A voltage calculator for circuits and cable runs
This free voltage calculator covers the three voltage questions electricians, engineers and students ask most. First, volts from what you know: V = I × R, V = P ÷ I or V = √(P × R). Second, the voltage drop along a cable — DC, single-phase or three-phase, copper or aluminium, in mm² or AWG — compared with the usual design limits. Third, the output of a voltage divider, or the resistor you need for a target voltage.
Every answer lists the constants used — resistivity at 20 °C and at working temperature, conductor resistance and the formula — so you can check it against a datasheet. Results are planning estimates; final cable sizing must follow your wiring rules and be checked by a qualified electrician.
How to use the Voltage Calculator
- Pick a tab: Volts from I, R, P, Cable voltage drop or Voltage divider.
- For volts, choose I × R, P ÷ I or √(P × R) and enter the two values with their units.
- For voltage drop, enter the supply, load current, one-way length, conductor material and size, power factor and conductor temperature.
- Choose a limit to compare with; you get the drop in volts and percent, the load voltage, the smallest passing size and the longest allowed run.
- For a divider, enter Vin, R1 and R2 — or switch to “Find R2” and give a target output — and optionally the load resistance.
Voltage Calculator features
Three kinds of supply
DC and single-phase use the out-and-back loop (factor 2); three-phase uses √3 with the line voltage.
Real cable resistance
mm² sizes use the IEC 60228 maximum conductor resistance by default, or resistivity ÷ area; AWG sizes use ASTM B258 diameters.
Temperature corrected
Resistance is corrected from 20 °C to the conductor temperature with the IEC 60228 coefficients for copper and aluminium.
Sizing help
Shows the smallest listed size within your limit and the longest run the chosen size allows.
Divider design
Loaded and unloaded output, divider current, output resistance and the nearest E12/E24 resistor.
How voltage drop is calculated
Current flowing through a cable meets the resistance of its conductors, so some voltage is lost before it reaches the load: ΔV = 2 × L × I × R for DC, where L is the one-way length and R the resistance per metre of one conductor. Single-phase AC uses the same loop with the impedance term (R cos φ + X sin φ); three-phase uses ΔV = √3 × L × I × (R cos φ + X sin φ) for the line-to-line drop.
Resistance depends on the metal and its temperature. Copper’s resistivity is 0.017241 Ω·mm²/m at 20 °C (the IEC 60028 annealed-copper standard) and aluminium’s about 0.028264 Ω·mm²/m (61 % IACS). Both rise about 0.4 % per °C — so a PVC cable at its 70 °C working temperature has roughly 20 % more resistance than when cold.
| Copper size | Max. resistance at 20 °C (IEC 60228) | At 70 °C | Drop per amp per metre, single-phase |
|---|---|---|---|
| 1.5 mm² | 12.1 Ω/km | 14.48 Ω/km | ≈ 29 mV |
| 2.5 mm² | 7.41 Ω/km | 8.87 Ω/km | ≈ 18 mV |
| 4 mm² | 4.61 Ω/km | 5.52 Ω/km | ≈ 11 mV |
| 6 mm² | 3.08 Ω/km | 3.69 Ω/km | ≈ 7.4 mV |
| 10 mm² | 1.83 Ω/km | 2.19 Ω/km | ≈ 4.4 mV |
Voltage drop limits in IEC 60364, BS 7671 and the NEC
- IEC 60364-5-52 (informative Annex G) and BS 7671 (Appendix 4, Table 4Ab) suggest at most 3 % for lighting and 5 % for other uses when the installation is fed from a public low-voltage network, and 6 % and 8 % when it has its own private supply. On 230 V that is 6.9 V and 11.5 V.
- The US National Electrical Code gives its guidance in informational notes to 210.19 (branch circuits) and 215.2 (feeders): about 3 % for a branch circuit or feeder and 5 % in total to the farthest outlet. Informational notes are advice, not enforceable requirements.
- Low-voltage DC systems (12 V or 24 V solar, vehicles, boats) suffer most, because a few volts is a large percentage.
Worked examples
Single-phase circuit: 20 A through 20 m of 2.5 mm² copper on 230 V. At 70 °C the conductor is 7.41 × 1.1965 = 8.866 Ω/km, so ΔV = 2 × 20 × 20 × 0.008866 = 7.09 V (3.08 %). That passes a 5 % limit but not 3 %; 4 mm² brings it down to 1.92 %.
Voltage divider: 12 V across R1 = 10 kΩ and R2 = 4.7 kΩ gives Vout = 12 × 4.7 ÷ 14.7 = 3.837 V. Hang a 10 kΩ load on the output and it falls to 2.907 V, because the load shares R2’s current. To get 3.3 V from 12 V with R1 = 10 kΩ, R2 = 10 × 3.3 ÷ 8.7 = 3.793 kΩ; the nearest standard E24 resistor, 3.9 kΩ, gives 3.367 V.
For the current, resistance and power side of a circuit, the Ohm’s law calculator solves any two values; for three-phase power and appliance energy, use the watt calculator.
Voltage Calculator FAQ
How do I calculate voltage with a voltage calculator?
Choose what you know. With current and resistance, V = I × R; with power and current, V = P ÷ I; with power and resistance, V = √(P × R). The voltage calculator shows the result with the working.
What is an acceptable voltage drop?
For installations fed from the public network, IEC 60364 and BS 7671 suggest 3 % for lighting and 5 % for other circuits; the NEC’s informational notes suggest 3 % for a branch circuit and 5 % overall. Equipment makers may ask for less.
How do I reduce voltage drop in a long cable?
Use a larger conductor, shorten the run, or raise the voltage — doubling it halves the current for the same power and quarters the percentage drop.
Why does the result depend on temperature?
Metal resistance rises with temperature by about 0.39 % per °C for copper. Cables are usually checked at their maximum operating temperature — 70 °C for PVC and 90 °C for XLPE insulation.
Does the calculator size cables for current-carrying capacity?
No. It checks voltage drop only. A cable must also carry the current without overheating and work with its protective device; both depend on installation method and are covered by your wiring regulations.
What is the formula for a voltage divider?
Vout = Vin × R2 ÷ (R1 + R2), where R2 is the resistor connected to ground. It holds only when the load draws much less current than the divider.