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Tom Veatch

(Ross Hebeisen) said:

the formula for figuring amp output is divide voltage into watt output
and you will have the max amp output like this 4000 watt divided by 110
volt = 36.36 amp, also 12ga wire has a 20 amp capacity and should be
fine on a 100ft. run
if you have a 15 amp chop saw 15amp x 110 volts gives you 1650 amp
draw.
ross

12 ga, 100 ft, 15 amp is close to the limit but should be OK with
amost 6v drop (5%). 12ga, 100ft, 20 amp is borderline at best with
amost 8v (6.7%) drop. Much better to use 10ga for that load and
distance and suffer less than a 5v (4.2%) drop. (Percentages figured
on 120VAC service)

As someone else said, for a given load, the motor will draw whatever
wattage it needs (volts X amps). So when the voltage drop in the line
is excessive, it will draw more amps to get the needed watts which
increases the voltage drop which increases the amps drawn which
increases the voltage drop which increases the amps, etc.

That said, you will be OK as long as you don't start popping breakers
or, Heaven forbid, let the magic smoke out of the motor. Only you can
decide whether you'd rather spend your money for a for a heaver
extension cord or for a new motor AND a heavier extension cord.