you MUST use a relay!!! you need to run a fused (place fuse within 12 inches of the battery)heavy gauge wire from your battery to the terminal number 87 of a 40 amp relay. from terminal #30 you run a heavy gauge wire to the (+) of the compressor. hook around a 16 gauge ground wire to terminal #85. run a 16 gauge wire from either your battery (or a toggle switch) or from a wire thats hot when the ignition is on(prefered) to one teminal of the pressure switch. from the other terminal of the pressure switch run a 16 gauge wire to terminal #86 of the relay. the center terminal of the relay (87a) will be unused. i recommend using a relay doing this for each compressor. you can use a jumper wire from one relay to the next for terminal 86 from the pressure switch.