(1) Not talking about the "B" to Batt cable, that cable is the 4 awg (CTY OEM size) battery cable.
OK, if it's the tap for the panel, agreed; but most of these setups work much better with a glow plug relay, in which case the current in that wire is pretty small.
(2) We are discussing a fuse on the power supply to the panel, NOT on the AO wire (which is to the "B" post anyway, NOT back to the batty.) Read up on your harness at my link and (if OEM) its fire hazard and you'd understand the problem and what we're talking about.
I don't have that harness.
(3) 4 awg is way undersized for the battery cable (but CTY saved $$) but that's a different discussion -- should be sized for no more than 3% and closer to 1% V loss to the starter.
Are you referring to the alternator to starter cable, or the battery to starter cable?
I was talking about the alternator B+ to the battery, not the starter. I know on some Catalinas they just jumped this wire, using the starer terminal as a junction point; but the wire form the alternator to the starter is going to have zero current flowing until the engine is - s
tarted - it's the battery to starter wire that matters here, and at typical starter motor currents, under load, you'd need something like 00 welding cable to stay inside of a 3% drop. At less than 20' round trip 4AWG works, of course #2 is better, and so on. If you're saying that you want to maintain a 3% drop on a jumper from the B+ to starter (which is where the battery is connected), for battery charging current: how long is the jumper? If on the order of a foot, 4AWG will likely suffice. For the starter to battery cable, again, at a max charging current of 50A, and a round trip distance of 20', 4AWG will give you a 3% drop.
(4) He doesn't have and we're not discussing an ammeter in the cockpit panel (leftfield.)
He said he was going to reinstall the Ammeter, I thought he meant at the panel.