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Boat is my 2001 Vista 328. I need and want to replace the 3 output current charger (1 20 watt output and 2 of 5 watt outputs for the starting batteries).
I'm going to a 3 output, 40 watts on each--charger is ProMariner, ProNauticP 1240P.

ProNautic says to run it direct to the batteries. Yet I want to continue to use the battery isolator, which I believe will mean continue to feed it up to the the Ships System circuit breaker and Port and Starboard engine breakers on the battery control panel in the cockpit.

However, it now has 12 ga. wire for the two 5 amp lines and 10 ga. wire for the 20 amp house and genset batteries (I have two grp. 31 AGM's in parallel for that).

Problem is it is recommended I go to 8 ga. wire for all three batteries, so I have to find the proper 8 ga. spade terminals to fit the breakers and snaking wire through nearly inacessible places. I may even have to reduce the wire at the terminals from 8 ga. to two strands of 12 ga. in parallel for a few inches to go to two smaller spade terminals.

Would there be any issues with this setup?

Or do I HAVE to run direct from the charger to the 3 different battery banks? And if so, won't that bypass the battery isolator?

Yes, I'll run 3 of 50 amp fuses within 8" of each battery as suggested, in addition to the main breaker(s)

Thoughts everyone?

You need to use ring lugs on that size wire. You might try passing it the terminal. BUT, if your run length is short enough you may get away with 10g wire.

As to the isolator, as long as you stay on the battery side its OK. Meaning that you could connect at the back of the battery switch or use the wiring that came with the boat.

On "fit the breakers and snaking wire through nearly inacessible places." Use a in-line fuse and the charger should be mounted close to the batteries.

I have the same charger but in 30amps. It displays the Volts & Amps which tells you what is going on.

Also you need a replacement of am old ProMariner. They will sell you a new one close to cost.

40 watts is only about 3.3 amps. I wouldn't worry about upgrading the wires from OEM. I think the OEM chargers are much higher rated. I doubt the OEM wiring even went through the isolator; they may have merely used the battery side of the isolator for a convenient tie-in spot to charge the batteries.