Free Online Calculators
Check the voltage drop on a cable run – copper/aluminium, 1~ and 3~
This calculator determines the voltage drop on a cable based on conductor cross-section, length, current and material. It uses the resistivity of copper or aluminium and accounts for both single-phase and three-phase systems.
Multiply the current by the total resistance of the conductor loop: take the conductor's resistance per unit length, multiply it by the one-way run length, double it for the return path on a single-phase circuit, and multiply by the load current. For three-phase circuits a different multiplier is used instead of doubling, and the result is usually expressed as a percentage of the supply voltage.
Voltage drop per 100 ft depends on the wire gauge, the conductor material, the load current and the circuit type, so there is no single figure. Cable tables list it as volts dropped per 100 ft at a given amp value, and you scale that linearly with your actual current and run length.
The common rule of thumb is to keep the drop on a branch circuit small enough that lights and motors still see nearly full voltage, and to check the total drop from the service to the farthest outlet. Practically, electricians go up a wire size whenever the calculated percentage exceeds the target for the run.
The NEC does not enforce a general voltage drop limit; the recommended maximum values appear only in informational notes for branch circuits and feeders. Mandatory limits exist for a few special cases, such as fire pump circuits and sensitive electronic equipment, and local codes or specifications may set stricter requirements.
The drop on 12 AWG copper depends on the current and the length of the run, so it has to be calculated rather than looked up as a fixed number. Enter the load amps, one-way length and system voltage to get the volts lost and the percentage of the supply voltage.
10 AWG has a lower resistance per foot than 12 AWG, so it drops proportionally less voltage at the same current and length. The exact value still comes from the current, the run length and whether the conductor is copper or aluminium.
The maximum run for 12 gauge wire on 120V is set by the load current and the voltage drop percentage you accept, not by a fixed distance. The lighter the load, the longer the run can be before the drop becomes a problem, so calculate the length at your actual amp draw.
Calculate the drop for the wire size required by ampacity, and if it exceeds your target percentage, step up to the next larger gauge and recalculate until it fits. Remember that the equipment grounding conductor must be increased proportionally when the ungrounded conductors are upsized.
Every time you run a cable from your consumer unit or main panel out to a garden shed, a detached garage, a workshop, an EV wallbox, or a site distribution board, you're fighting more than just current-carrying capacity. The real enemy on long runs is voltage drop — the gradual loss of voltage along the conductor caused by its own resistance. A cable can be perfectly rated for the amps flowing through it and still leave your workshop lights dim, your wallbox undercharging your EV, or your power tools underperforming, simply because too much voltage has been lost before it ever reaches the load. Our Voltage Drop Calculator lets you plug in the length, current (or power), cable material, and cross-section to instantly see how many volts — and what percentage — you'll lose, so you can size the cable correctly before you buy a single metre of it.
Voltage drop depends on the length of the cable, the current flowing through it, the power factor of the load, the cross-sectional area of the conductor, and the conductivity of the material used. The calculator applies the standard electrical formulas:
Where L is the one-way cable length in metres, I is the load current in amps, cos φ is the power factor (typically 1.0 for purely resistive loads like heaters, or around 0.8–0.95 for motors and mixed loads), A is the cross-sectional area of the conductor in mm², and κ (kappa) is the conductivity of the metal — 56 m/(Ω·mm²) for copper and 35 m/(Ω·mm²) for aluminium. The result, dU, is the absolute voltage drop in volts. To express it as a percentage of the supply voltage, the calculator simply computes dU% = dU / U × 100.
If you know the power of the appliance rather than the current, the calculator converts it automatically: I = P / (U × cos φ) for single-phase circuits, or I = P / (√3 × U × cos φ) for three-phase circuits. This is especially handy when you're sizing a cable for a wallbox or a workshop machine rated in kW rather than amps.
Rather than trial-and-error testing different cable sizes, you can flip the calculation around. If you set a target voltage drop — commonly 3% — the calculator works out the minimum cross-section needed: A = (factor × L × I × cos φ) / (κ × U × 0.03), where the factor is 2 for single-phase or √3 for three-phase. The result is then rounded up to the next standard cable size — 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120 or 150 mm² — because you can't buy a cable in an arbitrary in-between size, and rounding down would leave you exceeding your target drop.
Example 1 — Garden workshop: You're running a 230V single-phase supply 45 metres from the house to a garden workshop, feeding a load of 16A with cos φ = 1. Using 2.5mm² copper cable, the drop works out to roughly 2 × 45 × 16 × 1 / (56 × 2.5) ≈ 10.3V, which is about 4.5% — already above the usual 3% guideline for lighting and pushing the limit for general power. Stepping up to 6mm² copper reduces the drop to around 4.3V, or roughly 1.9%, which is comfortably within spec.
Example 2 — EV wallbox on a long driveway: A single-phase 7.4kW wallbox at 230V draws about 32A at unity power factor. If the driveway run is 30 metres, a 6mm² copper cable gives a drop of about 2 × 30 × 32 × 1 / (56 × 6) ≈ 5.7V, or roughly 2.5% — acceptable, but close to the edge if the supply voltage sags. Many electricians will jump to 10mm² for this kind of run to build in a safety margin, especially since wallboxes draw sustained current for hours at a time.
Example 3 — Three-phase site distribution board: A construction site distribution board is supplied 60 metres from the main panel at 400V three-phase, feeding a 20A load at cos φ = 0.9. With 10mm² copper, the drop is roughly √3 × 60 × 20 × 0.9 / (56 × 10) ≈ 3.3V, which is under 1% — well within any reasonable limit, showing that three-phase runs generally suffer less percentage drop than equivalent single-phase runs over the same distance.
| Application | Maximum voltage drop | Typical standard |
|---|---|---|
| Lighting circuits | 3% | DIN 18015-1, BS 7671, NEC 210.19 |
| Socket outlets and general power | 5% | DIN 18015-1, BS 7671 |
| House connection to meter | 0.5% | German TAB |
| Branch circuit (US) | 3% | NEC 210.19 informational note |
| Feeder plus branch circuit (US) | 5% | NEC 215.2 |
Whether you're wiring to BS 7671 in the UK or following NEC guidance in the US, the practical thresholds converge on the same numbers: keep lighting circuits under 3% and general power circuits under 5%. These aren't hard legal limits in every jurisdiction, but they're widely used design targets that keep equipment running reliably and prevent flickering lights, sluggish motor starts, or undercharging at EV wallboxes.
A cable can carry its rated current safely from a thermal perspective while still losing so much voltage over distance that appliances at the far end receive an inadequate supply. Voltage drop is a separate calculation from current-carrying capacity, and on runs over roughly 15–20 metres it usually becomes the deciding factor in cable sizing.
Yes — increasing the cross-sectional area reduces resistance and therefore reduces voltage drop proportionally, since A appears in the denominator of the formula. However, always round up to the next standard size and re-check that the new cable also meets current-carrying and short-circuit protection requirements, not just the voltage drop target.
Copper has a higher conductivity (κ = 56 versus 35 for aluminium), so a copper cable of the same cross-section will always produce a smaller voltage drop. Aluminium is lighter and cheaper for large feeder runs, but you typically need to increase the cross-section by roughly 60% compared with copper to achieve an equivalent voltage drop performance.