Voltage Drop CalculatorCable size · AWG · mm²
Voltage drop is the voltage lost along a cable because of its resistance: Vd = 2 × I × ρ × L ÷ A for a single-phase or DC run (√3 instead of 2 for three-phase). Most codes recommend keeping it under 3% for a branch circuit and 5% overall. Enter the wire size in AWG or mm², the length, current and supply voltage to get the volts lost, the percentage drop and the smallest cable that stays within 3%.
Formula verified against OpenStax — University Physics, Volume 2 · NFPA 70, National Electrical Code — Article 210.19 informational note
By Manzoor Ahmad · Pharmacist & calculator developerReviewed & updated
Cable run
Length is the one-way distance from the supply to the load.
Step-by-step working
What is voltage drop?
Voltage drop is the reduction in voltage between the source and the load, caused by the resistance of the connecting wires. The longer the cable and the higher the current, the bigger the drop.
Too much drop makes lights dim, motors overheat and electronics misbehave, and it wastes energy as heat in the cable. Electricians size cables so the drop stays within a few percent.
Quick answers
How do you calculate voltage drop in a wire?
Vd = 2 × I × ρ × L ÷ A for single-phase or DC. 10 A over 20 m of 1.5 mm² copper drops 2 × 10 × 1.724×10⁻⁸ × 20 ÷ 1.5×10⁻⁶ = 4.6 V.
What is the maximum allowable voltage drop?
The NEC recommends no more than 3% on a branch circuit and 5% total (an informational note, not a hard rule). IEC 60364 typically allows 3% for lighting and 5% for other uses from the origin of the installation.
What size wire do I need for 100 feet?
It depends on current and voltage. For 20 A at 120 V over 100 ft, 3% (3.6 V) needs about 13.3 mm² (6 AWG) at 75 °C. The tile above gives the minimum size for your run.
Does voltage drop waste energy?
Yes. The cable dissipates I²R as heat. Halving the drop by doubling the cable area also halves that loss. The watt calculator shows what the lost power costs.
Voltage drop formula
Copper ρ₂₀ = 1.724 × 10⁻⁸ Ω·m, α = 0.00393/K. Aluminium ρ₂₀ = 2.82 × 10⁻⁸ Ω·m, α = 0.00403/K.
| Symbol | Meaning | SI unit |
|---|---|---|
| Vd | Voltage drop | V |
| I | Load current | A |
| ρ | Conductor resistivity | Ω·m |
| L | One-way cable length | m |
| A | Conductor cross-sectional area | m² |
| α | Temperature coefficient of resistance | 1/K |
How to calculate voltage drop
- Measure the one-way length from the panel to the load.
- Find the conductor area (AWG to mm² from the dropdown).
- Multiply 2 (or √3) × current × resistivity × length, then divide by the area.
- Divide by the supply voltage and multiply by 100 for the percentage. Aim for 3% or less.
Worked example: 16 A over 30 m of 2.5 mm² copper
A 230 V single-phase circuit feeds a 16 A load 30 m away on 2.5 mm² copper at 20 °C.
% drop = 6.62 ÷ 230 = 2.88%, just inside 3%
When the cable warms to 75 °C under load, resistance rises about 22% and the drop reaches 8.05 V (3.5%). In US terms, 20 A over 50 ft of 12 AWG at 120 V and 75 °C drops 3.86 V (3.2%).
AWG to mm² conversion and resistance
Copper resistance per 1,000 ft and per km at 20 °C (solid conductor).
| AWG | Area (mm²) | Ω per 1,000 ft | Ω per km |
|---|---|---|---|
| 14 | 2.08 | 2.53 | 8.29 |
| 12 | 3.31 | 1.59 | 5.21 |
| 10 | 5.26 | 1.00 | 3.28 |
| 8 | 8.37 | 0.628 | 2.06 |
| 6 | 13.3 | 0.395 | 1.30 |
| 4 | 21.2 | 0.249 | 0.815 |
| 2 | 33.6 | 0.156 | 0.513 |
Common mistakes to avoid
- Using the round-trip length. Enter the one-way distance; the formula’s factor of 2 accounts for the return conductor.
- Sizing for voltage drop only. A cable must also meet its ampacity rating for the installation method.
- Ignoring temperature. Hot conductors have about 20% more resistance than the 20 °C datasheet value.
Manzoor Ahmad
Pharmacist turned Web Developer & Calculator Engineer · 13+ years in web developmentI’m a pharmacist by training who turned into a full-time tech geek, and for 13+ years I’ve designed and coded web tools. My pharmacy degree included physical pharmacy and pharmaceutics — states of matter, heat, flow and reaction kinetics — so the physics behind these tools is familiar ground, not a new hobby.
I’m not a physicist, so I don’t ask you to take my word for it: every formula on this page is traced to an open university textbook or NIST reference data, and the calculator is tested against published worked examples before it goes live. Spot an error? Tell me and it will be fixed and dated on the next update.
Voltage Drop Calculator: frequently asked questions
The resistance of the conductors. Current flowing through a wire with resistance R loses V = IR along the way, and longer, thinner and hotter wires have more resistance.
3% for a branch circuit and 5% for feeder plus branch is the common guideline in North America and similar in IEC-based codes.
Yes. Resistance is inversely proportional to area, so doubling the cross-section halves the voltage drop.
Copper has about 61% of aluminium’s resistivity, so for the same drop aluminium needs roughly 1.6× the cross-sectional area.
In a balanced three-phase system the line-to-line voltage drop is √3 times the drop in a single conductor, instead of 2× for a go-and-return pair.
References & sources
- OpenStax — University Physics, Volume 2 — Ch. 9.3, Resistivity and Resistance
- NFPA 70, National Electrical Code — Article 210.19 informational note — 3% / 5% voltage drop recommendation
- IEC 60364-5-52 — Selection of wiring systems — voltage drop limits (Annex G)
- HyperPhysics, Georgia State University — resistivity of copper and aluminium
Disclaimer: This calculator estimates resistive voltage drop for planning and education. Cable sizing must also satisfy ampacity, installation method, protection and your local electrical code. Use a licensed electrician for installation work.