PDA

View Full Version : Cable sizing - efficiency, cost, convenience



podolefsky
14 June 2011, 0843
There seems to be a lot of black magic involved in cable sizing. I'm going to try and break it down into bite-sized chunks of science.

Here's an example - Alltrax and Sevcon both spec 4 awg cable for theri 450(ish) amp controllers. I went with 2/0 on my bike (with an Alltrax 450A). What did that buy me, besides big, fat, awesome looking cables?

Here's a table of power dissipated (Watts) in 8 ft of cable, at 450, 150, and 50A draw.

AWG........450A........150A........50A
2/0..........126...........14............2
2.............181...........28............3
4.............403...........45............5
8............1018..........113..........13
10...........1618.........180..........20

At low current, doesn't make much difference. But at high current, going from 4 awg to 2/0 can save you about 280W. If you have a 28 kW system, that's about 1%. Not that much, but to put it in perspective, it's more than you save by installing an LED headlight.

Cost: 2/0 cable is about $3.50 / ft. 4 awg is about $1.50 / ft. For 8 ft, you save $16. Is $16 worth a % or so of efficiency? Up to you.

The lugs also cost more, so it depends on how many battery lugs you need. Still, it's not much compared to the cost of everything else.

2/0 is thicker and harder to bend. But I've done very tight bends (less than 2" radius) with it.

There it is. That should pretty much wrap up the cable debate (not) :D

GUFF
14 June 2011, 0927
What about a weight comparison between the cable gauge sizes? Weight is everything in an EV.

sentinel
14 June 2011, 0933
Howe many times are going to use a 8' section?

I would think that a 2 to 3 foot would be the longest?

teddillard
14 June 2011, 0947
I did a post a while back on that:
http://evmc2.wordpress.com/2011/05/13/wire-size-awg-and-funny-numbers/

There's a great table there that I took from the 'pedia.

1328

crap, sorry, can't copy it here...

jazclrint
14 June 2011, 0951
I am trying to remember from my Navy days, but isn't it a balance between having a big enough cable that heat won't build up but as small as possible to reduce resistance. Also, couldn't using the smaller awg4 wire be part of the safety design? The bigger wire will let anything through because it can. Do you really want that?

teddillard
14 June 2011, 1007
... as small as possible to reduce resistance.

How could a big wire increase resistance?


The bigger wire will let anything through because it can. Do you really want that?

Yes. :D

podolefsky
14 June 2011, 1013
The formula for resistance is R = rho*L/A, where rho is the resistivity (http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity), L is the length, A is the cross-sectional area. A longer cable will have more resistance, a fatter cable will have lower resistance.

It's all about the cross sectional area in terms of current handling - a longer wire has more resistance, hence more power dissipated (P = I^2*R), but it also has more surface area to dissipate the heat. In fact, both resistance and surface area are proportional to length, so the length doesn't matter for current handling. Where it does matter is how much energy you are wasting overall...a longer cable will waste more energy.

If you want safety, you should use a fuse. A smaller cable will limit current somewhat, but it's not a safety solution. It will still allow enough current to overheat and melt (that's exactly what a fuse does).

electriKAT
14 June 2011, 1014
Remember, those "watts" are being dissipated as heat. You don't want your wires or terminals getting too hot.


Howe many times are going to use a 8' section?

I would think that a 2 to 3 foot would be the longest?

His numbers are hypothetical. But you've got more than one wire in your system, right? Add them up, plus the resistance between each connector and battery terminal.

Oh yeah, and what Ted said.

podolefsky
14 June 2011, 1014
Howe many times are going to use a 8' section?

I would think that a 2 to 3 foot would be the longest?

It's all the cabling used in the entire system. If you have 4 2' sections, that's 8' total carrying the full current.

And what electriKat said ^^^^^^^ (and what Ted said)

podolefsky
14 June 2011, 1023
What about a weight comparison between the cable gauge sizes? Weight is everything in an EV.

Weights of copper cable here (http://www.allaboutcircuits.com/vol_5/chpt_3/1.html).

8 feet of 4 awg is about 1 lb, 8 feet of 2/0 is about 3.2 lb.

sentinel
14 June 2011, 1428
How many of you are really using 450 amps in your motorcycles?

If so for how long?

DaveAK
14 June 2011, 1436
Me. (420 amps which is the max for my controller.) Not sure for how long though. And that's controller to motor. Not sure of my battery to controller peak yet, but I know it's at least 180A and probably closer to 300A. If it's above 300A then I'm royally screwed. :) For what it's worth I'm using 4AWG welding cable.

podolefsky
14 June 2011, 1513
I pull around 500A under hard accelerations. For most of the time, it's 500 motor amps, and battery is less, but there are a couple seconds in there where the batteries are putting out 500A.

Plot below is real data (with fit curves overlaid). Batteries are putting out >400A for about 4 sec. (The only reason current stops at 500A is because the controller is limiting - if I had a bigger controller, it could put out more.)

One caveat to the whole cable thing is that the 500A is mostly through the motor cables, not the battery cables. My motor cables are short, only about 1 ft each, so very low loss from controller to motor.

http://www.colorado.edu/physics/EducationIssues/podolefsky/EV_project/batt_motor_amps_data.png

sentinel
14 June 2011, 1527
I am planning using a 300 amp Alltrax so I gather 4GA is good? Batteries unsure of, but no lead. 72 volt.

podolefsky
14 June 2011, 1546
Yup, even with 450A, 4 awg would be fine. That's what Alltrax specs.

sentinel
14 June 2011, 1624
Cool, Thnx!