A lot depends upon the length of the cable - if you're going stock with the battery next to engine, or if you're going trunk mounted for the battery.
I'm sure our resident Guru will have his slide rule out ready to explain the why's and where's about the calculations required....in his opinion.
IF you're referring to moi, I still have my slide rule but the batteries are dead....so I must resort to the computer, a canned program, and the ever popular Ohm's Law. To this end, each wire size has a specific resistance, expressed as Ohm's per foot, at the maximum stated amperage of the conductor X the overall length.
There are several examples of how to calculate the overall resistance then using Ohm's Law, calculate the voltage drop of the wire to see if the included voltage drop does not exceed the voltage at the use point (starter motor with battery in the trunk).
I suggest the user perform their own calcs, as each application is different. Additionally, if cost is a consideration, or not, use the largest size wire t
hat you can afford. I can supply typical formulas to do the calcs. Just my opinion of course.....
Engineers often ask how to calculate the resistance of a cable, there is a very simple formula that works well within an ohm or so with copper cable.
19 ÷ by 2.5mm² will give you 7.6 ohms per km. ÷ by 1000 for resistance per meter. 19 ÷ by 4mm² will give you 4.75 ohms per km.Apr 12, 2017
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Hints and Tips - How to calculate resistance of a copper cable
Use this link...i've deleted the previous reference as I errored and posted the wrong list