Wire Voltage Drop Calculator
Calculate the voltage drop along any wire run.
Enter wire gauge, length, current, and material to find voltage loss and check if wiring meets code.
Voltage Drop
Voltage drop is the reduction in electrical potential as current flows through a wire. Long wire runs, thin wires, and high currents all increase voltage drop. Too much voltage drop causes equipment to malfunction, motors to overheat, and lights to dim.
The formula:
Voltage Drop (V) = I × R_total
R_total = 2 × L × ρ / A
Where:
- I = current (Amperes)
- L = one-way wire length (meters or feet)
- ρ = resistivity of the conductor material (Ω·m)
- A = cross-sectional area of the wire (m²)
- The factor of 2 accounts for the return wire (complete circuit)
Resistivity of common conductor materials:
- Copper: 1.724 × 10⁻⁸ Ω·m (most common, lowest resistance)
- Aluminum: 2.65 × 10⁻⁸ Ω·m (used in large panels and long runs)
- Silver: 1.59 × 10⁻⁸ Ω·m (best conductor, rarely used due to cost)
AWG (American Wire Gauge) cross-sectional areas:
| AWG | Diameter (mm) | Area (mm²) | Max Current (A) |
|---|---|---|---|
| 18 AWG | 1.02 mm | 0.823 mm² | 7–10 A |
| 16 AWG | 1.29 mm | 1.31 mm² | 13 A |
| 14 AWG | 1.63 mm | 2.08 mm² | 15–20 A |
| 12 AWG | 2.05 mm | 3.31 mm² | 20 A |
| 10 AWG | 2.59 mm | 5.26 mm² | 30 A |
| 8 AWG | 3.26 mm | 8.37 mm² | 40–55 A |
| 6 AWG | 4.11 mm | 13.3 mm² | 55–75 A |
Acceptable voltage drop limits:
| Application | NEC Recommendation | Typical Limit |
|---|---|---|
| Branch circuits | ≤ 3% | 3.6 V on 120 V |
| Feeders | ≤ 3% | — |
| Total (combined) | ≤ 5% | 6 V on 120 V |
| DC systems (solar, automotive) | ≤ 2–3% | Varies |
Practical notes:
- Voltage drop matters most on long runs — a 50-foot (15 m) circuit rarely has issues; a 200-foot (61 m) run often does.
- If voltage drop exceeds 3%, use the next larger wire gauge.
- Voltage drop causes heat in the wire, which is both inefficient and a fire hazard.
- Use copper for residential wiring; aluminum is acceptable for large service entrance conductors.