"G.R. Patterson III" wrote in message
...


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?

Two 12ga wires should be adequate; one is not. As kontiki said, the
maximum
steady load for a 12ga wire is 20 amps.

In the absence of a breaker on the supply, you are looking at the problem
correctly, IMO. You could total the actual maximum current draw of the
avionics
and add an appropriate safety factor, but this will be very close to the
total
of the avionics CBs anyway.

George Patterson
The desire for safety stands against every great and noble enterprise.

The maximium draw of the avionics (speaker on, transmitting on both coms) is
20A, the continous draw (not transmitting) is under 15A. I realize that the
max for *continous* use is 22A with 12ga, but how much overload is
permissible for short term use (long enough for the avionics CBs to pop.
Starter cables are often overloaded to 250% of their continous rating..

Mike
MU-2
Helio Courier