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 = 2,400 watts; 2,400 x .80 = 1,920). An average-sized hair dryer is typically rated at 1,000 watts, and the rating of a curling iron can be even higher; plug them in and turn both on at the same time and you've already exceeded the circuit's safe capacity without even switching on the combo unit. That's why some manufacturers specify a dedicated circuit (which doesn't require GFCI protection) for some of their combo units.

Not only is wiring a separate circuit good practice, but section E3601.2 of the IRC specifies that branch circuits must have ampacities equal to the loads expected on the circuit. An installation must comply with all parts of the code, not just one provision.