nuckolls.bob(at)aeroelect
Guest
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 Posted: Sat Jan 16, 2010 7:43 pm Post subject: Wires, wires, so many choices . . . |
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| Quote: |
It seems that sometimes people size the wire simply based on the
Volts, current draw of the device, and the length of the wire. You
give some lengthy example calculations that go deeper to include
resistance of various devices in the circuit. How do i know when
it's ok to "grab any wire chart and hook things up accordingly"?
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It's not that random a process. In EVERY wire sizing task
one starts with design goals. For example, the case for
navigation lights (used to be the most energy hungry
system on the airplane - before LEDs), we consider
a high-duty cycle system where we want the bulbs to
operate close to their design values. Here, voltage
drop rules-of-thumb for 5% sort of drive the decisions.
For getting the engine started, its a very low duty
cycle, highest POWER task on the airplane with
variables such as temperature of engine oil and
battery, potential for partially discharged battery,
mixture management for lighting off the fires, no
alternator to help out, installed weight, etc.
Now our design goal is to minimize the time it takes
to get the engine running irrespective of how much
loss is tolerated. If we were to obsess over losses,
perhaps AWG 0 wire would make us feel better, but
that wire's ability to perform is only taxed while
cranking the engine . . . the rest of the time it's
dead weight on the airframe.
For establishing design goals, you STILL want to be
aware of where the energy comes from (chemical and/or
electro-mechanical), who needs it and how you're going
to use the various ingredients for success to get it
all wired up while always aware of the holy grail of
airframe systems design . . . minimum WEIGHT to still
get the job done.
| Quote: | I hadn't noticed Z-32 before. Is that the standard for a rear
battery? I hadn't considered using a relay. If the bulk of the ebus
load is resistive, shouldn't a switch be able to handle the 10 amps
that the #14 wire can cary? Wouldn't a relay just be another
possible point of failure? I'll go that route if it's necessary, but
I would rather keep it simple at this point.
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The relay becomes a 'mini-contactor' for driving a remotely
mounted bus. The rule-of-thumb for always hot wires in
airplanes is 5A breakered, 7A fused and MINIMUM length
for wires that carry more than that. LONG fat wires that
attach to batteries get protected with a contactor at
the battery. Your 14AWG feeder to a "heavy duty" e-bus
is too fat and too long to be allowed to run always
hot.
The history of the e-bus began before we had SD-8 standby
alternators and all electric panels. An e-bus load back
then was 4-5 amps max . . . no contactor necessary.
As the e-bus got bigger (supported by an 8A engine
driven alternate power source) it looks more like
the main bus in terms of feeders between the battery
bus and the e-bus . . . and the mini-contactor
is called for.
| Quote: | Ok, I'll use #2 wire to the starter, but why #4 at the battery? That
seems strange. Won't that negate the benefit of the longer piece of
#2? Let me guess at your answer: Is it because you're worried about
fatiguing the battery terminals and the voltage drop of those short
pieces of #4 is not very significant to the overall circuit, and the
temp rise is not really an issue?
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YES!
| Quote: | Is fatigue as much of an issue with more flexible welding cable?
The picture I have in my mind though is those pieces of #4 acting
like a traffic-jam to electron flow. is that incorrect?
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Yes, the voltage drop is greater . . . but if your
battery were up front, you would wire the entire
the entire suite of fat wires with 4awg. The batteries
really like 4awg mechanically so there's a reason
to retain them even if you up-size to 2awg for a remote
battery.
Actually, there are a lot of all metal airplanes with
rear mounted batteries that use 4AWG, I've even
seen a few Eze type aircraft with round-trip 4AWG
wiring . . . but these are 'warm weather' airplanes.
Now, could you use 2AWG battery jumpers . . . sure
and they're probably fine too in WELDING cable.
But I've seen two failures of the battery connector
tabs when the batteries were wired with 2AWG, 22759
wire.
Bob . . .
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