How the voltage drop calculator works
For a single-phase circuit it multiplies 2 × K × current × one-way length and divides by the wire’s circular mils. The 2 accounts for the current going out and coming back. Three-phase circuits use the square root of 3 instead of 2, because on a balanced three-phase circuit the line-to-line drop is √3 times the drop in one conductor. K is the resistivity of the conductor in ohm·circular mil per foot: 12.9 for copper and 21.2 for aluminum. The percentage is the drop divided by the system voltage, times 100, and the voltage at the load is the system voltage minus the drop.
Example: 16 A on 100 ft of 12 AWG copper (6,530 circular mils) at 120 V is 2 × 12.9 × 16 × 100 ÷ 6,530 = 6.32 V. That is 5.27% of 120 V, leaving 113.68 V at the load.
What the estimate leaves out
The K-factor method is a shortcut, not a full calculation. It treats resistance as a fixed number, but copper and aluminum resistance rises as the conductor gets hotter, so a heavily loaded wire can drop a little more than shown. It also ignores reactance, which matters on large conductors and long runs, especially in conduit. The result is therefore a guide for ordinary branch circuits and feeder runs, not a substitute for a design calculation.
The NEC informational notes suggest keeping branch-circuit voltage drop to about 3% and the combined feeder and branch drop to about 5%. Those are recommendations for good practice, not requirements, and they do not override any other rule. This calculator marks a result as within the 3% guideline when the percentage is 3 or less.
Wire size, ampacity and who should check it
Voltage drop is only one test of a wire. The conductor must also be sized for ampacity: the current it can carry continuously without overheating, allowing for the insulation, the terminals, the ambient temperature and how many conductors share a conduit. A wire that keeps voltage drop low can still be too small for the breaker, and a wire large enough for the current can still drop too much over a long run. Use the load current you actually expect, not the breaker rating. Have a licensed electrician confirm the circuit, the breaker and the connections before you install anything.
Frequently asked questions
How is voltage drop calculated?
Single phase: 2 × K × current × one-way length ÷ circular mils. Three phase: √3 × K × current × one-way length ÷ circular mils. K is 12.9 for copper and 21.2 for aluminum.
What is an acceptable voltage drop?
The NEC informational notes suggest about 3% for a branch circuit and about 5% for the feeder plus branch combined. They are recommendations, not requirements; check the circuit for ampacity as well.
How much does 12 AWG copper drop over 100 ft at 16 A?
At 120 V single phase it drops 6.32 V, which is 5.27%, leaving 113.68 V at the load. The same run in aluminum drops 10.39 V.