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Running from my house panel to my garage I currently have a 12-3 wire split into two circuits with a common ground, each circuit has a 15 amp breaker. I know 12 gauge wire can support a 20 amp but my question is can I run one circuit on a 20 amp breaker and the other on a 12 amp breaker with a common ground.

My reason for doing this is I want to run a slightly larger heater in the garage on colder days when I am working in there and once in a while it will trip the 15 amp breaker when it kicks on.

The garage currently has one 15 amp outlet on one circuit (the one I want to change to 20 amp) and the other circuit has a two 15 amp outlets and 2 50 w lights.

I hope this is clear. I look forward to the answers

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How long is the 12/3 cable run between your house and the garage subpanel? Does the cable run underground, through conduit, or is it suspended above the ground (ariel cable) at any point? Is the cable unbroke (a single cable) from panel to panel? What size grounding conductor is included in the cable? –  Tester101 Apr 22 '13 at 19:55
    
Cable is about 40 feet unbroken from panel to garage, there is no sub panel in the garage the cable runs directly into an outlet on both circut. The bare ground wire seems to be 14 guage but I am not 100% on that the actual wire is CSA Type NMWU 3-13 300V FT1 LL39965 –  John Apr 22 '13 at 20:37
    
My other thought was to simply change the two 15 amp breakers to a 20 amp two pole breaker (probably GFI) and upgrade all 3 outlets to outlets to 20 amp in the whole garage. Or just a regular breaker and make the first outlet in each circuit a GFI. All wiring in the garage is 12 guage. –  John Apr 22 '13 at 20:50
    
I wouldn't use 2 different sized breakers, using a 20A double pole GFCI breaker sounds like a better idea. Just keep in mind at a higher current the voltage drop will be larger, which could lead to undesirable results. –  Tester101 Apr 23 '13 at 11:59
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Is the garage detached? –  Tester101 Apr 23 '13 at 12:03
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1 Answer

I believe that it would be against the NEC to have these two seperate circuits with a shared neutral and not have them on a double pole breaker (or at the very least a handle tie between a 15A and a 20A breaker).

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system

210.4 Multiwire Branch Circuits. (B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

A double pole breaker will simultaneously disconnect both circuits and satisfy the code requirement.

20A breaker is required and 12 gauge copper 10 gauge aluminum is the minimum depending on how long the circuit run is.

Furthermore even if it is on a 20A circuit, you are not required to upgrade your 15A outlets to 20A outlets. This is only required if there exists a single receptacle on the circuit. One can only plug in 15A max devices into a 15A outlet, therefore the 20A load circuit load would not be achieved on a single device.

It might also be a code requirement now for a detached building to have its own grounded sub-panel as well but that I will have to look into.

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210.4(B) can be satisfied with a handle tie, so in theory different sized breakers could be used. 20A=#12 is not always the case, voltage drop can be a factor. –  Tester101 Apr 23 '13 at 12:07
    
@Tester101 Thank you I edited my answer to include the additional information. –  maple_shaft Apr 23 '13 at 12:24
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