Voltage Drop Calculator
Enter voltage, current, run length and conductor size to get the voltage drop of a branch circuit or feeder — single-phase or three-phase, copper or aluminum, AWG or metric mm². No sign-up, instant results.
AWG to mm² cross-section table
| Wire size (AWG/kcmil) | Nominal area |
|---|---|
| 14 AWG | 2.08 mm² |
| 12 AWG | 3.31 mm² |
| 10 AWG | 5.26 mm² |
| 8 AWG | 8.37 mm² |
| 6 AWG | 13.3 mm² |
| 4 AWG | 21.2 mm² |
| 3 AWG | 26.7 mm² |
| 2 AWG | 33.6 mm² |
| 1 AWG | 42.4 mm² |
| 1/0 AWG | 53.5 mm² |
| 2/0 AWG | 67.4 mm² |
| 3/0 AWG | 85 mm² |
| 4/0 AWG | 107 mm² |
| 250 kcmil | 127 mm² |
| 300 kcmil | 152 mm² |
| 350 kcmil | 177 mm² |
| 400 kcmil | 203 mm² |
| 500 kcmil | 253 mm² |
Resistivity values are the 20 °C IEC constants for annealed copper (0.0172 Ω·mm²/m) and aluminum (0.0283 Ω·mm²/m). See the temperature note under the result for running conductors near 75 °C.
How the Calculator Works
The calculator uses the standard resistance method from Ohm’s law. Loop resistance of the run is R = ρ × L / A, with resistivity ρ = 0.0172 Ω·mm²/m for annealed copper and 0.0283 Ω·mm²/m for aluminum at 20 °C, L the one-way length and A the cross-section in mm². The voltage drop formula is then:
Single-phase: Vd = 2 × L × I × ρ / A
Three-phase: Vd = √3 × L × I × ρ / A
The factor 2 accounts for the out-and-back conductors of a single-phase circuit; √3 replaces it on a balanced three-phase circuit because the phasor sum of the drops across the three conductors is √3 × the per-conductor drop. The same equation for voltage drop expressed with a resistance constant in Ω/km or Ω/kft gives identical results — only the units change.
Three limitations to keep in mind: resistivity is the 20 °C value, so a conductor running at its 75 °C ampacity temperature shows roughly 22 % more drop (the calculator prints this corrected estimate under every result); power factor is assumed near unity — on heavily inductive loads the resistive and reactive components combine as a phasor sum; and skin effect is negligible below 500 kcmil at 60 Hz.
When Engineers Use This Calculator
Sizing branch circuits and feeders. NEC 210.19(A) and 215.2(A)(1) informational notes recommend a maximum of 3 % drop on branch circuits and 5 % on the combination of feeder and branch — the two thresholds this tool checks against.
Checking long motor and heater runs. A 12 AWG copper branch that is fine at 50 ft can exceed 5 % at 200 ft; running the numbers before ordering wire avoids an undersized pull and nuisance undervoltage trips at the equipment.
Verifying existing installations. Measuring low voltage at a machine? Compare the nameplate voltage with the calculated drop for the installed conductor — if the calculated drop is small but measured sag is large, the problem is usually a loose termination rather than the conductor.
Specifying panel feeders. Feeder length from the main switchboard to a distribution panel is fixed by the building layout; the calculator shows which standard size (8 AWG up through 500 kcmil) keeps the drop inside the recommended window at the calculated load current.
Frequently Asked Questions
What is the voltage drop formula?
Vd = 2 × L × I × ρ / A for single-phase and Vd = √3 × L × I × ρ / A for three-phase, where L is the one-way run length, I the load current, ρ the conductor resistivity (0.0172 Ω·mm²/m copper, 0.0283 Ω·mm²/m aluminum at 20 °C) and A the cross-section in mm². Example: 20 A over 100 ft of 12 AWG copper (3.31 mm²) single-phase at 120 V → 2 × 30.48 × 20 × 0.0172 / 3.31 ≈ 6.3 V, or 5.3 % — over the 3 % recommendation.
How much voltage drop is allowed by the NEC?
The NEC informational notes (210.19(A), 215.2(A)(1)) recommend no more than 3 % on branch circuits and 5 % combined feeder plus branch. These are recommendations rather than enforceable limits, but most specifications and inspectors treat them as the design target; sensitive equipment often has its own tighter tolerance in the datasheet.
Does the calculator handle three-phase circuits?
Yes — switch System to three-phase and the loop factor changes from 2 to √3, which reflects the phasor sum on a balanced three-wire or four-wire circuit. For a three-phase load fed with a full-size neutral, the phase-conductor drop still governs, so √3 × L × I × R remains the right first-order estimate.
Why does aluminum drop more voltage than copper?
Aluminum’s resistivity is about 65 % higher (0.0283 vs 0.0172 Ω·mm²/m), so at the same cross-section an aluminum conductor drops roughly 65 % more voltage. In practice you compensate with the next-larger standard size — the wire size calculator on this site applies both materials against the NEC 310.16 ampacity table.
Should I use the 20 °C or the hot-conductor resistance?
Conservative practice sizes with the hot value: copper at 75 °C has about 21.6 % more resistance than at 20 °C (temperature coefficient ≈0.00393/°C). Every result above therefore prints both the 20 °C drop and the estimated 75 °C drop, so you can check the circuit against the 3 % and 5 % windows under running conditions.
Related Guides
- Wire Size Calculator — NEC 310.16 ampacity-based conductor sizing with a voltage-drop check.
- Conduit Fill Calculator — minimum EMT, RMC and PVC conduit size by conductor count.
- Transformer kVA Calculator — full-load current for the source end of the feeder.
- Industrial Voltage Stabilizer Selection Guide — what to do when the supply itself sags.
EN