A.Lynn Underwood, an engineer, licensed contractor, and building code official in Norfolk, Va., responds: Section E3603.4 of the 2006 IRC requires that at least one 20-amp circuit supply the bathroom's GFCI-protected outlets. And while outlets in other rooms can't be placed on this circuit, other minor equipment within the bathroom (like an exhaust fan) can be, according to the code — but only if the circuit serves just one bathroom. So a combo unit placed on the circuit may technically meet code.

In practice, however, electricians almost always add a separate circuit matching the rating for the ventilation fan motor and demand from the unit's heat lamp (or blower) to avoid callbacks for tripping failures. A 20-amp circuit can safely deliver 80 percent of its load, or 1,920 watts, before running a risk of tripping (20 amps x 120 volts =...

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