[ausev] cable sizing

[Christopher Robison]

On Thu, June 14, 2007 6:48 pm, TERRY KING wrote:
> I plan to have a 96 volt battery pack with 1/0 cable connecting the
> individual batteries. Between the full battery pack and the controller I
> plan to use 4/0 cable as well as between the controller and the motor. Is
> this a good idea or should all the cabling be matched? I have about 20' of
> 1/0 cable available to me which is the main reason I want to use it, and
> I've located 4/0 welding cable for $5.96 per foot @ a Dallas welding
> supply.

The reason most people use the same size cable for everything might be
economy. Beyond that, what size cable you use is a matter of acceptable
losses, and thermal management.

You can actually calculate exactly what you stand to lose by going with
1/0 cable. The resistance of 1/0 is nearly 100 microohms per foot. So
applying Ohm's law at 200 amps every foot of cabling is reducing your
available voltage by about .02V. Doesn't sound like much, but then that
also means that every foot of cable is dissipating 4 watts of heat at that
current. They'll get pretty warm and smelly if you keep this up.  Of
course, at 400 amps the losses and heating double, and so on. The feet of
cable and total losses add up.  There's a rate at which the cable can
dissipate heat into the surrounding air. This goes up and down with
ambient temperature, and if you're generating more heat than can be
dissipated, the result is a continuing temperature rise in the cable which
then also increases the cable's resistance and the effect compounds.

With 2/0 (the usual choice), the numbers are a little better and the
cables stay cooler. 78 microohms per foot gives you .015 volts lost per
foot, burned up as about 3 watts per foot, 25% less wasted.

I personally would not use 1/0 for the usual DC-powered car design (AC is
another story), but if you do, at the very least use 2/0 for the
connection between the motor and the controller. I think 4/0 is overkill,
as long as you keep those cables short (which you should to control
inductance and noise, anyway).


-- 
Christopher Robison
chris at ohmbre.org
http://ohmbre.org          <-- 1999 Isuzu Hombre + Z2K + Warp13!