Req'd battery switch amp capacity?
 

I have Perko Med Duty 8501 battery switches which are rated at 250 amps continuous & 360 intermittent. They're new (installed a year ago). I'm concerned though that for my 540 Dart Big M block motors, these are too small to adequately handle the needs of the starters. I'm getting a larger than desirable voltage drop when cranking, and I think the switches may be the culprit. Have already load-tested all of the batteries and they're fine. And when I hook a jumper directly from a battery to the starter and bypass the switch, the drop is minimal. So it's either the switches or the cables, but the cable & terminals all look in very good shape. Does anyone know how many amps a typical Mercury (or GM) starter for a 540 should pull? Sorry, but I don't know the exact model of the starters.

Try removing the cables from the back of the switch and bolting 1 battery at a time to the common cable and verify drop. If drop persists you have a cable problem, if drop disappears you have a switch problem. Hope this helps.

Thanks, great suggestion. We were just talking about doing exactly that test.

Heck you could turn switch to off and move 1 battery terminal to common (out) at a time and test.

Something else you might want to check, especially if the cable connections have been in place for a while, is the integrity of the connections. Sometimes hidden corrosion can build up between the wire lugs and the terminals. It might be worthwhile to pull the cables off at the starter, battery switch and the battery, clean them and use a bit of dielectric grease when reattaching them. Don't forget the ground cables too.

use a volt meter and do a voltage drop test on the switch and cables. Just google "voltage drop test ".

Yes, of course we've been using a volt meter. That's how we detected the specific voltage drop from the beginning. And yes, we know how test to try to isolate the problem. We've also checked all of the connections and terminals, and there is no corrosion on any. I'm just hoping somebody on the forum can give me more specific information about the general amperage draws on starters for my sized motors and what types/capacities of battery switches they're using.

Get a clamp style amp meter and put it around the starter cable and crank the motor. Vwa Lla.

Those Perko Switches arent the greatest quality ..... some have a zinc Tinned contact and the contact tends to build up an oxide layer , creating increase resistance . Most modern gear reduction starters will have an Inrush amperage of 200-300 amps then taper off to around 150.
That all depends on the integrity of you batteries , cables , switches , etc.