I'm going to skip right over the electrical question for now, and ask how well the duct work in the attic is insulated and sealed? Also, how well ventilated is the attic that you will be blowing all that air into? You may be better suited for a couple motorized dampers on each end of the house and an attic fan on the roof in the middle. The dampers keep the attic warmer in the winter than extra passive vents, and the fan keeps the attic and ductwork cooler even when you are running the AC.

I have an attic vent fan on a thermostat, not sure what that has to do with this. I'm looking for a way to actively draw cool air from outside into the living areas of my house on cool summer evenings (instead of using the AC). I understand I may have to increase the ventilation in the attic to do this.

I believe the ductwork in the attic (installed about 5 years ago) is very well insulated and sealed. My attic is well insulated and gets very cold in the winter, but I've never noticed any kind of cold-air ingress at the ceiling vents.

Motorized dampers - I'm guessing you mean in the walls of the attic? I have a hip roof and thus no walls in the attic. (This also complicates achieving adequate ventilation for a whole-house fan.)

You last statement is exactly what one has to do with the other. If you don't have enough natural ventilation in the attic, you should take care of that first. A hip roof definitely makes things tougher. A whole house fan is only around 4 amps, especially if you keep it small enough for the attic vents you have. The attic fan could help is you run a switch to bypass the thermostat when you want it. In that case, I might run the whole house fan on the same circuit as the attic fan so that the bypass switch and the whole house switch are the same thing. (Still thinking on how that might be done to code)

I haven't really thought of anything wrong with your original wiring plan yet, other than the will of the local inspectors.

You likely don't need a switch, but unless the circuit you want to tap into is 15A you can't do it.

I don't see where a 15 amp/ 120 volt circuit would have anything to do with it. There are some conditions where there is a 20 amp limit, but that can usually be handled with an SSU.

430.24 Several Motors or a Motor(s) and Other Load(s).

Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads.

FPN: See Annex D, Example No. D8.

Exception No. 1: Where one or more of the motors of the group are used for short-time, intermittent, periodic, or varying duty, the ampere rating of such motors to be used in the summation shall be determined in accordance with 430.22(E). For the highest rated motor, the greater of either the ampere rating from 430.22(E) or the largest continuous duty motor full-load current multiplied by 1.25 shall be used in the summation.

Exception No. 2: The ampacity of conductors supplying motor-operated fixed electric space-heating equipment shall comply with 424.3(B).

Exception No. 3: Where the circuitry is interlocked so as to prevent operation of selected motors or other loads at the same time, the conductor ampacity shall be permitted to be based on the summation of the currents of the motors and other loads to be operated at the same time that results in the highest total current.

430.32

(B) One Horsepower or Less, Automatically Started. Any motor of 1 hp or less that is started automatically shall be protected against overload by one of the following means.

(1) Separate Overload Device. By a separate overload device following the requirements of 430.32(A)(1).

430.36 Fuses — In Which Conductor. Where fuses are used for motor overload protection, a fuse shall be inserted in each ungrounded conductor and also in the grounded conductor if the supply system is 3-wire, 3-phase ac with one conductor grounded.

430.42 Motors on General-Purpose Branch Circuits. Overload protection for motors used on general-purpose branch circuits as permitted in Article 210 shall be provided as specified in 430.42(A), (B), (C), or (D).

(A) 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 430.32(D) and 430.53(A)(1) and (A)(2).

430.53 Several Motors or Loads on One Branch Circuit. Two or more motors or one or more motors and other loads shall be permitted to be connected to the same branch circuit under conditions specified in 430.53(D) and in 430.53(A), (B), or (C).

(A) Not Over 1 Horsepower. Several motors, each not exceeding 1 hp in rating, shall be permitted on a nominal 120-volt branch circuit protected at not over 20 amperes or a branch circuit of 600 volts, nominal, or less, protected at not over 15 amperes, if all of the following conditions are met:

(1) The full-load rating of each motor does not exceed 6

amperes.

(2) The rating of the branch-circuit short-circuit and

ground-fault protective device marked on any of the

controllers is not exceeded.

