ppilotmike

ppilotmike

Well Known Member
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)?
 
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.
 
rleffler said:
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.
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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?
 
ppilotmike said:
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?
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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.
 
"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.
 
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
 
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?
 
Scott Hersha said:
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.
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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..
 
Sounds like a review of Ohms Law is in order but I can't do that from the phone I'm writing on...
 
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
 
David Paule said:
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
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Well, actually it is based on temperature. The "charts", or code, could care less about the devices attached to the end. The only goal is to protect the wire. This is why bundled wires must be de-rated compared to wires hung in "free air". So, the charts are just a handy guide.
Voltage drop does affect your current, P=IE, and plays a role in heating the wire.
Got some liquid nitrogen handy? I can show you how a little tiny wire can carry a huge amount of current.....
 
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ppilotmike said:
"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?"
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For all intents and purposes, you can consider the short pigtail leads that a device comes with to be functionally the same thing as internal connections, just ignore their size and concern yourself mostly with the wire you're installing to "go the distance" .
 
Neil,

That is now my understanding too. Right now I'm planning and running strings/wire through conduit, not actually hooking things up. Thanks everyone for you typical, speedy replies.:)
 
So Mike, you have it all figured out! Now - should you crimp the male connector to the wire coming out of the device or the supply wire?

Just messing with you! Have fun.
 
Don't confuse the two things that you are covering here.

1) A smaller wire is still rated for the current that the load will require. The fuse is selected based upon this wire size which intern is selected because of the load.

2) The larger wire is used to reduce the voltage drop for the required current. Its OK to have a wire that could handle more current than is required because the fuse will blow before you can damage the larger wire or in fact the smaller wire.

Don't confuse the two things here. The fuse protects the smallest wire in the circuit. The larger wire reduces the voltage drop. As mentioned earlier Ohms law comes into play. Voltage drop = Wire resistance times current. Larger wire has a lower resistance so for a fixed current the voltage drop will be less. This means you will see a higher voltage at the load.
 
An example might help clarify why this might be important for some wiring runs.

Lets say you have a load that nominally requires 10 Amps at 14.6volts- say some sort of lamp. Lets say you have to run a wire 10 feet to that lamp from the source. Lets say you use an 18 gauge wire to do so.

The amount of resistance of an 18 gauge copper wire is 0.06385 ohms per foot or 0.6385 ohms for the 10 foot run of wire. Now the effective resistance of your load at rated voltage is 1.46 ohms (14.6V/10A). Now when served by a 14.6 volt source (your alternator voltage) the combined resistance of the 10 foot run of wire and and the lamp is 2.095 ohms (1.46+.6385) as they are wired in series. Now the amount of current the combined wire and load will draw is only 6.95 amps (14.6/2.095 of just under 70% of the rated amount. So the lamp will work fine but obviously not burn as bright, etc.

Go up to a 14 gauge size wire with a 0.002525 ohms per foot resistance and you end up with 85% of rated current being drawn.

So higher current this all matters. If you work out the details you find that in the first case the lamp is actually seeing less than 50% of its power rating while in the second you are seeing almost 73%. This is why they use high voltage for power transmission and offers up a good argument for 28V systems, etc.

The good news is the LEDs are more efficient and help alleviate this problem among others.
 
One More Silly Electrical Question...

...So when using the wonderful wire sizing charts, is the wire LENGTH, that determines the wire size required, the length of the run TO or FROM the device (i.e. about 25' from panel to wingtip), or is it the COMBINED length of the TO AND the FROM (i.e. the whole length of the circuit from power to device and back to ground = 50')?
 
ppilotmike said:
...So when using the wonderful wire sizing charts, is the wire LENGTH, that determines the wire size required, the length of the run TO or FROM the device (i.e. about 25' from panel to wingtip), or is it the COMBINED length of the TO AND the FROM (i.e. the whole length of the circuit from power to device and back to ground = 50')?
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It is the total path that the electrons travel. So in your example 50'.
 
The distance in most wire charts refers to the round trip length of the wire. If the load is 25 feet away and two of 25 foot wires are used, then the total length to look up in the chart is 50 feet. However, if only one wire is used and the aluminum skin of the aircraft serves as the return path, then I would use 25 feet for determining the wire size, because the aircraft skin has much less resistance than a wire. If there is any doubt about what size wire to use, then go with one size larger.
 
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