nuckolls.bob(at)aeroelect
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 Posted: Tue Jan 11, 2011 6:31 pm Post subject: Very Simple Question about landing lights, wire size, and |
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At 04:26 PM 1/10/2011, you wrote:
Hey, everyone.
I have a pretty simple (ha!) question regarding the set up of a
landing light arrangement for my in-work RV-7.
Yes my friend, it IS pretty simple . . .
I've just ordered a set of 2 55W HID lights
(http://www.ddmtuning.com/Products/Apexcone-Raptor-HID-Kit) and I'm
starting to think about wiring for one light in each wing (or
wingtip). (Note: The lights haven't arrived, and the instructions
there might give some guidance, but I thought I'd ask anyway...)
Question: These are listed as "55W" kits obviously
intended to drop-in replace halogen 55W incandescent
bulbs. But do they DRAW 55 Watts of power in operation?
I'm not familiar with this product so I have no
first hand information. It occurs to me that the power
demand may be different than what the kit title suggests.
Using page 1-3 of the 'connection as a guide, I'm going to assume
these lights are really designed for 13.0 volts. Nominally, My 55W
lights will pull A=W/V or 55/13.0 or 4.23A. I see from the wiring
table later that 22 gauge will hold 5 A and has a resistance of 16.1
Ohms per 1000 feet. At 24 feet, I get 0.3864 Ohms.
<snip>
That seems like a nice small loss. So, I'll go with 18 gauge wire
(not that much heavier than 22ga, which could technically hold the
4.23A, but maybe not with startup loads.
You're to be commended for going through this exercise.
It demonstrates your willingness to understand the
simple-ideas that govern performance and to craft
a plan that is not compromised by some overlooked
variable.
In practice, the legacy breaker vs. wire selection
criteria is VERY conservative. For small airplanes,
the system integrator very seldom needs to worry
about voltage drop in wires. You cited 24 foot runs,
is this a round-trip for power+ground? In a metal
airplane it's fine to ground remotely located accessories
locally. Dropping to 12 foot runs REALLY quashes
worries about voltage drops.
So now I have two runs of 18 ga wire each carrying about 5 amps. I
think the default answer of the list will be to separately protect
the circuits with their own fuses in a fuse-block, but, for the sake
of discussion, let's pretend I really want to run them through a
singe breaker on the panel (or a breaker switch)...again, just for discussion.
This isn't a current ratings discussion. It's
a failure tolerance discussion. If you'd like to
have no single point of failure for both lamps,
then separate fuses is called for. A single 2-pole
switch adds some risk for single-point failure but
its tiny.
At 4.23 Amps per side, I could use a fuse or circuit breaker (per the
table) for 18 ga, which is 10A. But I may trip the breaker if warm-up
current is higher than 4.23A+4.23A. If I bump up the breaker to 12.5A
or 15A, I'm now not adequately (more rise in wire temp before
tripping) the 18 ga wire. Right?
You cited inrush currents as a concern. If you
were using real, 55W incandescent lamps, inrush
can be 10x normal running current. Fuses are subject
to "wear out" by repeated overloads even if it takes
100 cycles to ultimately fail the fuse. Your HID
lamps almost certainly don't exhibit this magnitude
of inrush. So a pair of 7A fuses/breakers driving 20AWG wires
is just fine.
So, if I really want to run them both through the same breaker or
switch-breaker, I need to bump up the wire size so the warm-up loads
(is assuming 1.5x the steady state load a good estimate for startup
loads?) of ~7A x 2 lights or 14A can be handled. Does that push me to
a 15A breaker and therefore 14 ga wire?
Wouldn't do that to maintain widest practical separation
between the two lamps.
Am I on the right track here? I seem to be leading myself to 20 ga
(7A) or 18 ga (10A) wire with independent fuse protection, but a DPST
switch controlling both, but I'm curious about the logic behind
combining them in a breaker anyhow.
You've zeroed in on it . . . The only suggestion
I have is to confirm there REAL running current.
But in any case stay with 20AWG/7A protection.
Bob . . .
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