Wire size question

[Arie]

Good day all.
I have a question regarding wire size. If I look at the tables in AC 43.13 I see that for a wire length of 20 feet and a current of 6 amps I would need a #14 wire. Now for my question, If I decide to not locally ground the device , for example a landing light in the wingtip , and decide to run a dedicated ground wire as well does the "effective" wire length then go up to 40 feet ( out and return ) and therefore a larger diameter wire will be needed. Or do I only have to consider the wire length from the switch to where the current will be consumed? I hope my question makes sense ...

[krw5927]

The entire path that the electrons travel should be considered when sizing wires. If the supply from the bus (not just the switch, unless your bank of switches IS the bus, like the Vans plans) is 20 feet, and the return path to ground is 20 feet, then the wire should be sized for a 40 foot total length per the 43.13 charts.

When grounding to the airframe structure, the resistance of the airframe is considered to be negligible, so only the supply wire length is used with the chart. Whether grounding to the airframe is wise or not seems to be a bit of a debate.

[Arie]

Thanx Kurt.
It seems that to run a Vans landing light (75 watt) in a wingtip would require a #10 wire for the power and a #10 wire for the ground wire ,if you choose to install a ground wire. If you don't, you could use a #14 wire just to provide the power and use the airframe as a return path. Is my understanding correct?

[krw5927]

Originally Posted by Arie

Thanx Kurt.
It seems that to run a Vans landing light (75 watt) in a wingtip would require a #10 wire for the power and a #10 wire for the ground wire ,if you choose to install a ground wire. If you don't, you could use a #14 wire just to provide the power and use the airframe as a return path. Is my understanding correct?

Generally, yes, you're correct, but I'm not sure I see the need for #10 in this case. It really depends on the total run length. 40 feet total run sounds pretty long, just getting from the panel to the wingtip and back, and would probably be a very conservative estimate. 40 feet and 6 amps falls very near the #12 wire line on the chart in 43.13. If one wants to get really technical there are other considerations buried in 43.13 such as bundle de-rating (the charts assume a single wire in free air) and temperature/altitude de-rating. For a landing light that won't be on all the time, though, I don't think I'd be concerned with those details. I believe #12 would be adequate, especially if your total run ends up being somewhat less than 40 feet.

Landing lights, with their typically high current requirements and long run lengths, are one area where switching to LEDs can really have a dramatic effect on electrical system design.

[Arie]

Thank you for the reply. Next question , do you see any problem with local grounding of a landing light at the most outboard wingrib. What does the most guys do....

[Mel]

For anything except the most sensitive avionics using airframe ground is perfectly acceptable and has been standard practice for many decades.

[krw5927]

Originally Posted by Mel

For anything except the most sensitive avionics using airframe ground is perfectly acceptable and has been standard practice for many decades.

I agree with Mel (big surprise). Even though I chose to go the overkill route and ran dedicated grounds to pretty much everything, you will not have a problem with local grounding for landing lights.

[rbibb]

Originally Posted by Mel

For anything except the most sensitive avionics using airframe ground is perfectly acceptable and has been standard practice for many decades.

This would particularly be true in the case of a constant intermittent load like a landing light. Think of it this way. The current is only going to be flowing while the light is in use which is limited in time. It is going to be for all practical purposes a constant value (yeah I know their is a VERY brief transient that can be quite large in the case of an incandescent lamp), and therefore will not introduce any alternating current noise that can be coupled to other sensitive circuits nearby that might be the case if this were say a pulse-width modulated power circuit to flap motor, for example.

So using the airframe as ground return makes perfect sense here. Yet another reason not to build an airplane out of plastic?.

But I agree with the point that LEDs are a big advance here if only for the fact they should burn out with the frequency of incandescents which always happens when you just really feel you need them.