Wire length when wire sizing?

[ppilotmike]

Ok. I've asked this question once before, but I've been getting conflicting answers. I am just starting to plan & pull wires down my wing conduits, and I plan to have all grounds go back to the forest of tabs (FOT) to avoid a "noisy" airplane, electrically speaking.

Here's the question: When sizing wires, based on expected amperage, continuous vs. intermittent, voltage drop, etc., what is the wire length? Is it A) the length of the wire from the power source to the unit (i.e. batt to wingtip = 25') or is it B) the length of the entire circuit (i.e. batt to wingtip, then back to batt ground = 50')?

Secondary question: If the charts are considered conservative, is there another way to test the actual wire run for resistance to see if the size is correct? If so, how would one go about doing this?

As you can imagine, with a 14V electrical system, if the length doubles when using the sizing chart, the wire size (and weight) can really add up, especially if planning not to ground things locally...

P.S. Please don't drift this thread into the pros/cons of a 14V vs. a 28V electrical system. Thanks everyone.

[az_gila]

It's the total length of the wires that would be used for the calculations.

The electrons don't know were the load is in the wire and have to flow down the entire length...

No need to measure the actual wire resistance, it should meet the specifications in this table -

http://www.daburn.com/2430-ETFE-Hook...-22759/16.aspx

[BillL]

Mike, having answered the power and ground length issue, this post (in your other thread) best summarized the wire selection/fusing question.

http://www.vansairforce.com/communit...5&postcount=15

to summarize:
1. Initially size wire for amps ( temperature rise)
2. select fuse to protect the wire
3. use a larger wire if the voltage drop is unacceptably low.

Thanks for asking the power AND ground question! I have not heard that in my years of building and study - I am carrying a ground to each component too.

[Mich48041]

Look up the total round trip distance in a chart, 50 feet in your example. There are two considerations when choosing wire sizes: temperature rise and voltage drop. A wire that gets too warm could be dangerous. If the voltage drops too much over a long distance wire, then the load might not function properly.
The answer to your secondary question is to turn on the load and measure the voltage right at the load to see if it meets the manufacturer's specifications. Many devices will operate on a supply voltage anywhere between 11 and 15 volts. If the wire does not get warm and the load operates to your satisfaction, then it is good enough.

[ppilotmike]

I had to ask, as I was getting mixed responses from knowledgable people. Now my next question for you electrical gurus. Do you know whether or not the LED lights suffer much (in brightness) from voltage drop? I would think this would be more apparent in incandescent bulbs, but that's just my gut. Anybody?

[JonJay]

Depends on the LED driver being used. Many allow for a range of input voltage but put out the same amount of power. If this is so, you will have higher current at lower voltages. P=I*E or E=P/I
Power (Watts) = Current (Amps) x Voltage (volts)

Check with your LED system supplier.