[vettestr]

I think as the others have suggested a new circut should be used. Depending on wire gauge, length of run and of course the fan motor size your surge amperage could be in the 30 amp rang with a continious duty amperage of about 18 amps. Wire via a 30 amp relay and use 10 gauge wire on supply side for a trouble free install. I also like the manual override switch so you have option to turn on/off as desired. many kits are available with fixed or adjustable on and off set points at reasonable cost as well.