Originally Posted by blkcorvair
This is correct. There is no reason to go with #4 gauge wiring. A golf car which pulls mega amps only uses #6. Your trailer is usually wired with #8 so the wire that ties the two batts together needn't be any larger. In my case I just used a #8.
The OP decided to upgrade to 2 6V golf cart batteries, and as part of his upgrade, wanted to know if a different gauge wiring should be employed. The correct answer is: it all depends. The reason people upgrade the wiring has nothing to do with a larger battery bank per se or the amp draw (e.g., #8 wiring can handle a max of 73amps which is more than sufficient for amp draw), but instead, has everything to do with voltage drop. Voltage drop is very important as it pertains to the time needed to recharge the larger battery bank (or any battery bank for that matter). First, the length of the circuit needs to be determined. For example, let's assume that Jayco put the converter (near the power panel most likely) about 15 ft from the battery bank, which is usually at the front end of the trailer. The total circuit length is 30 ft. Second, determine the gauge wiring used for the circuit. Let's also assume that the OP has #8 AWG wiring. The result is a calculated 8.88% voltage drop (via one of the many voltage drop calculators on the web), which in turn, means that it will take many hours to recharge the battery bank. If the OP has a converter with a multistage charger, the battery bank will never see the higher voltage numbers for the bulk and/or absorption phases (typically ranging from 14.8V to 14.2V) due to the voltage drop. Therefore, to decrease the recharging time and take full advantage of the multistage charging features, larger wire sizes are necessary. Again, the wire size will depend upon the circuit length (note: if we used 4 AWG for the above example, the voltage drop would be about 3.5%). Ideally, the voltage drop should be less than 3%; the lower the better.
As stated by Seann45 and others, at a minimum, 4 AWG should be used. If you decide to upgrade to 4AWG, don't forget to use 4AWG for the cable used to connect the batteries in series.