Your should use the biggest wire you can comfortable use so it will contribute
the least voltage drop under the full load of all avionics and electrical
systems. 12Ga wire is typically rated for 20amps continuous. I would try to
go with al least 10GA or even 8 if you can work with it. Basically, in
that section of the circuit (before the breakers) the bigger the better,
within reason of course.

Mike Rapoport wrote:
In my 1974 Helio Courier, the avionics bus is powered by a contactor that is
activated by the avionics master switch. There is no circuit protection
between the contactor and the avionics bus. The wiring between the
contactor and the bus consists of two 12ga wires. There also doesn't appear
to be any circuit protection in the field circuit for the avionics
contactor. It seems to me that since there is no breaker between the
contactor and the bus, the wire size should be sufficient to supply the
rating on all the avionics CBs (32.5A total), but I don't know what the
absolute maximium permissible temperature rise is before the avionics CBs
pop. Would one 12ga wire be sufficient between the contactor and the
avionics bus? Also, am I looking at the problem correctly, sizing the wire
considering all the avionics CBs to reach their limits simultaneously?

Mike
MU-2
Helio Courier