Voltage drop calculator
Enter the circuit and conductor to get the voltage drop in volts and percent, the voltage at the load, and the smallest wire size that meets your target.
Inputs
Result
Voltage drop for every wire size
Same circuit, same length, each conductor size. Ampacity is the 75 °C column of Table 310.16 before any correction or adjustment.
| Size | Drop, V | Drop, % | Ampacity, A |
|---|
How it works
Resistance comes from NEC Chapter 9, Table 8, for stranded conductors at 75 °C. Parallel sets divide the drop by the number of sets. The 2 in the single-phase formula covers the current's path out and back, which is why the length you enter is one way.
What the NEC says about voltage drop
For most circuits, the 3% and 5% figures come from informational notes, such as the one under 210.19(A): a branch circuit sized for no more than 3% drop, and no more than 5% combined across feeder and branch circuit, gives reasonable efficiency. Informational notes are recommendations, not enforceable requirements. Some articles do set hard limits, fire pumps among them, so check the article for the equipment you are wiring.
Limits of this method
Table 8 gives DC resistance. On AC circuits, reactance and skin effect add a little drop that DC resistance does not capture. It is negligible on small conductors and grows on large ones, so for AC feeders of 2/0 and up, treat the result as a slight underestimate or use the effective impedance in Chapter 9, Table 9.
Questions
What is an acceptable voltage drop?
The NEC's informational notes recommend no more than 3% on a branch circuit or feeder and no more than 5% for the two combined. Many designers use 2% for feeders to leave room for the branch circuits.
Is 3% voltage drop a code requirement?
Not for general circuits. The 3% and 5% figures are informational notes, which are advisory. A few articles, such as those for fire pumps and sensitive electronic equipment, set enforceable limits.
How long can a 12 AWG circuit run at 20 amps?
About 45 ft one way on a 120 V circuit before copper 12 AWG reaches 3% drop. A 100 ft run at 20 A drops 7.9 V, or 6.6%, and needs 8 AWG to stay under 3%.
Is the length one way or round trip?
One way. Enter the distance from the panel or source to the load; the formula doubles it for the return path on single-phase and DC circuits.
Why does aluminum need a larger size?
Aluminum has about 61% of copper's conductivity, so for the same drop it needs a larger cross-section, usually two sizes up from copper.
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