Universal M-25/XP starter amperage?

[Noah]

Old pics of my battery under bunk. No top needed. Plywood shelf epoxied in.  Wood cleats bolted with wing nuts and lift off to slide battery out. Straps go around.

[Jon W]

When I rewired my MK1 8 years ago, I installed the reserve battery just forward of the starboard water tank where you're talking about and used 1 AWG marine cable. My M25XP starts no problem on reserve or the house bank. After much discussion on this forum, I sized the start and charge cables to the max alternator output of 105A, but have an MRBF of 250A on the reserve and house bank for the inrush current of the starter. My alternator has never come close to putting out 105A. I have a write-up of my electrical upgrade in the Tech Wiki.

[KWKloeber]

Jack

ABYC has a nifty free AWG calculator app.  Check your app store.  What I don't like is it dumbed-down and makes all the decions for you. Blue Sea used to have one as well.

I prefer this one where it is easier to "gauge"   the trade off of cable size vs V loss. 
https://www.boatwireusa.com/pages/voltage-drop-calculator
And below it are ABYC ampacity tables.
HOWEVER I'm not sure why they are giving me different answers - I will look into that SNAFU.

You are ok with not sizing for the "bottom of the voltage curve" (avg A not peak A?) 
(I'm asking, not judging!) 

I'm thinking the starter needs the voltage heft during the worst instance.  Seems that might also allow the starter to last longer.

Ken

[Carbon]

Jack

You are ok with not sizing for the "bottom of the voltage curve" (avg A not peak A?) 
(I'm asking, not judging!) 

I think so. The average should drive the voltage drop and the temperature rise. Sizing for the peaks doesn't make sense to me, the peaks and valleys happen too quickly. I think of it this way; you have to have a high-end amp meter or an oscilloscope to even see it. My Victron Smart Shunt reports 150 amps and I'm sure a decent Fluke amp meter would agree.

I think of it this way; A standard 110 Volt AC circuit peaks at around 155 volts. Since it's peaking 60 times a second, no one ever worries about it and we just work with the average. The amps are peaking when the voltage peaks and it's also above the average, but we size the wire for the average.

I appreciate your link to the Boatwire calculator and the ABYC app. It's interesting that they don't all agree. I'm not sure what to make of that; it seems like it should be "settled science."

I'm not too worried about damaging the starter. At least on my boat, the engine fires right off with the 4Ga wire the factory put in. Whatever I do, the wire will be substantially bigger than the factory wire. I've not heard reports of starter motor failure even with the small wire and large voltage drop.

Jack