Voltage Drop Question

[jfls41]

Am I doing this voltage drop formula correctly?

I am running a feeder line to a detached garage which is about 85 feet from the load center. It is 120volt line, 20 amp, using the voltage drop formula I come up with running an No. 8 awg to maintain a 3% drop in voltage. Does anyone have any comments about using a No. 10 or even No. 12 for this circuit? It seems to me a No. 10 would be sufficient but according to this formula it is more than 3%.

8awg: 26.4 x 85ft x 20a / 16510mils = 2.72 volts
10awg: 26.4 x 85 x 20 / 10380mils = 4.32 volts
12awg: 26.4 x 85 x 20 / 6530mils = 6.87 volts

[sky1]

Are you installing a 20amp breaker as the overcurrent protection for this circuit? If so the maximum load allowed on the circuit is 16 amps. A # 10 would then only drop 3.46 volts which would be less than the 3.6 volts @ 3% suggested by the NEC. Keep in mind this is only a suggested percentage Vd, not a NEC rule.

[jfls41]

What if I use 12/3 with a 20amp gfi breaker, what kind of allowable amps would I have then on this configuration?

thanks

Jeff

[iam14sure]

In this case I would first decide what is the actual load required at the garage. If there are only lights general purpose receptacles and a garage door opener 20 amps is not required. Your actual load would be closer to 8-10 amps. In this case # 12 will work.