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Voltage Drop Calculator for Copper and Aluminium Cable
Enter the supply voltage, the current, the one-way cable length and the conductor size. The calculator finds the voltage lost along the cable, the percentage drop, the power lost as heat and the smallest conductor for your target drop.
How voltage drop is calculated
Every conductor has some resistance, so current flowing through it loses voltage along the way. The resistance of a conductor is R = ρ × L ÷ A, where ρ is the resistivity of the metal, L the length and A the cross-sectional area. Copper is about 17.2 nanohm-meters at 20 °C and aluminium about 28.2, so for the same size aluminium has roughly 64 percent more resistance.
For two-wire DC or single-phase AC, the current passes out and back, so the drop is 2 × I × R over the one-way length. For a balanced three-phase system the line-to-line drop is √3 × I × R. The percentage drop is the lost voltage divided by the supply voltage.
Conductor sizes
Metric cables are labeled by cross-sectional area in square millimeters. American wire gauge (AWG) sizes are defined by a diameter formula, d = 0.005 × 92(36 − n) ÷ 39 inches, so each step of three gauge numbers roughly halves the area. For instance 12 AWG has a diameter of about 2.05 mm and an area close to 3.31 mm², while 10 AWG has about 5.26 mm². The calculator converts AWG to area with the formula, so a size listed here is the plain solid conductor, and stranded cable may differ slightly.
A larger conductor lowers the drop but costs more and is harder to terminate, so the choice balances cost, safety and performance.
Reading the result
- Percent drop. Lower is better. Many designers aim for a few percent at most on a single circuit, but the figure that applies to a project comes from its code or specification.
- Power lost as heat. This is the I²R loss in the cable, and it adds to the running cost and warms the cable.
- Area needed. The smallest cross-section that meets your target on drop alone. Round up to the next standard size.
Low-voltage circuits, such as 12 V lighting, solar and automotive wiring, suffer most because the same few volts represent a big share of the supply.
Estimating, codes and safety
This tool is for estimating and learning. It checks voltage drop only. Current-carrying capacity, derating for ambient temperature and grouping, short-circuit protection, earthing, and the permitted installation method are separate requirements, and local electrical codes govern all of them. A qualified electrician or engineer must design and approve a real installation. An undersized cable can overheat and start a fire even if its voltage drop looks acceptable, so never choose a cable from this result alone.
Worked example
A 15 A load 100 ft from a 120 V single-phase supply, wired in 12 AWG copper.
Inputs
| System | AC single-phase (two conductors) |
|---|---|
| Supply voltage | 120 V |
| Load current | 15 A |
| Units | Imperial (feet, AWG) |
| One-way cable length (feet or meters) | 100 |
| Conductor size (AWG) | 12 AWG |
| Conductor material | Copper (17.2 nΩ·m at 20 °C) |
| Target maximum drop | 3 % |
Result
Voltage at the load: 115.25 V.
The drop is above your target of 3%. Use a larger conductor or a shorter run.
| Conductor area | 3.31 mm² |
|---|---|
| Resistance per meter | 5.198 mΩ/m |
| Voltage drop | 4.753 V |
| Drop as % of supply | 3.96% |
| Power lost as heat | 71.3 W |
| Area needed for a 3% drop | 4.37 mm² |
- Resistance is taken at 20 °C and reactance is ignored. Hot cables, long AC runs and low power factors can drop more.
- Cable ampacity, insulation and installation method are separate checks that this calculator does not make.
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
How do I calculate voltage drop?
Multiply twice the current by the one-way length and the conductor resistance per unit length for a two-wire circuit. For three-phase use √3 instead of 2. Divide by supply voltage for the percentage.
What voltage drop is acceptable?
It depends on the code and the application. A few percent is a common design goal, but the limit that applies to your project comes from local regulations and the equipment maker.
Is aluminium worse than copper for voltage drop?
Aluminium has higher resistivity, so for equal size it drops about 64 percent more voltage. It is often used in larger, lighter feeders with a bigger cross-section to compensate.
Does the calculator account for temperature?
Resistance is taken at 20 °C. Conductors run hotter in practice, which raises resistance and drop, so treat the figure as a lower bound for heavily loaded cables.
Why is the length one-way?
The calculator automatically doubles it for two-wire circuits to include both the outgoing and return conductors, so you only need the distance from source to load.
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References and sources
- IEC 60228: conductors of insulated cables. ASTM B258: standard specification for nominal diameters of round wires used as electrical conductors, including AWG sizes.