<< In Bob Nuckolls' book he talks about using the actual voltage rating of the
<< device in question to determine the allowable voltage drop. Is this procedure
<< not acceptable? This procedure would allow a much larger voltage drop than .7
<< volts. For example a system might be putting out 14.6 volts while a lamp was
<< rated for 13 volts. This would seem to allow a 1.6 volt drop.
The issue I was speaking to there was a special case wherein I cited examples
of landing lights characterized for operation at 13.0 volts. Obviously, for
a system that runs nominally 13.8 to 14.2 volts, the lamp would be quite happy
with a drop of .8 to 1.2 volts in the system's wiring. This figure falls
nicely
into the 5% allowable system losses figure I cited in my earlier posts. The
voltage RATINGS of various devices are NOT requirments but simply notification
to the system designer as to how the device was characterized . . . so many
lumins over so much pattern at 13.0 volts. Since 13.0 was the design point,
this tells a designer what to expect in the way of performance under nominal
system drops.
/Using a good digital voltmeter (or the manufacturer's specifications
/if available), determine what the appliance draws sustained worst
/case under load. If the appliance is a transmitter, measure the draw
/while transmitting. If the appliance is a lamp, measure the draw after
/it has stabilized at temperature. If it is a motor, put a sustained typical
/load on it.
Good advice . .
/Then determine the required wire gauge based on wire point to point length
/assuming a half to a volt drop from the supply to the appliance by using
/the curves shown in AC 43.13-1A (I think). I got my copy out of a buddy's
/Glasair Builder's Manual.
Also wire table I've published in the past few days. . .
/Select the breaker based on 135-165% (or the next available size up) of
/the continuous maximum current for the appliance.
Actually, breakers are selected according to wire size but maybe that's
splitting hairs. Picking the breaker according to applicance load is okay
too, especially if you've upsized a wire for voltage drop considerations
and left the breaker sized for the load, the trend is to protect bigger
wires with smaller breakers which is not a hazazrd.
/You do not want to chose too low or you will experience nuisance
/trips. We are really trying to protect the wiring, not the appliance.
Exactly!!!! Tens of thousands of Cessnas have 60 amp breakers protecting
their 60 amp alternator b-lead feeds . . . this breaker is DESIGNED to
nuisance trip. I complained about it when I worked there 30 years ago
but nobody cared. Now, that breaker causes more unhappy electrical surprises
for pilots than any other breaker installation in history. When in doubt,
upsize the wire and breaker a step. It never hurts to make it a tad bigger!
Regards,
Bob . . .
AeroElectric Connection
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