#10 wire 30 amp 140 feet
If you draw the full 30 amps, the voltage drop is about 8-1/2 volts or seven percent of 120. That's considered high considering we have not included voltage drop elswhere in the system such as between the utility pole and the house. Voltage drop should not be more than 3-1/2 to 4 percent. If the voltage at the start of the 140 foot run was the full 120, you could get away with 7% drop. But if the source voltage was a little less, like 114 which is still within tolerance from the power company, you would have brownout conditions out in the garage.
Two forty is less sensitive to voltage drop issues than 120 volts. It's the same 8-1/2 volts lost to voltage drop in this example but that is 3-1/2 percent of 240, within acceptable voltage drop within the home.
Don't forget that, if you use a 30 amp breaker for the 10 gauge wires, you need a subpanel with 15 amp and 20 amp breakers out in the garage to feed the lights and receptacles and garage door opener.
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Last edited by AllanJ; 06-10-2012 at 06:30 PM.