Does NEC require a separate circuit for a whole house fan?
I don't believe the fan mfr will call for a dedicated circuit but you may want to contact them to be sure. More than likely, they'll defer to local codes so also check with your local inspector.  As with any other device, consider the other loads on that circuit, and which ones could be operating at the same time, to ensure you don't overload the circuit.

Anecdotally... My own house fan is on the same circuit with my dining room &amp; hallway.  In normal use, I'll have up to 11 led lights (154 Watts, about 1.5 Amps) and the 1/3 hp fan (6.7 Amps published, unknown actual) running and can add a vacuum cleaner with no problem on a 20 A circuit.

Fan specifics... The install instructions for a Dayton 1LXN7, 1LXN8, 2EAX4, and 2EAX5 don't say anything about a dedicated circuit. Their manual just says...
"Install the fan control(s) in the desired location and wire according to the appropriate wiring diagram (following applicable NEC and local codes).  Connect power to motor and 2-speed switch with timer using approved wiring methods."

The instructions for an Attic Cool/Ventamatic CX242DDWT, CX302DDWT, CX24BD2SPD, CX30BD2SPD, CX36BD2SPD, CX242DDWTHUB, CX302DDWTHUB, CX24BDM-2SPDHUB, CX30BD-2SPDHUB, and CX36BD2SPDHUB also don't mention a dedicated circuit...
"Note All wiring supplies and installations must meet or exceed the requirements of local electrical and fire codes.  Wire to 120 Volt, 60 Hz circuit only, using 14-3 two-conductor wiring with ground." ... "Install included switches in a double gang UL listed wiring box, in a wall close to the fan."