Wire Size Calculator

[Stu Jackson]

Nifty little trick:  http://boatstuff.awardspace.com/awgcalc.html

Change the 10% to 3%.

I just checked our alternator output (50A) at 30 feet total distance = #2 wire, which is what we installed.  Easy as looking it up in the WM catalog or other tables.

Thanks to Maine Sail again.

[Ron Hill]

Just remember that if you have a 90 - 120 amp hi-output alternator, the difference between #4 and #2 wire for charging, is nearly negligible as far as amperage drop for the length of wire used in a C34  - when considering the new wire cost/effort to do the job.  Just make sure you have a good crimped, clean connectors and I also solder the connections.

If you really think you need a heavier wire - rather than removing the #4 factory wire and adding new #2 wire; I'd just add another #4 wire to the same connectors which is a little less costly. 
A few thoughts