Wire size vs current incongruity

The amp values they provide are essentially the fuse sizes, so the max amps. I can tell you that the fuel pumps on my aircraft, and other iS engines I have seen, actually draw 5-6 amps each, and are fused at 10 amps. Typical fuse sizing would be at least 1.25 x the running amps, and often 1.5 x.  That said you can’t go wrong by going a bit larger on the wire as long as the connectors being used will accept the size you choose.  Rotax does not know your wiring lengths, so they can only really specify up to the HIC connector (end of their harness). From the HIC connectors to your devices you should size the wire based on the current and length of the run - just as you are doing.  I don’t like voltage drop. Specifically regarding the fuel pumps, assuming 7 amps, I believe 16 is a good choice and should yield a VD of .5 volts at 12’.  

Jeff Blakeslee wrote:

The amp values they provide are essentially the fuse sizes, so the max amps. I can tell you that the fuel pumps on my aircraft, and other iS engines I have seen, actually draw 5-6 amps each, and are fused at 10 amps. Typical fuse sizing would be at least 1.25 x the running amps, and often 1.5 x.  That said you can’t go wrong by going a bit larger on the wire as long as the connectors being used will accept the size you choose.  Rotax does not know your wiring lengths, so they can only really specify up to the HIC connector (end of their harness). From the HIC connectors to your devices you should size the wire based on the current and length of the run - just as you are doing.  I don’t like voltage drop. Specifically regarding the fuel pumps, assuming 7 amps, I believe 16 is a good choice and should yield a VD of .5 volts at 12’.  

I agree, but why would you use the larger gauge wire on the smaller fused circuit?  That makes no sense.  Makes me wonder if either the current tables is incorrect or the wire size on the schematic is wrong.

Matt,

The wiring diagram I’m looking at in the 912iS installation manual shows 16 awg for both the lane select switches and the fuel pump switches.  The only 18 shown is for the lane warning lights.  

Jeff Blakeslee wrote:

Matt,

The wiring diagram I’m looking at in the 912iS installation manual shows 16 awg for both the lane select switches and the fuel pump switches.  The only 18 shown is for the lane warning lights.  

I was looking at the 916 manual since my 915 manual lacks the diagram.