Voltage Drop Calculator — Single & Three Phase
Who this is for: For electricians and solar installers sizing long runs — detached garages, well pumps, EV chargers, PV home-runs — who need the 3% branch-circuit answer without opening a slide rule.
Enter voltage, load amps and run length — get the voltage drop in volts and percent for every wire size, with the smallest size that holds 3%.
Quick answer: Voltage drop = 2×K×I×L ÷ circular-mils (single phase), with K = 12.9 for copper and 21.2 for aluminum. Example: 120 V, 20 A, 100 ft on 12 AWG copper drops 7.9 V — 6.6%, above the 3% guidance; stepping up to 8 AWG drops it to 3.1 V (2.6%), which passes. Enter your numbers below to see the full table for every size.
| Size | Drop (V) | Drop (%) | vs 3% |
|---|---|---|---|
| 14 | 12.55 | 10.46% | ✗ over |
| 12 | 7.9 | 6.58% | ✗ over |
| 10 | 4.97 | 4.14% | ✗ over |
| 8 | 3.13 | 2.6% | ✓ pass |
| 6 | 1.97 | 1.64% | ✓ pass |
| 4 | 1.24 | 1.03% | ✓ pass |
| 3 | 0.98 | 0.82% | ✓ pass |
| 2 | 0.78 | 0.65% | ✓ pass |
| 1 | 0.62 | 0.51% | ✓ pass |
| 1/0 | 0.49 | 0.41% | ✓ pass |
| 2/0 | 0.39 | 0.32% | ✓ pass |
| 3/0 | 0.31 | 0.26% | ✓ pass |
| 4/0 | 0.24 | 0.2% | ✓ pass |
| 250 | 0.21 | 0.17% | ✓ pass |
| Formula (1φ/DC) | VD = 2 × K × I × L ÷ cmil |
|---|---|
| Formula (3φ) | VD = 1.732 × K × I × L ÷ cmil |
| K — copper / aluminum | 12.9 / 21.2 Ω·cmil/ft |
| Design targets | 3% branch · 5% total (NEC informational) |
| Example above | 120 V · 20 A · 100 ft: 12 AWG = 6.6% ✗ → 8 AWG = 2.6% ✓ |
| Status | Figures follow NEC 2017+ reference tables (310.16 ampacity, Chapter 9 conduit fill, 314.16 box fill, 240.4(D) small-conductor caps) and standard equal-friction duct sizing, compiled September 2026. Estimates only — final designs belong to a licensed electrician / HVAC designer per local code. |
What does this voltage drop calculator show?
Voltage drop is the voltage the conductors themselves eat before power reaches the load. This calculator applies the standard K-factor method — VD = 2×K×I×L ÷ circular-mils for single-phase and DC, √3×K×I×L ÷ cmil for three-phase, with K = 12.9 for copper and 21.2 for aluminum — across every conductor size from 14 AWG to 250 kcmil. You get the drop in volts and as a percentage of source voltage, plus a pass/fail against the 3% branch-circuit and 5% total guidance the NEC prints in its informational notes. Southwire's own calculator only covers its own cable scenarios; this one shows the full table so you can see exactly where the 3% line falls.
Common uses
- Sizing a feeder to a detached garage, shed or barn before trenching
- Checking an EV charger or heat-pump circuit on a long residential run
- Comparing copper vs aluminum cost on a 150 ft+ service lateral
- Verifying a PV string home-run stays inside 3% before final inspection
How the K-factor method works
K is the resistance of one foot of a conductor with a cross-section of one circular mil, expressed in ohm-cmil per foot. Multiply by four real-world quantities — the circuit multiplier (2 or √3), current in amps, one-way length in feet — and divide by the conductor's circular-mil area, and you get volts lost in the conductors. The method assumes the conductor is at about 75 °C, which is the normal operating condition for a loaded branch circuit; a cold, lightly loaded wire drops slightly less. Because it's an approximation good to a few percent, treat results within half a point of the limit as 'step up one size' rather than 'pass'.