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I want to buy an electric chainsaw, rated at 15A (125V). Manufacturer specifies max 50' extension cord (no gauge spec). There are 50' 14 gauge extension cords that are rated at 15A/125V. Could I use a 100' 10 or 12 gauge cord? Is the basic problem just heat + power loss due to resistance in the cord? Any measurements I could do with my volt meter to make sure it will work okay?

I plan to use chainsaw for short periods of time, and when its cold out, to prevent overheating. The outlet is right below my main panel. Any other tips?

The issue here is voltage drop in the circuit supplying the saw. The saw's motor has an optimal voltage range, and will not run well if the voltage is too low. The voltage drops over the length of the extension cord; the longer the cord, the greater the drop. However, a heavier gauge cord will have less voltage drop than a lighter gauge cord.

(Voltage drop is a factor right at the receptacle, even without an extension cord - the gauge and length of the wiring from the panel to the receptacle, the load on that branch circuit, the load on your whole service, the gauge and length of your service conductors / feeders, heck even the load on neighbors service can affect your voltage at the receptacle.)

You didn't mention the manufacturer specifying a gauge for the 50' max extension cord, let's assume they figure you'll use a 14 gauge cord, and let's assume the saw draws a full 15A (which it probably does not). The voltage drop for that 14 gauge cord at 15 amps is about 3.79 volts. A 12 gauge cord 100' long at 15A will drop about 4.75 volts. So I'd feel completely confident with a 12 gauge 100' cord - one volt is negligible.

If you really want to test the voltage drop, testing at the end of the cord doesn't tell you anything because you need to see what the voltage is under load. If you really want to test this, and if you can do this safely, you could plug a receptacle splitter or power tap (rated for the load, of course) and check voltage while someone runs the saw full bore cutting wood.

Yes, the critical issue is voltage drop due to resistance, which is both a function of conductor size and plug connections. One 100' #12 cord is better than two 50' #12 cords, for example, because there will be fewer contact connections.

I wouldn't hesitate to use your saw on a 100' #12 or #10 cord, assuming that you're not already at the end of a very long outlet circuit as well. You can check voltage across the plug contacts to make sure you're within about 5% of nominal (120v). As brhans pointed out in the comments, this would need to be done while the tool is under load.

You can decide what is the maximum cord length you should use for yourself, when you figure your max cord length for a given guage wire with :

V drop = ( K x P x L x A )/( M )
Where:

• K = aproximate specific resistivity in ohm circular mils per foot




• P = phase constant ( 2 fir 1 phase, 1.732 for 3 phase )




• L = Wire length ( one way not there and back )




• A = current in amperes




• M = wire area in circular mills

Values for K are:

• 11 for solid or stranded copper 77 - 121 F




• 12 for solid or stranded copper 122 - 167 F




• 18 for solid aluminum 77-121 F




• 19 for stranded aluminum 77-121 F




• 20 for both aluminum 122-167 F

The wire circular mils can be easily looked up.

You want the V drop to be 2% or less.

Calculate that with ( V drop ) / ( V input )

Answers to you 3 questions...

1. 
   

   Can I use a 10 or 12 guage cord?

   
   
   
   
   

   • 10 guage = ( 11 x 2 x 100 x 15 ) / 10400 = 3.17308 V and 3.17308/120 = 0.026442 (2.64%)

   
   

   • 12 guage = 9(11 x 2 x 100 x 15)/6530 = 5.0536 V and 5.0536/120 = 0.042113 (4.21%)

   
   
   
   

So I'd say No (a drop greater than 2% severely decreases efficiency and life of the equipment.)

2. is heat + powerloss the big problem? Again no, a drop greater than 2% will shorten the chainsaw motor's life.




3. Voltmeter measurements.
   Well yes, you could measure the locked motor current to find the maximum current draw the chainsaw will ever use, but the chances of that happening irl are small. You could also measure running draw at load to get a precise number, but that's a good bit of work for little return.

Overall I'd say move your power source closer (a generator?), get a gas chainsaw, or live with the shortened life expectancy.