I think we're getting into some number errors here folks.

10 sq mm broken back into wire diameter would be 10 / pi or 10 / 3.14.
If I'm thinking this through right in round figures that would give us
3.18 and the square root of that would be a wire with a diameter of 1.78
mm. Way too small for a starter wire. So something is wrong with our
numbers.

Forgetting the numbers for a minute lets go to the real heart of the
question. Someone has a starter wire that is running over 15 feet from
the starter to the battery. I'm guessing we are talking about a run
from the battery to the shut off switch and then to the starter. The
first thing to do is try to reduce that distance somehow. Then look at
wire size. The bigger, the better. For an auto conversion engine like
those found in most boats we should be thinking 4, 6 or at least 8
gauge. If we're trying to start and outboard motor (which may be the
other reason for the long wire distance) we could use a smaller wire.

Earl