I currently have two bathrooms serviced by one circuit. Each bathroom has lights, a single GFCI counter-top receptacle, and one bathroom has in-floor resistance heating. I will be adding in-floor heating to the other bathroom. As I understand it, code requires a two circuits for two bathrooms, but allows two configurations: 1) all lights on one circuit (but not shared with any lights outside the bathrooms. and the other circuit serving the outlets for both. 2) one circuit for each bathroom.

I prefer to keep lights on a separate circuit from the receptacles, since 14 gage wire is easier to run and I am forced to do some some long, difficult runs (vaulted ceilings, solid log home), Also If a breaker trips, it's nice if the lights stay on. But I also think putting both bath receptacles on the same circuit increases the chance of nuisance tripping (two hair dryers at the same time > 20A?). I'd like to put the bathroom lights on a bedroom light circuit and give each bath receptacle it's own circuit, but this appears to violate NEC.

I'm curious what the rationale for these rules are. I'm also interested in the pros and cons of each configuration. Also would like some input on whether or not the heaters should go on the receptacle or light circuit for config #1. I would prefer putting them on a dedicated circuit, but unfortunately that is not an option. The heater load fits easily in the 20A available.

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You are correct that the first two options are allowed by Code (citing from the 2014 NEC here) -- the relevant passage is 210.11(C)(3) along with its Exception:

(3) Bathroom Branch Circuits. In addition to the number of branch circuits required by other parts of this section, at least one 120-volt, 20-ampere branch circuit shall be provided to supply a bathroom receptacle outlet(s). Such circuits shall have no other outlets.

Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance with 210.23(A)(1) and (A)(2).

However, this passage only applies to receptacles unless you take advantage of the terms of the exception; putting the bathroom light on a shared lighting circuit is Code-legal as long as the receptacles are on a dedicated circuit.

For the heaters, you can either put them on a per-bathroom circuit as long as they satisfy 210.23(A)(2) by not using up more than 50% of the circuit's ampacity:

(2) Utilization Equipment Fastened in Place. The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied.

or you can have them on a shared lighting circuit, if you decide to have a separate circuit for bathroom receptacles. Unfortunately, putting them on the same circuit as the bathroom receptacles when that circuit is shared between bathrooms is prohibited by the Exception to 210.23(A):

Exception: The small-appliance branch circuits, laundry branch circuits, and bathroom branch circuits required in a dwelling unit(s) by 210.11(C)(1), (C) (2), and (C)(3) shall supply only the receptacle outlets specified in that section.

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