# Ampacity of battery wiring



## major (Apr 4, 2008)

Caps18 said:


> http://evwest.com/support/Shielded_Battery-Irradiated_XLE.pdf
> 
> What should I look for when it comes to buying the battery cables for connecting the battery pack to the controller? I'm not really sure what the ampacity rating means either. For a 2/0 cable it is 390. Is that a continuous amperage for a long time going through the wire? Or is it an instantaneous limit? And does having a voltage at 25% or less of the rated voltage limit impact the amp ratings at all? I'm not really sure how many constant amps it would take to power my S-10 conversion down the highway at 75mph-80mph, or to accelerate to that speed. (Although I drive slower than that and accelerate normally 99.5% of the time.)
> 
> ...


Use the forum search feature for AMPACITY. It will bring up numerous threads discussing the subject. Here is one I like: http://www.diyelectriccar.com/forums/showpost.php?p=245897&postcount=6 

And FWIW, one common failure mode for the DC motor controller is full on. There have been discussions about this and appropriate disconnects on this forum as well. Perhaps other members will chime in on that subject.


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## Sunking (Aug 10, 2009)

OK first up is economics. Your link is for a Shielded cable. Very expensive stuff and unless you have a specific requirement for shielded cable don't use it and spare yourself the expense.

As for how much current a given guage of wire can safely conductor is determined by many factors that involve heat dissipation. I am from the Building Power and Lighting (NEC) and we have the most conservative limitations because our applications require the use of raceways that hold heat and many more lives are at risk vs an automotive application. In Building wire the maximum current depends on the cable insulation type, how many conductors in a raceway, ambient temps,free air, and so on. Example a 2/0 conductor can be as low as 50 amps with a conductor insulation rated at 60 degree C in a race way with 30 conductors in an ambient temp of 25C. On the other end can be as high as 590 amps in free air with a 205C cable insulation at 40C ambient temp. 

Automotive applications have more liberal ratings and tend to use much higher temp cable insulation of up to 250C using nickel plated copper that allow up to 591 amps on a 2/0 conductor.

So to answer your question it depends on the operating voltage and temperature rating of the cable you select. All approved wiring and cable is going to have a UL cable insulation rating like AVA, AVL, AI, Z, AA, FEP, XLE, XLPE, TFE and so on. These are available on charts for you to select an appropriate cable that meets all the criteria of your design. 

I am not aware of any specific EV industry standard capacity charts. My guess is it falls under SAE applications which I cannot find for you. What I am comfortable with saying is if you use the NEC application being conservative in nature would be more than adequate. Once you get above a cable insulation temp of 105C is safe to say 300 amps on a 2/0

Here looks to be a fairly comprehensive charts in the temperature and voltage range of interest. CLICK HERE to see. Use NEC Table 310-19.


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## dougingraham (Jul 26, 2011)

Unless you are racing 1/0 cable will be fine for your application even if you are doing 1000 amps. 2/0 is overkill for most conversions. In a car you just cant do 1000 amps for more than 10 to 15 seconds at a time or you will be doing felony speeds. The fusing current on 1/0 cable is 1900 amps for 10 seconds. it will get warm at 1000 amps for 10 seconds but not excessively so. If you want to be conservative use 2/0 and feel comfortable in the knowledge that the fusing current for 10 seconds is 2300 amps. 1/0 is quite a bit less expensive and a lot easier to work with. Consider that at highway speed a smaller vehicle needs 18kw to maintain 60mph. At 144 volts this is a current level at the battery of 125 amps. At 96 volts it would be 188 amps. At 300 volts it would be only 60 amps. The advantage of going to higher battery voltages is clear with this example.

The one exception to this recommendation is the motor loop. The connection between the controller and the motor. The currents found there are higher and last longer. If your motor currents are more than 500 amps use 2/0 there and if more than 1000 amps you should use 4/0 or larger. But even here we are usually talking about fairly short runs and I doubt you would have any problems with 1/0 under normal conditions.

For best practice there should be a fuse in each battery box.

As mentioned one of the failure modes of a DC motor control is Full on. I have my main contactor open when the key switch is turned off. Contactors are designed to break the current. Almost nothing else that people tend to use for these emergency disconnects will. If you are really paranoid about this you can install a slap switch that will open your main contactor. Lots of people place something like this in series with an inertia switch so in the event of an accident the power is automatically disconnected.

Best Wishes!


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