Regulator/rectifier question

I have a Rotax 912s in my SS7 build and am struggling with all things electrical. I am hoping to use only the internal generator for power and not an add on alternator. First question: I thought I needed a voltage regulator and over voltage protection so I bought a Plane Power reg with OV when I bought my avionics last month. As I read and learn more, I am not so sure I need the Plane power reg. Does the Rotax regulator/rectifier perform this function? Maybe I dont need another regulator but still need OV (like a crowbar)? As far as connections...I have wired the C, R, and B+ outputs from the Reg/Rect to a shunt (for the Flight dek 180 to read the amperage). Is this a recommended location or should the shunt go between different points? Where would the Plane power regulator go or the Crowbar if that is all that is needed. Then from the shunt, a 14awg wire goes to a 30A circuit breaker and then to the main buss. If set up this way, does the CB have to be opened and closed during the engine start sequence? Right now my switches look like the factory light sport array with a Master switch, lights, heater, etc. I have read the Aeroelectric connection and attended an EAA wiring class, but I still need help. Thanks all.

"Need" in many cases is defined by the builder :)
That said I chose not to have an OV, put my shunt where you did and do not have an external alternator. I also do not disconnect the regulator for starting (I think Rotax recommends against running the engine with no load on the reg).

A 30 amp circuit breaker seems to large for a 14 awg wire. As I recall, 14 awg wire is rated for 15 amps.

The first thing you need to do is add up all the potential loads. Often, equipment specifies an average and a peak load. Write them both down, but for determining whether the rectifier-regulator alone can carry the load, use the average values. The Rotax (Ducati) rectifier regulator can source 20 Amps at 5000 RPM. If your average load is 15 Amps or below, you probably don't need anything additional.

I'd follow the Rotax recommendations pretty closely. The latest installation manual specifies a 25 Amp fast blow fuse and 14 AWG minimum wire. 14 AWG should be just fine as long as the run length is reasonably short.

The comment regarding a 15 Amp breaker for 14 AWG conductor size applies to AC home wiring per NEC code. It is not applicable to vehicle wiring.

Thank you to all who have replied. I learn a lot from the different points of view and I like to hear how others have done things. I got the 30A idea from my SS7 build manual from two years ago as shown in an example wiring diagram. I didn't know whether to use peak or average Amp values, so thanks for answering that question. I talked to John McBean who indicated that the LSAs have neither OV protection or adjustable voltage regulators (my apologies to John if I am rephrasing something he didn't say) I learned that modern avionics are often rated up to 30 VDC hence the need for the VR and OV wasnt as critical. A Lockwood advisor told me to wire the BC and R wires together at the shunt along with the Pos. Capacitor lead. Should I run the capacitor ground to the "ground forest" mounted on the fire wall , or should this potential radio interference be grounded to a separate point? And one last question for tonight...should I exchange the 30A CB for a 25A CB? Or not. Thanks, jeff

Jeff,

We decided to stick with the Rotax recommendation on our S7 and went with a 25 amp breaker......the total running load on this side of the system is more like 15 amps max.

The 25 amp breaker is one of those you can pull to open in the event somethng goes kafutz and I would want to be able cut the alternator out. The alternator is left connected all the time, starting as well as running.

In this case, we installed a voltmeter rather than an ammeter to monitor the charging system condition.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp Drive

I use a 30 amp fuse (I did not use breakers on my build) and grounded to mother ground (ground forest as you say :) )

A good electrical system concept is - If in doubt use a bigger wire, and a
smaller fuse. That helps eliminate smoke.

It's simplistic, but it is as easy to install using 18ga wire as for example 22ga
wire so why use 22ga? Now for example if a load is 7amps use a 10amp
fuse, rather than a 15amp fuse ... A small fuse feeding a big wire is a very
safe way to do wiring.

These planes don't have enough wire that you need to get weigh conscious
so I wire everything with mostly 18, 16, 14 gauge tefzel, and the load
handling feeds to the buses, landing lights, etc. with bigger yet.

Yes, use Tefzel also, as you can almost arc weld with it before it smokes.
Don't get your wire at the homedepot aircraft electrical department ...

I use to do this stuff for a living, so I know it's overkill, but who cares? I'd
rather be safe. If you aren't an expert, it's an even better idea to do things
the safer way as well.

Jeff Hays