What size circuit breaker for generator / sample wiring diagrams

Shaun,

I see your point that the battery relay (38) would disconnect the battery from the regulator so what is the relay in front of the voltage regulator (58) doing.  I’m going to venture a guess that it’s there to protect the regulator and main fuel pump from a catastrophic failure of the aircraft electrical bus.  If there was a short in main bus and the voltage went low, then relay (58) would drop out and protect the regulator/alternator so it would continue to power the Main Fuel Pump. This would be the only thing keeping the engine running since with the airframe bus dead the aux fuel pump would not work.  This would be unique to the 914 since there is no mechanical fuel pump. 

I don’t think a fuse between the regulator output and airframe bus is needed, but if one is used it should be after the fuel pump not before.  The fuel pump would want to run until failure.   

That makes sense, but I wonder how we are supposed to satisfy

"During engine stop break circuit between battery and terminal C to avoid discharge

of battery"

I guess with the engine off but the master on, the generator drains the battery?

Edit I found an old thread discussing the relay

Rotax-Owner.com - 914 Wiring Diagram Question

 "The rationale behind the relay (58).
The relay was implemented into the wiring diagram for ground fault protection of the main fuel pump (connected to the internal generator).
A ground fault (short to main bus) leads to blowing of the Battery circuit breaker (44) and the discharge of the 22,000mf capacitor (25). At this point the regulator stops supplying power and this defeats the GEN 25A circuit breaker (#42 in obsolete drawing), resulting in very high current to the main pump.
Why have a 1A fuse (#42)? Reason: An in-line fuse protects this 20G wire. Otherwise it is hot and un-protected."

Sounds like we are saying the same thing, the relay protects the fuel pump circuit.  The 1A fuse in series with the relay coil makes sense and is shown in the latest diagram, but this is not the fuse the OP is asking about.  That would be the 25 amp (42) fuse shown in the obsolete diagram.  I’m thinking they did away with this for a reason, likely because it’s an unneeded failure point. It’s a personal choice to install it or not.     

If the master switch is left on with the engine off, then yes the regulator may cause a slight battery drain, but probably inconsequential compared to other loads on the bus with the switch left on.