Another wire size question..

[ppilotmike]

I think I understand how to size wires for run lengths and voltage/current now, after reading a bit, taking a SportAir class on electrical systems, etc. However, my "potentially stupid" question is this:

Many electrical items such as pumps, lights, etc come with short lead wires poking out of them which we, as the builder, must connect to longer wires to incorporate them into our electrical systems. What if, after careful determination, you end up with a wire size for your run that is larger in size than the short lead wire into the device? Given a steady power consumption situation, wouldn't this mean the lead wire would act like a fuse now (bad)?

[rleffler]

The short wire isn't necessarily going to act as a fuseable link. You are going to supply a breaker value for that short wire.

Let's say motor x, requires 5amps. You are going to ensure that all wire used is more than capable of handing 5amps (including the short piece).

However, the longer run wire may be fatter because of the length of the run, not necessarily how much current it will need to handle. If I recall correctly, there are charts in AC 43.13 that will tell you what size wire you'll need given a set current requirement and the length of the run.

[ppilotmike]

Originally Posted by rleffler

The short wire isn't necessarily going to act as a fuseable link. You are going to supply a breaker value for that short wire.

Let's say motor x, requires 5amps. You are going to ensure that all wire used is more than capable of handing 5amps (including the short piece).

However, the longer run wire may be fatter because of the length of the run, not necessarily how much current it will need to handle. If I recall correctly, there are charts in AC 43.13 that will tell you what size wire you'll need given a set current requirement and the length of the run.

Bob,

So you're saying that it doesn't matter that the short lead wire is smaller, due to the fact that it's short..? Is that right?

[rleffler]

Originally Posted by ppilotmike

Bob,

So you're saying that it doesn't matter that the short lead wire is smaller, due to the fact that it's short..? Is that right?

Sort of, it matters less because it's short, but it still matters.

You need to ensure that the short wire is capable of handling the required current. That should be a safe assumption, since the manufacturer hard wired it.

The long wire may be capable of carrying a larger current load, but you will use the appropriate breaker size for the smaller wire.

[Scott Hersha]

"The long wire may be capable of carrying a larger current load, but you will use the appropriate breaker size for the smaller wire."

I agree with Bob. Put another way, you should fuse for the smallest gauge wire in the circuit. Generally speaking, wire run lengths in our small airplanes aren't long enough to require fatter wires than what is recommended by the device manufacturer.

[penguin]

Mike,

No it won't act like a fuse. The small length of wire provided by the manufacturer doesn't really figure in the calculations. A 100W landing light (draws 8A on the battery or 7A on the alternator) can be supplied by a short piece of 20AWG wire. A thicker wire will be required to avoid excessive volts drop on the long run from the bus bar. But the thin wire is quite capable of handling the current over a short distance - say a couple of feet.

You could be over thinking this! Discounting the battery, starter & alternator, there aren't many answers - the only sensible wire sizes are 22, 20, 18, 16 and 14AWG. For most services that are located in the cabin there are usually only 3 choices 22, 20 or 18. In an RV, 14 is usually only needed for really bright landing lights (100W). Use the continuous load to size the wire and the in-rush load to size the fuse/breaker. Wiring is heavy, use the thinnest wire that is sensible, but if you only need a few feet of one size go to the next larger, the weight penalty is minor and it will avoid buying a few feet of the intermediate size.

BTW AC 43.13-1B says a 4 Amp load requires 8 feet of wire before greater than 22AWG is required for a continuous load, and 16 feet for an intermittent load.

Pete

[Kahuna]

Your all mostly right. To be more accurate, you are protecting the wire at its length for a rated and desired voltage drop. A very short wire will need higher rated breaker to protect it and a longer wire a smaller one at the same gauge. Get it?

[ppilotmike]

Originally Posted by Scott Hersha

Generally speaking, wire run lengths in our small airplanes aren't long enough to require fatter wires than what is recommended by the device manufacturer.

This quote sums up the source from whence this question was formulated. I was thinking: "What if the chart tells me the use 16g to go the distance, but the device manufacturer has supplied the device with 18g lead wires?" Thanks for answering my question..

[rbibb]

Sounds like a review of Ohms Law is in order but I can't do that from the phone I'm writing on...

[David Paule]

The charts tell us what size to use so that there's no more than a certain amount of voltage drop. That's all they do, right?

Dave