(3) Individual overload protection conforms to 430.32.

(B) If Smallest Rated Motor Protected. If the branchcircuit short-circuit and ground-fault protective device is selected not to exceed that allowed by 430.52 for the smallest rated motor, two or more motors or one or more motors and other load(s), with each motor having individual overload protection, shall be permitted to be connected to a branch circuit where it can be determined that the branchcircuit short-circuit and ground-fault protective device will not open under the most severe normal conditions of service that might be encountered.

(D) Single Motor Taps. For group installations described above, the conductors of any tap supplying a single motor shall not be required to have an individual branch-circuit short-circuit and ground-fault protective device, provided they comply with one of the following:

(1) No conductor to the motor shall have an ampacity less than that of the branch-circuit conductors.

(2) No conductor to the motor shall have an ampacity less than one-third that of the branch-circuit conductors, with a minimum in accordance with 430.22, the conductors to the motor overload device being not more than 7.5 m (25 ft) long and being protected from physical damage.

(3) Conductors from the branch-circuit short-circuit and ground-fault protective device to a listed manual motor controller additionally marked "Suitable for Tap Conductor Protection in Group Installations" shall be permitted to have an ampacity not less than 1⁄10 the rating or setting of the branch-circuit short-circuit and ground-fault protective device. The conductors from the controller to the motor shall have an ampacity in accordance with 430.22. The conductors from the branch-circuit short-circuit and ground-fault protective device to the controller shall (1) be suitably protected from physical damage and enclosed either by an enclosed controller or by a raceway and be not more than 3 m (10 ft) long or (2) have an ampacity not less than that of the branch-circuit conductors.

I was presuming that his original statement indicated that the instructions for the unit he was installing mandated a 15A circuit. But absent, that, the code guidance you posted is applicable.

OP here. I was looking at Triangle Engineering/Dayton fans; they seem pretty typical. I haven't bought anything yet. The manual says, "This fan requires a 120 Volt AC. power source with at least 15 amp capacity. All electrical wiring must be in accordance with local electric codes. Use 12 gauge or
larger for all the fan wiring."

Here's what I'm gathering from the above:
1) Hire an electrician.
2) Does a DPDT switch (like I mentioned above) count as an "interlock" per Exception No. 3? If so I can use a circuit with less total amperage available then I need, so long as I ensure it is impossible to run the air handler and the fan simultaneously.
3) The statement in the manual seems to indicate a greater-than-15 amp circuit is OK; maybe the motor has a push button circuit breaker or something.

Thanks

220.21 Noncoincident Loads.
Where it is unlikely that two or more noncoincident loads will be in use
simultaneously, it shall be permissible to use only the largest load(s)
that will be used at one time, in computing the total load of a feeder
or service.

The motors in question are 1/3 or 1/2 hp. So probably < 5A I guess while running, maybe 7-8A at startup?
Just went and looked at the breaker box. There's one 15A circuit labeled "attic" and "fan" (or maybe "attic fan") and (added later) "furnace." So I guess that one breaker is for everything up there. So probably a Good Idea to pull a second circuit up if possible. Plus that would give me an opportunity to add a couple ethernet cords and a TV cable for another project I'm thinking about.

2) DPDT actually turns two circuits on at the same time. A standard 3-way switch would turn on only one or the other. Local inspectors may want something more.

3) Practically everything is made to go on a 20 amp circuit even if it only needs a 15 amp service. Just look in your kitchen. The receptacles are most likely a 15 amp configuration, and the circuit is 20 amps. If the appliance needs something else for protection below 20 amps, it is customary to use a disconnect switch with a fuse in it within sight of that appliance.

We are not really talking about feeders or service loads here.

Maybe I mean SPDT. The kind where center is off, up is circuit1 on, down is circuit2 on.

"You don't want to run communication cables too close to line voltage,
especially parallel. You will get "noise" from the line voltage."

Yeah, I know, but I only have so many plumbing stacks where I can pull wires up 2 stories!