Help determining AWG size

[crabandy]

I'm confused as to what awg wire I should use for the vans 75watt 2 bulb per wingtip landing lights. I figured the total wire length from panel to wingtip roundtrip was 40 feet (wire runs from panel to firewall to floor back through the spar to wing root 10 feet +wing length 10 feet *2).

At .00253ohms per foot (14awg)* 40 = .1012ohms
75 watts / 12 volts = 6.25 amps
6.25 amps * .1012ohms = .6325 voltage drop

Conductor chart page 11-30 figure 11-2 from AC43.13b shows I should use 10 awg. If my total length was 35 foot instead of 40 I could use a 12 awg.

Wire Table, Figure 8-3 of the Aeroelectric Cpnnection shows 14 awg running 15 amps within the 35C temp rise.

Am I way overestimating the distance of my wing wiring runs?
Do I run 1 14 awg wire to power both landing/taxi per wingtip totaling 12.5 amps and take the 1.265 volts lost?
Do I run 1 14 awg per light it's .6325 volts lost?
Seems like overkill but would you run the 10 awg?

Thanks,
Andy

[vlittle]

Originally Posted by crabandy

I'm confused as to what awg wire I should use for the vans 75watt 2 bulb per wingtip landing lights. I figured the total wire length from panel to wingtip roundtrip was 40 feet (wire runs from panel to firewall to floor back through the spar to wing root 10 feet +wing length 10 feet *2).

At .00253ohms per foot (14awg)* 40 = .1012ohms
75 watts / 12 volts = 6.25 amps
6.25 amps * .1012ohms = .6325 voltage drop

Conductor chart page 11-30 figure 11-2 from AC43.13b shows I should use 10 awg. If my total length was 35 foot instead of 40 I could use a 12 awg.

Wire Table, Figure 8-3 of the Aeroelectric Cpnnection shows 14 awg running 15 amps within the 35C temp rise.

Am I way overestimating the distance of my wing wiring runs?
Do I run 1 14 awg wire to power both landing/taxi per wingtip totaling 12.5 amps and take the 1.265 volts lost?
Do I run 1 14 awg per light it's .6325 volts lost?
Seems like overkill but would you run the 10 awg?

Thanks,
Andy

The biggest saving is to ground the lights to the spar near the wingtips. This will reduce the voltage drop by 50%.

re: AC43.13 Figure 11.2 (continuous loads), a 10 foot connection for a 7.5 amp load should use a 16 AWG wire. This assumes a wingtip spar ground.

Each lamp will require it's own wire, assuming both a taxi and landing light in each tip. These wires will be connected in pairs near the circuit breaker ... a pair for landing and a pair for taxi lights.

Each circuit protection device (landing or taxi circuit) will require will have two lamp loads (2 x7.5 = 15 amps). AC43-13 Table 11.3 shows that 16 AWG wire shold be protected with a 15A breaker.

This all matches up nicely. If you want to run 14 AWG wires, you'd have a bit extra in reserve for wire routing and connectors and the breaker can be upsized to 20A.

Ground currents returned via the wing spars are perfectly fine since lamp loads are insensitive to electrical noise and also do not generate electrical noise. LED lamps, however should be wired with shielded cable.