Wire sizing

[rgeiss]

I have a 24 vdc motor, that will be running at 140 amps. I need less than 1/2 volt of voltage drop over the length of the wire. I have lengths of copper wire for the positive lead as follows:
0' to 5'; 5' to 8'; 8' to 12'; 12' to 16'; 16' to 20'; and 20' to 25'. What is the best way to calculate wire gauge sizes needed for each length range?

 

[roydm]

0.5 / 140 = 0,00357 Ohms, that's all you are allowing yourself.
You should be able to find a cable resistance per foot chart on-line.
Seriosly though, why so strict on voltage drop i suggest you re-think and allow at least 2V, it won't have a big impact on speed.
What is the load?
Roy

 

[DRWeig]

rgeiss,

Pardon me for asking, but is this either college homework or a college take-home exam? Your question here, when put alongside your other one (in the Pump engineering forum), sure do look like something a professor might put on a test... Not very practical, but lots of math to do...

Again, excuse me if I'm wrong --

Goober Dave

 

[rgeiss]

No, this is not a college homework problem. I work for a fire truck pump company, we have a 12 vdc and 24 vdc version starter motor, which incorporate a solenoid. These motors are engaged by pulling a valve handle to the open position which connects the circuit to ground, engaging the motor, which in turn spins a centrifugal vane, vacuum primer, which gets water into the pump. This is the reason the VD needs to be so low (about 0.400 volts), as to correspond with SAE standards, and the different lengths needed are for informative purposes for our customers, depending on the mounting location of the pump/primer assembly in relation to the power supply. I would normally not ask, but SAE books are ridiculous to follow and are not that helpful, there must be an easier way to do this.

 

[rgeiss]

so far I have come up with these gage wires for these lengths:
0' to 5' = #8
5' to 8' = #6
8' to 12' = #4
12' to 16' = #2
16' to 20' = #2
20' to 25' = #1

Now this is for copper positive lead wire for a 24 vdc starter motor, 140 amp draw, and a voltage drop of less than .500 volts over these different lengths. If someone could let me know if I am on the right track, it took me much longer than I would have liked to come up with what I have here.

 

[waross]

You may want to check some ampacity tables also. 140 amps is a little high for #8.
I would start with minimum ampacity.
For free air rating with 90 deg. insulation that would fall between #3 and #4. Find your maximum length with #3 or #2, and upgrade for longer lengths.
Don't forget the return path. Your frame may be a good conductor for a ground return, but don't assume.
Try measuring your ground return voltage drop under load and go from there.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

rgeiss,

Sorry then, for the homework inference -- it's an unusual application you have!

Here's a great resistance table for copper wire.

http://www.powerstream.com/Wire_Size.htm

However, I agree with waross -- go with the ampacity tables or your voltage drop calc, whichever gives you the largest wire size for each length. For your 5-foot example, #8 will give you a 0.4V drop, but you'll be dissipating over 60 watts in the wire, which will cause it to exceed the temperature rating of its insulation.

It's best to consult the NEC and pay attention to conductor insulation type and temperature rating, and also pay heed to ambient temperature correction factors. There's not a simple table I can point you to...

Good luck, and let us know how it goes!

Goober Dave