I'm working out my electrical power budget to determine whether I can get
away with a B&C 40amp alternator, or whether I have to pony up for the heavier,
more expensive 60amp unit.  (BTW, what's the difference between "case mount"
and "boss mount", or more importantly, which should I use on a Lycoming IO-360?)

First question -- alternator sizing.  I forgot the rule of thumb on this one
and couldn''t find it in Bob Nuckolls' book.  Constant current draw should be
less than 80% of alternator capacity?  Or is it more like 60%?

Second question -- current draw for Whelen strobes and position lights.  The
strobe power supply (A413-HDA-CF) draws 7 amps at 14 VDC.  I assume this is
just for the strobes, and the non-blinking Nav lights (the red, green and white
bulbs) have to be accounted for separately.  Correct?  

Third question -- same topic.  There is no listing in the Whelen catalog for
the nav light current draw.  I took my units apart (well, just one of them)
and found that the front bulb (the one under the red or green cover) is a 26W
bulb.  The rear bulb appears to be a 75W halogen bulb, but the watt rating was
hard to read so I'm not sure.  It's an A508-14 bulb but I can't find the
current draw anywhere.  So I added the wattage ratings (to get 101 W) and divided by
14VDC to get 7.21A per side.  This is a little suspicious because the Nav
Lite circuit on my EXP2-BUS system allocates 7 amps TOTAL for the nav lights.

Fourth question -- How much power does the alternator field draw?

Fifth question -- How much power does a battery contactor (constant duty)
draw?

I'm on the ragged edge of being able to use a 40 amp alternator.  In general
terms, my constant power draws are:

Avionics -- 6.5 Amp (very basic IFR)
Strobes -- 7 amp
Nav lites -- 7 amp
Recognition lite -- 2.5 amp (mounted in wing leading edge)
Housekeeping -- 5.5 amp

Total = 28.5 amp, which is 71% of 40 amps.

Notes:

1)  Housekeeping are all the things that come on when the master switch comes
on, like the directional gyro, Vision Microsystems engine monitor, CDI
ignition system, instrument lites and so forth.

2)  Note that the landing lights (mounted to gear leg) and fuel boost pump
are considered intermittent loads.

3)  Pitot heat is considered intermittent only because I rarely fly IFR, and
then not in conditions where icing is even possible.  But this may change and
if it does, I would have to upgrade to the 60 amp alternator. I'm not averse
to upgrading to the 60 amp alternator when I get some spare change to upgrade
my radio stack or change my IFR habits, since that's likely to be 5 years or
more down the line.

Thanks for any input y'all have to share.

- Rob Wolf
LNC2 51%