Is that 15 foot how far away your battery is from the TM?
If it is,the total distance is 30 foot(both wires complete circuit).
You could use 6 gauge at 30',that will give you around 5% loss for the wire resistance.
If you use 4 gauge,there would be a 3% loss.
3-5% losses,from what I've read are perfectly fine.
Lower % is better but size/thickness(wire) and price will come into play normally when finally deciding.
Heres a site I use,pretty easy just fill in the blanks(plus theres notes on what each thing means)
https://www.freesunpower.com/wire_calc.php#startGrn
I have a 50lbs power drive(draw alittle less amps) and the total distance was closer to 40'(20' like you?).I went with 6 gauge,which is alittle light(I'm probably like at 6-7% loss).I went with the 6 gauge because I got it free and it was easier to work with but mostly the free part.I use a 50amp circuit breaker and a winch type plug for the TM plug.The wire is never hot to touch and the breaker didn't trip after 5 years use.
That being said,this year(6th) if I go WOT for to long the circuit breaker(auto reset) trips but will come back imediatly,when I slow it up abit.I think it's a combination of my breaker being border line big enough, could be getting alittle old plus it's only one of those little cheapy auto circuit breakers($10).I havn't bought a new circuit breaker to see if my theroy is right though.
