Here's why. The issue is the voltage that will be presented to the electric lawn mower itself. The torque produce by the mower motor is proportional to the square of the voltage at the motor terminals. That voltage will be the voltage at the receptacle MINUS the voltage drop in the cable.
Now, the numerical gauge (size) of a wire is inverted - that is, larger conductors have smaller gauge numbers, so a 12 gauge wire is physically larger than a 14 gauge wire, etc. And the resistance in the wire is inversely proportional to wire diameter - so a smaller wire has more resistance. And if the wire has more resistance, then there will be more voltage drop in the wire.
So when the manufacturer tells you that you can use a 150ft 12 gauge extension cord, what he is saying is that the voltage that will be present at the lawn mower end of that cable will be sufficient to produce the torque required to cut grass based on the expected voltage drop in that cable. A 100 ft 12gauge cable will have about 2/3 the voltage drop of a 150 foot cable. So the shorter cable will be fine.
Always use a recommended size wire, or larger. A 14 is better than a 16 and a 12 is better than a 14.
Sometimes you don't know what the wire size is so when operating a lawn mower or hedge trimmer, Operate the tool for a minute and feel the cord. It should never be warm. Then use a larger size wire.
A 12 gage extension cord 100 foot long will be too adequate.
Yes, a smaller gauge is a thicker wire. There's generally no problem with that. If you use a thinner wire it could overheat.
The owner's manual of my 13-amp electric lawn mower says that if I use a 50-foot extension cord, it must be 16-gauge. If I use a 100-foot extension cord, it must be 14-gauge. And if I use a 150-foot extension cord, it must be 12-gauge.
My question is: Can I use a 100-foot 12-gauge extension cord? Will it be inadequate, or perhaps too adequate?
