# How long extension cord for jack hammer

twigpigNovember 21, 2007

I bought a used Bosch Brute jack hammer to help me put in post holes for a fence. I have lots and lots of rocks. The area I need to use it is 250 feet from my closest exterior outlet. I have a bunch of 10 gauge extension cords from when I built my house. Plenty to get to the fence location and then some. The best info I can find indicates the jack hammer takes 15 amps. All of my electrical outlets are 12 gauge wire with 20 amp breakers. Is there too much voltage drop to use the extension cords. I also have a 3500 watt honda generator that I have never used but if the extension cord thing won't work I'll have to fire it up for the first time.

Off topic but I wanted to give everybody here that helped me with my house wiring questions a few years a big thank you. I built and wired the house myself. My family has been living in the house for over a year now and thinks it's much better than the mobile home we were living in.

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rjoh878646

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.

November 21, 2007 at 3:04PM
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twigpig

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?

November 22, 2007 at 1:46PM
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brickeyee

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.

November 22, 2007 at 1:56PM
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twigpig

Thanks again to both of you. Generator it is.

November 22, 2007 at 11:30PM
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cobraguy

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

November 23, 2007 at 7:30AM
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