The Cessna 120-140 Association Forum

Hello,

I am upgrading the original fused electric installation in my C140 sn 10.260.

The radio was of course not original, and the power feeding was wired with three wires:

One AWG 16 for ground from AC chassis to Pin 21 of radio
One AWG 16 for +B 14 Volt from the RADIO fuse to Pin 20 of radio
One AWG 16 for +B 14 Volt from the RADIO fuse to Pin 1 NAV of radio

King installation paper specifies two different fuse ratings ( 2A and 5A) for NAV and COM lines.

I think that there is no reason to install two wires. A single AWG 16 from a 10 A Circuit breaker to the rear of the radio plus a bridge between pins 1 and 20 will be correct and cleaner.

I am missing something ?

Thanks for your help !
and kind regards,
Horacio

In theory your plan would work in most cases. But I would advise against wiring it that way. The radio manufacture separated those two lines for a reason and you could risk damaging the radio. Imagine the comm drawing 1-2 A and for some reason the nav draws 4-5A. This is under your 10A total and could fry a circuit. Keep in mind if the nav is having an issue and trips the circuit, you'll also lose your comm, rather than only losing the nav.

If you do plan to wire the two on the same breaker, I would use a 7.5A breaker, closer to the total of both.

8359 wrote: ↑Wed Dec 18, 2019 1:13 pm Keep in mind if the nav is having an issue and trips the circuit, you'll also lose your comm, rather than only losing the nav.

That's the best reason. Another is that, while a 10 amp breaker is fine for a 16AWG wire, the lower rating may reduce the amount of smoke coming out of the radio if it fails.