You don't need a relay to wire the Rubicon compressor. All you need to do is to provide the compressor a source of 12v that you wire an on-off switch into. Turn the switch on, the compressor turns on & locks the locker. The compressor has an internal switch to shut itself off when the required air pressure has been reached. The compressor doesn't draw much current, my two Rubicon compressors are protected by a pair of 5 amp fuses.