More on wire size

Here are some quick figures using the dc resistance of wire at 25 degrees c. It will get you in the ball park.

#14 has a resistance of 2.575 ohms / 1000 feet.
#12 has a resistance of 1.619 ohms / 1000 feet.
#10 has a resistance of 1.018 ohms / 1000 feet.

The below % drop values are figured from 120 volt supply.

With #14 at 15 amps (I*R) you will have a voltage drop of 38.6V /1000 ft. That = 3.86v /100 feet of wire. Times 2 for 2 wire circuit = 7.7 volts drop in a 100 foot circuit. That is = to about a 6% drop.

With #12 at 15 amps (I*R) you will have a voltage drop of 24.29 volts / 1000 ft. That = 2.43v / 100 feet of wire. Times 2 for 2 wire circuit = 4.86v drop in a 100 foot circuit. That is = to about 4% drop.

With #10 at 15 amps (I*R) you will have a voltage drop of 15.27 volts / 1000 ft. That = 1.53v / 100 feet of wire. Times 2 for 2 wire circuit = 3.06v drop in a 100 foot circuit. That is = to about 2.55% drop.

With #12 at 20 amps (I*R) you will have a voltage drop of 32.38 volts / 1000 ft. That = 3.2 v / 100 feet of wire. Times 2 for a 2 wire circuit = 6.4 v drop in a 100 foot circuit. That is = to about 5.5% Drop.

You can see by the voltage drop that if you went to 200 feet in the circuit that you would double that voltage drop. To maintain the Percentage to around the 5% range you would have to go up 2 wire sizes at double the length.

With #14 at 15 amps you have about a 6% voltage drop for a 100 foot circuit.

To maintain 6% drop at 15 amps you could go to 150 feet with the #12.
To maintain 6% drop at 15 amps you could go to 235 feet with the #10

Regards
Gary Schafer