# New Project - HV Wire Size Calculator...need help



## omonoid (Aug 30, 2009)

Hey it's been a while since i posted. I am now on a college level research team for converting a GM Malibu to hybrid. Look up Eco Car 2 if you're curious.

My question is, i need a full justification for choosing the wire size i did.

I created the calculator attached to give a good wire/fuse size, but it only uses the continuous current.

For the testing, i plugged in 124 A for continuous, 488 A for peak current, 288 for average voltage, as shown in the attached image. All of these numbers were calculated from various simulations.

The wire size it spit out was 3 AWG. We planned on 1/0, so 3 ga seemed surprising.

I got most the equations from http://www.openelectrical.org/wiki/index.php?title=Cable_Sizing_Calculation, if they are wrong, or there are better ones, let me know

I couldint find any way of sizing cable based off the time it spends at its peak. Obviously 3 ga wont cut it if it peaks at 488 for more than a few seconds.

Any advice or equations or links or whatever to help me make my calc better would be great. 


Thanks!
Jake


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## charliehorse55 (Sep 23, 2011)

I have a similar calculator - but I calculate the losses in a different way. I calculate the losses at both the nominal and peak powers and simply present them to the user. Also, the user is prompted for the AWG wire gauge while the cross section is calculated (I believe you do the opposite?) 

If you input a peak pulse duration, it will calculate the temperature increase in the cable (assuming no heat is radiated). 

I like to rate the cables to withstand a 10s pulse at maximum engine power with no more than a 20C temperature rise. 


With a 500A pulse I get an 18.5C temperature rise on the AWG #3 wire.


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## omonoid (Aug 30, 2009)

oh thanks! very helpful. I need to know where those equations came from since this car is part of a competition. Im trying to figure that out through the spreadsheet, but i'm having trouble.

Also, are the Density, Heat Cap, and Conductance all cable specs? I might have to call the supplier to get that info for our cable.

Thanks

....So according to your calculations 3 Ga wire is fine. Why is it so standard to use 1/0 or 2/0 cable then?


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## charliehorse55 (Sep 23, 2011)

omonoid said:


> oh thanks! very helpful. I need to know where those equations came from since this car is part of a competition. Im trying to figure that out through the spreadsheet, but i'm having trouble.
> 
> Also, are the Density, Heat Cap, and Conductance all cable specs? I might have to call the supplier to get that info for our cable.
> 
> Thanks


Density and heat capacity are fundamental properties of copper. Your cable probably won't list them as specs as they don't really change. As long as the cable is mostly made from copper those values will hold. 

The equations are just basic math - nothing special. Just Ohms law. Basically, the resistance figure is the resistance of a 1 square meter thick conductor 1 meter long. I divide it by the surface area of the wire to get the resistance per meter for the AWG wire gauge. Then, use ohms law to calculate the power loss. Temperature increase is just power loss * time / volume of cable * heat density per volume


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## omonoid (Aug 30, 2009)

Ok thanks alot


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## dladd (Jun 1, 2011)

5v over a 5m cable seems like a lot of voltage loss to me. How much cable total is there in the car? What is a typical voltage sag due to wiring to expect in an EV?


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## Ziggythewiz (May 16, 2010)

That would be why we all use 1/0 or 2/0, to get under 1V of sag (in the wire)

6 volts of sag at 500A is 4 HP lost


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## omonoid (Aug 30, 2009)

What voltage drop should i shoot for


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## major (Apr 4, 2008)

omonoid said:


> What voltage drop should i shoot for


Hi omo,

It's really not so much about voltage drop. Of course you want as little as possible, but it is a trade off with cable mass, ease of installation, cost, etc. There has been a lot of discussion about cable size. Here is an example: http://www.diyelectriccar.com/forums/showpost.php?p=245897&postcount=6 For most of these guys converting to EVs, the cable size should be in line with the motor and battery ratings. If that rule is followed, the voltage drop will be pretty much insignificant.

Regards,

major


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