Electrical – Does NEC require a separate circuit for a whole house fan

Sounds simple, but it's not. Assuming this is a "normal" attic fan, one manufactured recently and packaged for that purpose (not a super-charged home built model or a last-century model reclaimed from a 1950's estate sale), then let's start with the NEC:

430.42 (A) Motors not over 1 Horsepower. One or more motors without individual overload protection shall be permitted to be connected to a general purpose branch circuit only where the installation complies with the limiting conditions specified in 430.32(B) and (D) and 430.53(A)(1) and (A)(2).

Per that, you can add the attic fan to an existing circuit, unless it's limited by:

• 430.32(B), (D): These apply to auto-starting motors and requires they have a separate overload device. Modern, commercial fans with a thermostat on have a thermal fuse and comply this parts B and D.
• 430.53(A)(1), (A)(2): These apply to multiple motor arrangements and do not apply in this configuration.

However, when adding a device to a general circuit, you must perform either a VA/sf calculation (which isn't ideal for attic appliances, so I'll skip that) or a whole-circuit calculation. For that, see section 210.11, but in short divide the total calculated load in amperes by the ampere rating of the circuit itself. Ensure your calculation is for the fan on its most consumptive setting (ie "high"). It'll be something like 115V @ 7A.

If you have your attic lights, an outlet, and your attic fan on a 15A circuit, you're probably fine.

You only need an individual branch circuit (what's called "dedicated" in the trade) in specific scenarios. Kitchens and similar areas need a small appliance outlet dedication. Laundry rooms similarly. Neither of these dedications can loop to the lights: last thing you want when your washer faults is to be in a dark closet with an electrically hazardous situation around you. More restrictive rules apply to bathrooms, in that neither can the circuit supply lights nor other outlets elsewhere. GFCI considerations also apply in the bathroom and, sometimes, the laundry room.

That said, I would put the fan on a dedicated circuit with a manual disconnect no matter what, for a several reasons. First, unless it maxes out your panel, an inspector can't argue with this wiring. Second, fans generate a lot of line noise, which interferes with signal quality on downstream devices. Third, if the fan starts while I'm working in the attic, I don't want to have to worry about any tools I'm using tripping the circuit and leaving me in the dark.