You dont need real heavy wire 14 ga works fine, you wont get voltage
drop that will matter or hurt anything. On high amp draw RC cars I have
a 30 amp fuse is used but not because of motor damage but speed control
damage, which the drill has in the trigger VS. Here stalling with high
amp could fry a cheap or old drill since high amp was not in its design.
Safe would likely be 10a for a 12v drill. My new makita has a curcuit
that shuts down the drill and resets on stall surge. My old Makita
doesnt, Use a fuse and dont stall it, But drilling metal is hard not to
stall the drill. Maybe wire in an inline auto type fuse, experiment,
start with 5a, New RC controlers can handle 100a, but cheap drill are
cheap and built to the packs limitations. A 7 cell 1200ma can blow a 30a
fuse stalled. A good motor will just get real hot, but a cheap drill who
knows, Fusing is the only way to protect it, Any RC shop will know the
proper amp fuse.