Your power and Ground are sapposed to be the same size. If you think of the electrical system as a chaine it is only as strong as the weekest link. So if you run only 4ga wire to your ground then your system will only be able to draw current equal to what the 4 ga will allow. Do not bumm out if you did that as 4ga wire is more then enough for your starter. Myself I run 2/0ga this is way overkill but it is what I keep in stock. Just so that you are all aware it goes from small to large 6ga, 4ga, 2ga, 0ga, 1/0ga, 2/0ga, and so on. Most boats will run either 4 or 2 ga wire to the starter

Ampacity of wire depends on insulation temperature rating AND duration of loading. The continuous ratings are roughly:

#6 THWN wire 60 amps
#4 THWN wire 85 amps
#2 THWN wire 100 amps

Larger wire (Smaller wire number) is rated for more amps, and results in lower voltage drop with the same load.
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You can measure voltage drop with a voltmeter.

Set meter to read 12 volts DC. Connect the + lead to the battery post, the - lead to the point you are checking- slave solenoid terminal, starter solenoid terminal or whatever. crank the engine- whatever the voltmeter reads is voltage DROP from battery to that point.

The same can be done on the ground side- but connect + lead to test point, - lead to battery post.

Many starter circuit problems can be isolated by systematically checking for large voltage drops. Check at each connection point- solenoids, etc.. Voltage drops almost 100% of the time are at contact surfaces.

Bulldog AKA Ronnie

Bulldog

you left out the mention of length. Additional length requires heavier gauge.

You also only show Continuous ratings. Just want to make sure others do not take those numbers to mean that they need to size their starter cable for continuous duty (unless they regularly use it as a trolling motor).

I don't know the intermittent ratings of the cables mentioned- do you? - I was only trying to indicate the relationship between amperage and wire number. Yes, there will be more voltage drop for longer length.

Bulldog B.S.E.E.

I saw mentioned that someones regular crimping tool works fine. If you have a hydraulic or pneumatic crimper that's fine. If that's your regular crimper, great. What I consider "regular crimper" is my hand Klein crimper. And I don't mean the flat punched steel ones from Radio Shack that I see in many peoples tool boxes.
If you are crimping large gauge wire you need a real heavy duty crimper with specific dies for the wire size and crimps used. Crimping larger wire sizes is best left to those with the exact tools required. I also use color coded glue heatshrink for ends.

I use tinned 1/0 wire for all four batteries.

Good luck wiring.

I used 1-0 on my old boat when I rewired it. The difference in cranking speed was phenominal. I belive the old stuff may have been 4g.

As to the flexibility of the wire, I bought the cable from Mark Hamilton at MAD Enterprises. The guys is incredibly knowledgable and is more than willing to share it. Just don't let him sell you one of his solenoid kits-it would be redundant. Anyways, he has this battery cable that is incredibly flexible. I'm serious-I've had the stuff for several years and everytime I handle it, I'm amazed at how flexible it is! The cable only comes in black, but Boat US has red shrink tube that works nicely.