I believe you are better off firing up the generator. I believe you will have close to a 10% voltage drop from what I can calculate using the on-line calculators.

Thank you for looking that up. I didn't know there were online wiring calculators. Is there a way to test voltage with a multimeter without a load? If I tested the voltage at the end of 250 feet of extension cord it would probably not reflect the voltage drop with the jack hammer running? Is the voltage drop praportional to the amount of electricity being used as well as the length of extension cord?

Voltage drop ONLY occurs when current is flowing.
The drop is the resistance of the wiring times the current.
The longer the extension cord the more resistance (unless you switch to a larger diameter wire).

A common mistake is to look up the wire resistance per 100 feet (a common table value) and then multiply by the cord length.
The problem is the actual conductors are twice the length of the cord. One conductor out, one conductor back.
Both contribute to the voltage drop.

Induction (non-brush) motors pull more current at lower voltage (with the same load).
This leads to additional voltage drop and heating of the windings in the motor.
Heat is what kills most motors (cheap bearings are the next cause).

If you have a generator this would be a good time to fire it up.

Thanks again to both of you. Generator it is.

Not only that, I can't imagine dragging 250' of cord around would be much fun!