Voltage Drop Calculator
Voltage drop is the voltage a wire loses between the panel and the load. For a single-phase circuit it is 2 × K × I × L ÷ CM, where K is 12.9 for copper or 21.2 for aluminum, I is the current in amps, L is the one-way length in feet and CM is the wire's area in circular mils. The NEC's informational notes suggest keeping the drop to 3% on a branch circuit and 5% overall. These are recommendations, not requirements.
Voltage drop calculator
Enter your circuit. Results update as you type and are saved in the page address. Nothing leaves your browser. Lengths are in feet.
How the voltage drop calculator works
The calculator uses the circular-mil method. It multiplies the current, the one-way length and the material constant K, divides by the wire's area in circular mils and applies the multiplier for the circuit type. The percentage is the drop divided by the system voltage.
single-phase: VD = 2 × K × I × L ÷ CM three-phase: VD = 1.732 × K × I × L ÷ CM percent drop = VD ÷ system voltage × 100 voltage at load = system voltage − VD longest run for a target % : L = CM × (target % ÷ 100 × voltage) ÷ (2 × K × I) (three-phase: use 1.732 for 2) circular mils needed : CM = 2 × K × I × L ÷ (target % ÷ 100 × voltage)
- K
- 12.9 (copper) or 21.2 (aluminum), ohm-circular mils per foot at 75 °C
- I
- load current in amperes
- L
- one-way length of the circuit in feet
- CM
- conductor area in circular mils, NEC Chapter 9 Table 8
- 1.732
- the square root of 3, rounded as published
Worked example
Given: a 20 A load on a 120 V single-phase circuit; 75 ft one way; copper wire.
- 12 AWG (6,530 circular mils):
VD = 2 × 12.9 × 20 × 75 ÷ 6,530 = 5.93 V, which is 4.94% of 120 V - 10 AWG (10,380 circular mils):
VD = 2 × 12.9 × 20 × 75 ÷ 10,380 = 3.73 V, which is 3.11% - 8 AWG (16,510 circular mils):
VD = 2 × 12.9 × 20 × 75 ÷ 16,510 = 2.34 V, which is 1.95%
Answer: 12 AWG and 10 AWG are both above the 3% recommendation on this run; 8 AWG is within it. Whether the wire can carry 20 A and which breaker protects it are separate questions: see the wire size calculator.
Constants and where they come from
| Constant | Value | Source |
|---|---|---|
| K, copper | 12.9 ohm-cmil per ft at 75 °C | Source 1 |
| K, aluminum | 21.2 ohm-cmil per ft at 75 °C | Source 1 |
| Three-phase multiplier | 1.732 | Source 1 |
| Branch-circuit recommendation | 3% (informational note, not a requirement) | Source 2 |
| Total (feeder plus branch) recommendation | 5% (informational note, not a requirement) | Source 2 |
| Circular mils, 14 to 500 kcmil | 4,110 (14 AWG), 6,530 (12), 10,380 (10), 16,510 (8) and so on | Source 3 |
Every constant is stored with its source, link and access date in the site's data files.
Circular mils by wire size
| Size | Circular mils |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 3 AWG | 52,620 |
| 2 AWG | 66,360 |
| 1 AWG | 83,690 |
| 1/0 AWG | 105,600 |
| 2/0 AWG | 133,100 |
| 3/0 AWG | 167,800 |
| 4/0 AWG | 211,600 |
| 250 kcmil | 250,000 |
| 300 kcmil | 300,000 |
| 350 kcmil | 350,000 |
| 400 kcmil | 400,000 |
| 500 kcmil | 500,000 |
Assumptions and limits
- The calculation is the direct-current resistance method at 75 °C. It ignores reactance and power factor, which matter on large conductors and long AC runs. IAEI describes it as generally acceptable for this purpose.
- Uncoated, stranded or solid copper and aluminum as listed in Table 8. Aluminum 14 AWG is not offered because Table 310.16 lists no ampacity for it.
- The 3% and 5% figures are informational-note recommendations. Local amendments, the equipment maker's instructions and some specific NEC sections can set different limits.
- The result is a planning figure. A licensed electrician sizes and installs the circuit and follows the code edition your local authority has adopted.
Next: check the same circuit's size against the ampacity table with the minimum-AWG sizing tool.
Frequently asked questions
What is an acceptable voltage drop?
The National Electrical Code does not set one for ordinary branch circuits. Its informational notes to 210.19(A) say conductors sized to prevent a voltage drop exceeding 3% at the farthest outlet, with no more than 5% total on feeders and branch circuits, provide reasonable efficiency of operation. An informational note is explanatory material, not a requirement, but equipment instructions and your local rules may demand more. The calculator labels results against these recommendations.
Is L the one-way length or the round trip?
One way. Enter the distance from the panel to the load along the cable route. The formula multiplies by 2 for single-phase circuits to account for the current going out and coming back.
Why is the percentage taken against the system voltage?
Because what matters is the share of the supply voltage that is lost. A 3.6 V drop is 3% of 120 V but only 1.5% of 240 V. For three-phase circuits use the line-to-line voltage (208 V, 480 V and so on).
Does a larger wire fix a high voltage drop?
Yes. A larger conductor has more circular mils and so less resistance. Shortening the run or lowering the current also works. The wire must still be large enough for the current and protected correctly; use the wire size calculator for that and ask a licensed electrician.
Does this calculator check that the wire can carry the current?
No. It only calculates voltage drop. A wire can have an acceptable drop and still be too small for the breaker. Check the ampacity and the small-conductor limits with the wire size calculator, and remember that derating for bundling and ambient temperature may require a larger wire.
Why does it use 75 °C?
Table 8 gives resistance at 75 °C and the K values are derived from it. IAEI notes that the result is also generally acceptable for 60 °C conductors. Resistance changes with temperature, so a hot conductor drops a little more.
Sources
- IAEI Magazine, "Voltage Drop Formulas" (from Ferm's Formulas and Calculations, 2014): formulas, K = 12.9 and 21.2, 1.732 for three-phase, percentage method (iaeimagazine.org) Accessed 2026-11-14.
- NFPA 70 (NEC) 2020 text as reproduced in the 2022 California Electrical Code: 210.19(A) informational note (3% and 5%), p. 70-69 (law.resource.org) Accessed 2026-11-14. The 2023 NEC is unconfirmed here; check your adopted edition.
- Same document: Chapter 9, Table 8 Conductor Properties (circular mils), p. 70-819 (law.resource.org) Accessed 2026-11-14.
- ESFI, Do-It-Yourself (DIY) Electrical Safety (esfi.org) Accessed 2026-11-14.
- CPSC, Plug into Safety! (NSN 18-07) (cpsc.gov) Accessed 2026-11-14.