# Max length of #2 Cable

Discussion in 'Home Power and Microgeneration' started by Rob, Oct 1, 2004.

1. ### RobGuest

Hi all. I operate a CAA roadside assistance truck. (AAA in the
states).
Anyway, one of my tasks is to boost cars. The cables in my truck are
approx.
15 feet long, they are attached to 2 batts in the rear of the truck,
these batts are
charged through an isolator so as to be re-charged from the alt in the
truck. 25-30 foot
cables (#2 gauge) would make life better for me. Also, as they are now
I have to
back the truck halfway into my garage so as to attach this set up to my
home
batts which power my inverter during power outages. (Helps charge the
home
system, and adds a little more power to the mix).
My question is, can I replace my 15 footers with 25-30 feet (approx.
9-10 meters)
of #2 wire and still get 250 -300 amps out at the clamp ends. And yes
before you
ask, we are talking 12 volts. I'd prefer not to go with #1 wire because
of the extra
weight, and physical size. Thanks all.
Rob

2. ### William P.N. SmithGuest

Well, since the ampacity of #2 wire is only 100 amps
http://xtronics.com/reference/wire_gauge-ampacity.htm probably not.
However, 60 feet should only have about 12 milliohms resistance, which
will drop a volt and change at 100 amps, so for jumping a car and
charging it's battery you'll probably be OK. Just let it charge for a
minute or two before trying to crank it over.

3. ### EcnerwalGuest

2AWG copper is 0.156 milliohms per foot. V=IR.

You presently have (as I read your post) 2 15 foot cables, for a total
of 30 feet, for a total of 4.68 milliohms, or 0.00468 ohms. At 300 amps,
that's 1.4 volts lost in the cables, or roughly 10% loss on a (nominal)
12V system - at typical 13.5V, you'd still get 12.1 V out the far end at
300 amps.

Double the cables to 30 feet each and you double the resistance (9.36
milliohms), which doubles the voltage drop (2.8V), which doubles the
voltage loss (20%). When it's actually pulling 300 amps: 13.5-2.8=10.7
volts, which is probably low enough to cause problems on a 12V system.

Using 1AWG won't get you back where you started - you'll need 3 sizes
larger than 2AWG, or 00 (2/0) cable to have twice the copper and half
the resistance per foot of 2AWG cable, so that twice the length gets you
the same voltage out at the same amperage. 50-60 feet of 2/0AWG is
manageable - 100 feet gets a bit annoying to pick up. If you stick to 23
foot cables for 46 feet total, 1/0AWG cable would be close enough to the
same as 30 feet of 2AWG, and lighter than 2/0AWG.

It should be clear that with resistances this small having large effects
(due to high amperage and low overall system voltage), the importance of
clean, tight, low resistance connections to the cables and batteries is
paramount...and you need to keep the road crap out of them so they stay
good.

4. ### EcnerwalGuest

Well, since the ampacity of #2 wire is only 100 amps
That chart does permit 181 amps in the "free air" configuration, which
jumping certainly is - the wires can cool themselves better than if they
were stuck inside a conduit ("enclosed").

At low duty cycles (30%) 100 feet of #2 can be used for welding at 250
amps (the voltage is higher, which is one reason I didn't mention that
in my first post); you get different numbers for acceptable amperage
depending on duty cycle and heat dissipation - running power continously
inside a conduit leads to lower acceptable numbers, running jumper cable
for a brief jolt outside in the Canadian winter leads to higher numbers.

With your chart I now have 3 different resistance values published in
different places for the same copper AWG numbers, but those are probably
due to different assumptions about the operating temperature, which
affects the resistance (0.156, 0.190, 0.201 milliohms per foot for 2AWG).

5. ### Anthony MatonakGuest

Rob wrote:
....
Would adding a second connector for the cables in the back of your truck
help?

Anthony

6. ### SQLitGuest

Make sure that you use welding cable or MTW wire. (machine tool wire). There
are more strands and it will be easier to work with when it is cold. Larger
cable would be better but it will work.

7. ### Gordon RichmondGuest

Instead of replacing the cables on the truck with longer ones, why not
use some #2/0 wire to extend from your home batteries to terminals on
the front wall of the garage so you can tie on to them with the truck
when you need the extra capacity?

That way, the heavy wire is in a fixed installation, and you need
handle it only once.

You could use welder jacks, and hide them in an enclosure like a
mailbox, perhaps.

Gordon Richmond

8. ### Walter R.Guest

My car broke down recently. The AAA "Battery Van" used a fairly long cable,
I would say about 30 feet, certainly more than 12 feet. Looked fairly thick