (MWBC) is allowable too since you are using a 2-pole breaker to control it.

I would probably go with the MWBC since they are going to the same box... save a few dollars on wire, but 6 of one...

I would consider 14ga wire on a 15A breaker though since it's a lighting circuit, unless you have a need for 20A. 14ga wire is much easier to pull and use in boxes and is often used for lighting circuits since 15A is usually sufficient. Just a thought though.
 
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Old 05-21-13, 07:30 AM
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One issue you will have is the 2011 requires AFCI protection for the circuits you are adding. There is only one two pole AFCI and it may not be listed for use in your panel.

I would use 2 runs of xx-2 cable. Rarely are lighting circuits loaded to 20 amps so I would use #14.
 
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Old 05-21-13, 07:48 AM
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My original plan was for two 12/2 from two 120 volt breakers to go into the box. If someone later works on the box, they should turn off both breakers to de-energise the whole box. A possible new plan would be to run one 12/3 from one 240 volt breaker to go into the box. Is there a better plan?
While either method is allowable, the better practice is to use two individual 120V breakers to supply the two circuits in a MWBC, rather than one 240V breaker. The two breakers are mounted vertically adjacent to each other in the same two spaces that the 240V breaker would occupy and their handles are joined with a handle tie.

The advantage is that using the two individual breaker tied together allows either circuit breaker to trip independently of the other in the event of a fault. The handle tie insures that both circuits must be turned off before work can be done, thus de-energizing the box in this case, as you say.
 

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  #5  
Old 05-21-13, 11:01 AM
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I am rewiring my upstairs. I plan for two of the three lighting circuits to go through the same three gang box. My original plan was for two 12/2 from two 120 volt breakers to go into the box.
I'm curious. It sounds like you're planning to run three 20A circuits just for lighting on one floor of your house. Why so much power?

If we assume that lighting is a continuous load, those three circuits will be able to supply 48 amps, or 5,760 watts of lighting. What is the lighting load you're planning to install on this floor?

Others have already suggested running two 15A circuits, instead of two 20A circuits, for the two circuits you asked about here. Those two circuits alone can supply 24 amps of power. or 2,880 watts, to continuous loads. Do you need more than that? Why run a third circuit?
 
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Old 05-21-13, 11:05 AM
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Unless all the lighting was on for 3 hours or more you do not even need to limit the circuits to the 80%.
 
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Old 05-21-13, 01:28 PM
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Unless all the lighting was on for 3 hours or more you do not even need to limit the circuits to the 80%.
Of course, and it could be on that long. That said, I only used the 80% load calculation to show the minimum amount those circuits can supply. I'm willing to bet that one or two 15A circuits will supply everything the OP is planning to load on, with capacity to spare.
 
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Old 05-22-13, 06:51 AM
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I don't know what I was thinking when I suggested using a 240 breaker. I knew I would be using AFCI breakers. The three circuits will have a total of 22 recessed lights on them plus three fans, a chandelier, a bathroom sink light, and a porch light. The recessed lights say they can handle up to a 150W PAR38 bulb. I will probably be using compact fluorescent bulbs. I assumed that the code would require me to have circuits that could handle the max that could be loaded into those recessed lights. If this is not the case, then less circuits and 14 instead of 12 wire would sure be easier.
 
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Old 05-22-13, 07:38 AM
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You are correct about needing to size the circuit for the maximum bulb rating of the fixture, regardless of the actual bulb installed.
 
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Old 05-22-13, 11:37 AM
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I don't know what I was thinking when I suggested using a 240 breaker. I knew I would be using AFCI breakers.
Then you won't be able to pull 12-3/G and wire it as a MWBC unless you can find a 2-pole 20A AFCI breaker listed for use in your panel. Single-pole AFCI breakers each need their own neutral.

The three circuits will have a total of 22 recessed lights on them plus three fans, a chandelier, a bathroom sink light, and a porch light. The recessed lights say they can handle up to a 150W PAR38 bulb. I will probably be using compact fluorescent bulbs.
You're right. Since your recessed fixtures can be lamped for up to a total of 3300 watts, you need the three 20A circuits. You will, in fact, need to take care to divide your loads evenly between the three circuits.

I guess the good news is that you can install your three 20A single-pole breakers in any breaker position, and you don't have to worry about joining any of their handles.
 
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Old 05-22-13, 02:03 PM
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I will stay with the original plan of three circuits using 12/2 and three 20 amp AFCI breakers. I will label the inside of any wall plate covering a box with more than one circuit in it with the position of the breakers which need to be thrown to de-energise the box.
 
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Old 05-22-13, 03:03 PM
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I will stay with the original plan of three circuits using 12/2 and three 20 amp AFCI breakers. I will label the inside of any wall plate covering a box with more than one circuit in it with the position of the breakers which need to be thrown to de-energise the box.
Sounds like a plan. Do keep an eye on how you're distributing the loads on those three circuits.

BTW, I note the circuit(s) feeding any box on the back of the cover for that box as I discover them, even if it's only one circuit. Saves a lot of chasing and adds to certainty going forward.

FWIW

Sorry about all the blind alleys. Just trying to see if there was a more efficient way to do what you need.
 
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Old 05-22-13, 04:19 PM
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Be sure to tie the grounds from all the circuits in the box together.
 
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Old 05-22-13, 06:19 PM
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Thanks for the help and suggestions, especially the ones that might save me bucks or time, help me pass inspection, or increase safety.
 
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