A 16 amp load at the end of 100 feet of 12 AWG copper on a 120 volt circuit loses 6.32 volts, a 5.27% voltage drop, so only 113.68 volts reach the load. The NEC's informational notes suggest keeping a branch circuit under 3%, which on that run takes 8 AWG copper (2.08%). Voltage drop grows with current and distance and shrinks with the wire's cross section, so a long run is the case where you size up.
How the voltage drop is calculated
The calculator uses the K method taught by electrical inspectors. K is the resistance of a copper or aluminum conductor one circular mil in area and one foot long at 75 °C: 12.9 for copper and 21.2 for aluminum, the figures IAEI Magazine derives from NEC Chapter 9 Table 8. Multiply K by the current and the one-way length, double it because current flows out on one wire and back on the other, and divide by the conductor's circular mils.
VD = 2 × K × I × L ÷ CM; three-phase: VD = 1.732 × K × I × L ÷ CM
Percent drop is VD ÷ source voltage × 100. Turn the formula around and you get the circular mils a run needs: CM = 2 × K × I × L ÷ allowed volts. The calculator does this for you and picks the first size in Table 8 that is large enough.
Worked example: 16 A over 100 ft at 120 V
Enter the defaults: single-phase, 120 V, copper, 12 AWG, 16 A, 100 ft.
- Drop: 2 × 12.9 × 16 A × 100 ft ÷ 6,530 cmil = 6.32 V
- Percent: 6.32 ÷ 120 × 100 = 5.27%, leaving 113.68 V at the load
- For 3%, the run may lose 3.6 V: 2 × 12.9 × 16 × 100 ÷ 3.6 = 11,467 cmil. 10 AWG has 10,380 cmil (3.98 V, 3.31%), so the answer is 8 AWG with 16,510 cmil: 2.50 V, 2.08%
The same run at 240 V drops the same 6.32 V, but that is only 2.63%. Doubling the voltage halves the current for the same power, which is why long runs to a shop or well pump are often wired at 240 V.
Voltage drop per 100 feet, copper, 10 A
The table below is 10 amps through 100 feet one way; scale it by your amps and feet, since drop is proportional to both.
| Wire size | Drop | % at 120 V | % at 240 V |
|---|---|---|---|
| 14 AWG | 6.28 V | 5.23% | 2.62% |
| 12 AWG | 3.95 V | 3.29% | 1.65% |
| 10 AWG | 2.49 V | 2.07% | 1.04% |
| 8 AWG | 1.56 V | 1.30% | 0.65% |
| 6 AWG | 0.98 V | 0.82% | 0.41% |
| 4 AWG | 0.62 V | 0.52% | 0.26% |
Circular mils are from NEC Chapter 9 Table 8. Aluminum drops about 64% more than copper of the same size (21.2 ÷ 12.9): 12 AWG aluminum on the worked example would lose 10.39 V, 8.66%.
3% and 5%: what the NEC says
Voltage drop appears in the NEC only as informational notes under 210.19(A) for branch circuits and 215.2(A) for feeders: about 3% on a branch circuit, and no more than 5% for feeder plus branch circuit together. IAEI points out that informational notes are not enforceable code (90.5(C)), but the installation instructions of equipment, which 110.3(B) makes you follow, often require a minimum voltage at the terminals. Pick "5%, feeder + circuit" when you are checking a subpanel feeder and the circuit beyond it as one path.
Voltage drop never replaces ampacity. A size that passes the drop check still has to carry the current under Table 310.16, which is why the result also checks the 75 °C ampacity. The wire size calculator runs both checks and gives you the larger size.
12 V and 24 V DC
Low-voltage DC is where drop bites hardest, because every volt lost is a large share of 12. The same formula applies (the 2 counts the positive and the negative wire). A 10 A load 20 feet from a 12 V battery loses:
| Wire size | Drop | % at 12 V | % at 24 V |
|---|---|---|---|
| 16 AWG | 2.00 V | 16.67% | 8.33% |
| 14 AWG | 1.26 V | 10.46% | 5.23% |
| 12 AWG | 0.79 V | 6.58% | 3.29% |
| 10 AWG | 0.50 V | 4.14% | 2.07% |
| 8 AWG | 0.31 V | 2.60% | 1.30% |
At 12 V you need 8 AWG to stay under 3%; at 24 V, 10 AWG does it. Use the DC tab and the 12 V or 24 V preset for RV, boat, solar and landscape lighting runs.
Three-phase circuits
For balanced three-phase loads, the multiplier is √3 (1.732) instead of 2, because the phase currents return on the other phase conductors. A 50 A motor feeder 200 feet long on 6 AWG copper at 480 V drops 8.52 V, 1.77%. Enter the line-to-line voltage (208, 240 or 480 V).
Limits of this method
K is a direct-current resistance at 75 °C. For AC conductors of 2/0 and larger, skin effect raises the resistance a little, so the true drop is somewhat higher than shown; engineers use the impedance values in Chapter 9 Table 9 for those. Conductors running cooler than 75 °C drop slightly less.
Next steps
Once the size is set, find the conduit size for those wires, check the breaker size for the load, or convert a nameplate in watts with the amp calculator. The full set of tools is on the electrical calculators page.