It all boils down to the resistive load of the wire you are using. The multi conductor cables are great, but they are not all the same. I have seen some with charge lines as light
as 14GA, and some with charge lines as large as 8GA. To get the right wire you need to know the load expected on that circuit, and the length of the circuit. Wire is just a simple resistor. There are tables that list resistance for different gauges. Here are a few:
14GA = .0028 ohms per foot
12GA = .0018
10GA = .0011
Once you know the total resistance of the wire and the current load you can solve for voltage drop using Ohms Law. I don't mind a little voltage drop on some things. Generally I try to choose the wire size so that the volatge drop does not exceed .5 volts. That is not a magic number, it is just a value that I have found works well for most applications. On some applications like charge wires and brake wires I prefer to see less voltage drop because the voltage directly affects the performance of those items.
The headlights on my Tacoma came wired with some ridiculous wire size like 20GA. I installed a new harness using a relay and heavier wire. The lights
are noticably brighter because of it. There is an incentive for manufactures to use the lightest wire they can get away with: cost. It's not uncommon to see auto wiring that presents 1V of drop or more across a circuit.
If you want the best performance from your electrical
system it pays to know these things. The heavier wires are a bigger pain to stuff into a little socket. You will find the Hopkins unit a lot more forgiving. I run a 10GA charge and ground wire, 12Ga brake wire splitting to 14 for each wheel. Other circuits are figured using the same idea.
The last time I had to get into my socket it was 17 degrees and snowing lightly. The electronic converter had given up. I was in a parking lot on my back fighting with that thing and it took 2 tries to get it back so that the trailer plug engaged it properly. That is why the old one is getting the 3 pound hammer