Tom,
Since the Viar 380 draws 23A max, it needs a 30A fuse in the supply circuit, wired with 10 awg wire.. The relay is used to turn the Viar on and off based on the PSI in the supply tank as sensed by the check valve. I've attached a diagram of the circuit. Use 10awg from the 12v house supplied 12V to the the relay and from the relay to the Viar. As noted in the thread I provided, I strongly suggest vibration dampening to reduce the compressor noise. I've also attached photos of my installed Viair and the vibration damping mount(s).
Steve