Ah, the old voltage drop dilemma.
2 AWG is the standard response by most on this Forum. Seems to have been working for years so that's the answer but....
2 AWG will get you a 6.43% voltage drop based on a 150 amp current draw and give you about 11.2 volts at the starter. Do you know your current draw when cranking? Probably won't find it in a service manual. Might get it from some Merc folks at their headquarters or a Merc mechanic. Better yet, you could measure the draw with an amp meter (if you have clamp type, that works well).
That stated, I used 150 amps because my 200 draws 170 amps measured at cranking. If you follow electrical standards for motors, 3% is the accepted voltage drop so you're more than double that but....
Most use 2 AWG as the wire size.
Assuming 150 amps is your draw, to meet the 3% you would need a conductor sized at 2/0 AWG. That would be just over 3% and give you 11.6+ volts at your starter. 2/0 AWG is heavy, very big (think hard to fit under cowling) and most of all expensive so....
That brings you back to 2 AWG. Easier to work with and much easier on the wallet.
Now you know why manufacturers place the battery(s) next to the motor and use 4 AWG wire.
Good luck.
