wire sizing question

[plumbflyer]

I have a question on sizing wire when a circuit splits. I have a fuse that feeds to a switch then splits to feed a 75w light in the end of each wing. I understand how to size a single wire but what size should the wires to the wigs be? 7.5 amps to each wing is a lot smaller wire than the combined 15amp fuse would protect.

 

[N941WR]

What one person did (that would be me) was to run 14 AWG wires out there (See the sample schematic in your builder's manual) with 10 amp breakers for 100 W landing lights.

Some day I would like to replace the 100 W lights with LEDs but for now I will keep what I have.

 

[vlittle]

The breaker is sized to correspond with the capability of the smallest capacity wire in the circuit. Therefore, 14 AWG is the answer.

Hope this is clear.

Vern

 

[Isaac]

RE: Wire Size

The wire size from the switch to the lamp out in the wing needs to be sized to handle the current that the lamp will draw. If you use 12V (battery) as the supply voltage, the current supplied to a 75W lamp would be 6.25 amps. In actual operation the current will be less as the alternator voltage will be around 14VDC.

The length of the wire from the switch to the lamp affects the resistance of the wire in the circuit and you should size the wire for no more than a 0.7V drop in voltage due to the wire from switch to the lamp. If the wire length from the switch on the instrument panel to the lamp in the wing tip is no longer than 28 feet, then 16 gauge wire from the switch to each wing tip would be the minimum wire gauge to use. If you bump the size up to 14 gauge, then the volt drop due to the wire would be approx 0.44V.

Just as a comparison, Vans specs 14 ga wire for the landing light and taxi light in there wiring diagrams.

Wire resistance for different gauges can be found in this article on the Aeroelectric web site.

http://www.aeroelectric.com/articles/wiresize.pdf

 

[frazitl]

OK Back to your question

Based on my understanding of the Nuckolls Tome, if you're going to run two lights at ~7 AMPS each off the same switch (as I did), and you are feeding the switch with one fuse/breaker (as I did) you need to run a 15 amp breaker (minimum - some would say 14 *1.25 or 17.5 amps) and all the wire in the circuit should be capable of being protected by that 15 (17.5) amp breaker. If not, you could have a short in one wing and the wire would melt before the breaker popped. The fuse/breaker is there to protect the wire. Therefore, you need 14awg for a max 18 ft each leg run with 35C rise. If you want to limit the wires temp increase to 10C, you would need 10 AWG wire (47 ft max). In my case, I ran 12 AWG protected by a 15 amp fuse.

YMMV