What is the total draw your planing on pulling that's where i would start, (fans,Lights< ect)
and with an attic fan based on size and if you plan on utilizing it, (normally 10-12in 6 amps)
it needs to be in your figure before adding amps to you circuit. (20 amps now total)
an attic is a long draw from any receptical.

My advice,
figure out when you want run your grow, night better house consumption low.
go to power box get breaker out, and go to local hardware store and get 30amp replacement,
no larger because romex will get to hot during operation and it will end up causing a fire or
costing you double to run your grow cause of distance and draw.
pull light fixture out (remove breaker or turn off),
take with you and ask for a 30amp/120 gfi protected circut like in a kitchen,
ask how to install ( very simple) and their you have it electrical 101 and with safety.
cost approx 35bucks.

let me know i need to put a receptical for my new 12in in attic before summer ill do quick vid and post on you tube for ya.

@ 120 1000,hps draws 10amps with your distance
@ 240 1000, hps draws 5 amps

food for thought the longer the draw the the hotter the more it cost....