Voltage Drop Calculator
Calculate voltage drop across cables and wires for copper and aluminum conductors.
Component Values
Results
AWG Wire Size Reference
| AWG | mm² | Copper (Ω/km) | Aluminum (Ω/km) |
|---|---|---|---|
| 10 | 5.261 | 3.19 | 5.04 |
| 12 | 3.309 | 5.08 | 8.01 |
| 14 | 2.081 | 8.07 | 12.73 |
| 16 | 1.309 | 12.83 | 20.24 |
| 18 | 0.823 | 20.41 | 32.20 |
| 20 | 0.518 | 32.43 | 51.16 |
| 22 | 0.326 | 51.53 | 81.29 |
| 24 | 0.205 | 81.95 | 129.27 |
| 26 | 0.129 | 130.23 | 205.43 |
| 28 | 0.081 | 207.41 | 327.16 |
| 30 | 0.0509 | 330.06 | 520.63 |
Understanding voltage drop in cables
Voltage drop is the reduction in voltage across a conductor due to its resistance. Every wire has resistance proportional to its length and inversely proportional to its cross-sectional area. The total cable resistance for a round-trip circuit is R = p x 2L / A, where p is the material resistivity, L is the one-way length, and A is the cross-section.
The IEC 60364 standard recommends that voltage drop should not exceed 3% for lighting circuits and 5% for other loads. The NEC similarly recommends 3% for branch circuits and 5% total. Excessive voltage drop wastes energy as heat in the cable and can cause equipment to malfunction, especially motors and sensitive electronics.
Copper (p = 1.68e-8 ohm.m) is the most common conductor material due to its low resistivity. Aluminum (p = 2.65e-8 ohm.m) has about 60% higher resistivity but is lighter and cheaper, making it popular for overhead power lines and large installations. To compensate, aluminum cables use a larger cross-section than copper for the same current capacity.
For long cable runs -- solar panel arrays, industrial plants, EV charging stations -- voltage drop calculation is critical. Increasing wire gauge (larger cross-section) reduces resistance and voltage drop. The trade-off is cost and weight. Always verify that your cable sizing satisfies both current capacity (ampacity) and voltage drop requirements.
Cable Resistance
R_cable = ρ × 2L / AVoltage Drop
V_drop = I × R_cableDrop Percentage
Drop% = (V_drop / V_source) × 100Key Points
- Voltage drop is proportional to current and cable length
- IEC 60364: max 3% for lighting, 5% for other circuits
- Copper has ~40% less resistivity than aluminum
- Doubling the cross-section halves the voltage drop
- Round-trip length = 2 x one-way cable length
Applications
- Residential and commercial wiring design
- Solar panel array cable sizing
- EV charging station installation
- Industrial plant power distribution
AWG quick-reference table (copper wire)
| AWG | Diameter (mm) | Area (mm²) | Max Current (A) | Resistance (mΩ/m) |
|---|---|---|---|---|
| 10 | 2.59 | 5.26 | 30 | 3.28 |
| 12 | 2.05 | 3.31 | 20 | 5.21 |
| 14 | 1.63 | 2.08 | 15 | 8.28 |
| 16 | 1.29 | 1.31 | 13 | 13.2 |
| 18 | 1.02 | 0.82 | 10 | 20.9 |
| 20 | 0.81 | 0.52 | 7 | 33.2 |
| 22 | 0.64 | 0.33 | 5 | 52.9 |
| 24 | 0.51 | 0.20 | 3 | 84.2 |
NEC / IEC voltage drop guideline
NEC/IEC guideline: voltage drop should not exceed 3% for branch circuits or 5% total (feeder + branch combined). For sensitive electronics — microcontrollers, sensors, audio — keep drop below 2% to avoid measurement errors and logic instability.
Practical example
Calculate round-trip voltage drop for a 5-metre LED strip cable run using 18 AWG copper wire.
R = 2 × 5m × 20.9 mΩ/m = 209 mΩ
Drop = 3A × 0.209Ω = 0.63V (5.2%) — exceeds 5% limit
Solution: upgrade to 16 AWG (13.2 mΩ/m) → drop = 0.40V (3.3%)
Did you know? Voltage drop in long cable runs is a critical issue in solar installations and marine/automotive wiring. A 10 m run of AWG 18 wire at 5 A can drop nearly 1 V — enough to cause a 5 V microcontroller to brown-out or an LED strip to visibly dim at the far end.