Electrical – one circuit or two

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.

The easiest thing to do, would be to wire it up like this.

^{Grounding conductors excluded for simplicity. Make sure the grounding conductors are connected properly.}

• Install a 20 ampere breaker in the panel.
• Run 12/2 with ground from the panel to the switch box.
• Run 12/2 with ground from the switch box to the fan, and the light.
• Connect the black ungrounded conductor to the two switches and GFCI receptacle, using pigtails.
• Connect the white grounded conductor to the GFCI receptacle, and whites from the cables running to the fan and light.
• Connect the black from the fan cable to one switch.
• Connect the black from the light cable to the other switch.
• Connect all grounding conductors.