Originally Posted by Walt (Post 1441519)

Personally I don't see what useful function the 'clearance' bus provides, even during a quick-turn I don't have any problem getting the ATIS and a clearance with the engine running.

I don't like running primary nav/com items thru a diode unless you plan on leaving the 'clearance' bus on all the time, extra failure point and voltage drop.

I don't see any back-up battery system for primary instruments like PFD/GSU25/EIS. If you want you can add the com to this BU bus.

General purpose relays are far more prone to failure than high quality switches, so I use switches with no relays for avionics/radio bus.
Todays panels generally draw under 8 amps so 30A relays are overkill.

I like to install 'avionics' and 'radio' bus switches these days as many of the units like the GTN have no on/off switch so that allows controlling those items with separate switches and it also splits the load/failure points.

Originally Posted by Carl Froehlich (Post 1441530)

+1.
As you have now simplified this to a basic system, consider:
- Combining the Clearance Delivery and Avionics buses. Here you can run them from a relay that goes to the battery, not the output of the master solenoid (the main bus). Keep all non avionics loads on the main bus (e.g. fuel; pump, nav/strobe/landing lights, pitot heat, all the USB charging ports we now have, and anything else not basic to continued IFR flight). This make load shed and such easy.
- As this is now a basic system, you will need backup batteries on your avionics to be, in my opinion, adequate for IFR. Another option is to take the next step to have a backup battery for your avionics buss, that battery charged via a diode from your main buss. That way you have a battery for your required clearance deliver function and avoid the plethora of individual EFIS type batteries (as they tend to not last very long anyway, a double win).

I differ from Walt on the use of small relays for avionic type busses. I have four in each of the three planes I built. I like them as they are mounted at the battery so no long unprotected runs, the control switch is a simple DPDT toggle that needs only handle ~100ma, and it provides option to have each avionics buss powered from a different battery if needed.

Carl