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.
Best Answer
I have one in my house, never really had a problem with it. They are a bit loud though, so put it someplace out of the way - putting it in the kitchen or living room or anyplace people like to congregate and talk would not be a good idea.
The only other thing to look out for is what happens in the winter. The big problem with ours is that while the louvers close automatically when it isn't in use, in the winter heat has no problem escaping through those louvers into the attic.
It might seem like a minor thing, but it's easily noticeable - in the winter my whole roof is covered with snow except for a bare rectangle directly above the fan. They make styrofoam covers that you can place over the attic fan, but that requires you to get up in the attic every winter and put it on, then take it off every spring, which can be pain if you don't have easy attic access.
Another solution (the one we use) is to use one of those window insulating kits (with the tape and plastic, where you heat it with a hair dryer) and put that over the louvers. It works reasonably well, but not ideal.
If you could get an attic fan that had some kind of real solution for this built in - say an automatically retractable cover, or one with a pull chain, that would be ideal. Not sure if they even make such a thing, but it's something to look for.
ETA: Another thing to watch out for - make sure your attic has plenty of ventilation. Those fans move a lot of air, and you'll need plenty of ventilation in the attic to make sure all that air can get out.