Maybe I am not using the right names, but aren’t we talking about the same three, the two big ones that go through the bulkhead to the amp meter and the ignition feed?
I think you are probably correct that we are talking about the same wires.
Reading it over my disagreement is where they go and the current loads.
The big one from the alternator doesn't really go to the ammeter. It (usually) goes to the main splice. A branch from the main splice connects to the ammeter.
Another branch goes to the ignition switch.
All of those are subject to fairly high currents, but the branch to the ammeter only sees somewhat high current (20-25 amps) when the battery is recharging after start up.
The line to the key switch sees high current when the wipers and heater fan are both on.
The R6 line from the alternator through the bulkhead sees the same high current as above,
plus current to the headlights, brake lights, and marker lights when they are on.
The
ignition wire out from the key switch does not carry a lot of current. Ignition is only a couple of amps and the maximum field current is also 2-3 amps. Lets call it 4-5 amps.
But even with relatively low current flow, resistances up to and through the ignition line cause voltage drop. This can be a serious issue because the voltage regulator then 'sees' a lower voltage than the alternator is producing. The regulator then works to bring the voltage up. End result is something like the alternator producing at 16 Volts before the regulator 'sees' the 14 Volts on its input connection.
