Calculating Subpanel Loads

Once you've determined the circuit breaker size, the wiring feeding the subpanel has to match or exceed the rated circuit breaker size, not the projected load. That just means if the breaker is rated for 50 amps, so should the wire, unless there is a long distance run for this feed, when the next sized larger wire size should be used. This wiring size is determined from a wiring chart that lists the wire types and wire sizes based on applications. For your convenience, I've included this wiring size chart.