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Date: Sun, 06 Nov 2005 19:27:00 -0500
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From: "Robert R Pastusek" <rpastusek@htii.com>
Subject: RE: [LML] 14 vs 28 volt system
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Dan Ballin wrote:

I am going to be starting on an RG Legacy in a few months and wanted
to get opinions on whether to do a 14 volt system or 28.  I am sure
I'll end up with an all electric  dual alternator/bus/battery
system.  Love to hear + / - and if you had to do it over, what would
you do.


Dan,
Jim White and I built two IV-P's with 28 volt systems. One just flying, the
second soon. All the avionics we used were dual voltage, but some of the
other systems required additional work/considerations. The Ray Allen trim
system, for instance is 12 volts (only). Simple, but extra circuits to
provide 12 volts to the trim system--and the power outlet in the cockpit,
etc. One mistake we made on Jim's plane was to power the Ray Allen flap
position indicator system with 28 volts. Only burned out two of these before
the clue bird got through that we'd made a mistake in this wiring... There
are LOTS of automotive-type cockpit lights, etc in 12 volts; you can even
get many of them in 28 volts--if you go to Europe.

The reasons we went 28 volt were 1) a belief that we'd need the power for
our "all electric" airplanes, and 2)the hope that a good 28 volt electric
anti-ice and/or air conditioner compressor would be available by the time we
were ready to fly. Neither system has delivered to date, and the
avioinics/electrical systems we're using draw considerably less power than
those available when we started building. A 12 volt system at 25-20 amps
(steady state) is more than adequate for our night flying load. Pitot heat
and hydraulic pump run this value up, but these are intermittant loads, and
a 60 amp system should give you more than an adequate reserve. In any case,
I'd recommend you chart and total all the loads, both stead state and
intermittant, and give yourself at least a 20% reserve margin on power
generation capability.

Bottom line for me is that I'd recommend you stick with a 12 volt system,
and "buffer" your avionics with an emergency/standby battery that feeds the
avionics during engine start and primary electrical system shutdown. Lots of
previous discussion on this on the LML in past years. You may want to review
the views of others using the search engine.

Hope this helps,

Bob Pastusek