Suppose if the trailer battery was near dead or dead and tried to pull high current it would trip the breaker that is in the circuit....I`ve been using this system in RV use for 30 years without a problem.....Benny
The reason a battery "goes dead" is because its internal resistance increases to the point that very little current flows. It doesn't matter whether the current is going out, or coming in.
That is why you will notice, via your charger ammeter, that a dead, or near dead, battery won't take much charge when the charge is initiated. Then after a while, the ammeter will read higher and higher up to the limit of the charger output.
What this means in real life is the in-rush current when you first turn your ignition on would be very low with a fully discharged trailer battery. In fact, the worst case would be with a trailer battery that was just down a little.
Concerning the size of the charging wire -- I'm not convinced that you would ever want to jam anything like the full output of your tow vehicle alternator into a deep discharge trailer battery. For best battery life it is usually recommended to charge slower, say 6 to 10 amps maximum for a typical automotive lead-acid battery.
The reason for the large alternators on autos today is to power accessories, especially stereos that can suck huge amounts of power. Also accessory lighting
. Many automotive batteries have charge limiting devices built-in to protect them from huge charge rates.
So, as was previously stated, a 10 guage charging wire should be about right. That's about as large as you can reasonably get onto a trailer plug anyway. Also, I bet there's a lot of vehicles running around with factory trailer wiring setups that just use 14 guage and call it good enough. And it probably is.