Voltage drop is the reduction in voltage between the start of a cable and the equipment at the far end while current is flowing. Think of voltage as the electrical “push”. The cable has some resistance, so the equipment receives a little less voltage than is available at the supply.
This guide explains voltage drop without assuming electrical knowledge. You will learn what changes it, what the figures on a cable datasheet mean, and how to calculate a simple single-phase example with an ordinary calculator.
The quick version: with the same cable and current, a longer lead has more voltage drop. With the same cable and length, more current means more drop. A larger copper conductor generally reduces the drop.
Technical references checked: 4 November 2026. Examples explain the calculation; they do not approve a particular lead length or installation.
For the difference between 230 V to neutral and 400 V between phases, read our Three-Phase Power Explained beginner’s guide.
What does voltage drop actually mean?
Imagine 230 volts at the start of a lead. If the drop along that lead is 8 volts under load, the equipment receives approximately 222 volts. The supply voltage itself can also change, so 230 volts is an example starting point.
The cable is not using up some of the current on the way to the equipment. In a simple circuit, the same current flows through the outgoing and return conductors. The cable’s resistance causes a voltage difference and dissipates some energy as heat.
A small drop is normal. Too much can affect equipment performance or cause equipment to stop operating. The acceptable amount depends on the equipment and the complete electrical installation.
What makes the drop bigger or smaller?
| Change | What happens to voltage drop? |
|---|---|
| Use twice the cable length | The calculated drop doubles. |
| Draw twice the current | The calculated drop doubles, using the same cable coefficient. |
| Use a larger copper conductor | The resistance is generally lower, reducing the drop. |
| Run the conductor hotter | Copper resistance increases; the calculation must use suitable temperature assumptions. |
“Larger conductor” means more copper cross-sectional area, measured in square millimetres (mm²). A thick outer jacket does not necessarily mean there is more copper inside. Our CableSpec guide explains markings such as 3G2.5 and 3G4.
The three numbers you need
- The cable’s voltage-drop coefficient: a figure from an appropriate cable datasheet or design table, matching the cable, circuit arrangement and calculation conditions.
- The load current in amps (A): the current the equipment is expected to draw. A 32 A plug does not mean that the equipment is always drawing 32 A.
- The one-way route length in metres (m): for a 25-metre lead, enter 25.
For this method, use a coefficient published for the complete single-phase circuit. It already accounts for the outgoing and return path. Do not double the lead length again. A calculation starting from the resistance of one conductor uses a different approach.
What does mV/A/m mean?
It means millivolts per amp per metre. A millivolt is one thousandth of a volt. The coefficient tells you how much voltage drop to allow for each amp of load and each metre of route, under the stated conditions.
Some datasheets write the coefficient as V/A·km: volts per amp per kilometre. This has the same numerical value as mV/A/m. For example, 10.1 V/A·km can be used as 10.1 mV/A/m in the metres-based formula below.
The simple calculation
Voltage drop in volts = cable coefficient × current in amps × one-way length in metres ÷ 1,000
Then, to express that drop as a percentage:
Voltage drop percentage = voltage drop in volts ÷ starting voltage × 100
You do not need to memorise the symbols. Find the correct coefficient, multiply by amps, multiply by metres, then divide by 1,000.
Use the voltage-drop calculator
Open the Voltage Drop Calculator
Choose core count, conductor size, supply voltage, current and length. Includes single-phase, balanced three-phase and advanced cable-data settings.
A worked example: 25 metres at 32 amps
For this illustration, Nexans publishes a single-phase voltage-drop coefficient of 10.1 V/A·km for TITANEX H07RN-F 3G4, reference 10055534. That is a three-core cable with 4 mm² conductors. Nexans states that its voltage-drop data use a power factor of 0.8. Power factor describes the relationship between useful power and apparent power in an alternating-current load; use data appropriate to the actual load.
| Input | Example value |
|---|---|
| Starting voltage | 230 V |
| Cable coefficient | 10.1 mV/A/m |
| Load current | 32 A |
| One-way length | 25 m |
- Multiply the coefficient by the current: 10.1 × 32 = 323.2.
- Multiply by the length: 323.2 × 25 = 8,080.
- Divide by 1,000: 8,080 ÷ 1,000 = 8.08 V.
- Find the percentage: 8.08 ÷ 230 × 100 = 3.51%, rounded.
If the voltage at the start is 230 V, the estimated voltage at the equipment is 230 − 8.08 = 221.92 V.
On a calculator, enter 10.1 × 32 × 25 ÷ 1000. The answer is 8.08 volts. This is a cable-drop estimate using the cited coefficient, not a measurement of your lead or a check of its safe current rating.
What happens if the lead is longer?
| One-way length | Calculated drop | Percentage |
|---|---|---|
| 10 m | 3.23 V | 1.41% |
| 25 m | 8.08 V | 3.51% |
| 50 m | 16.16 V | 7.03% |
These are comparison figures, not recommended lead lengths. At 25 metres, reducing the example current from 32 A to 16 A halves the calculated drop to 4.04 V, or 1.76%. If equipment draws more current as its supply voltage falls, the real behaviour needs a more detailed assessment.
Does a low percentage prove a lead is suitable?
No. Voltage drop is one part of selecting a lead. Cable current capacity, connectors, protective devices, earthing, operating conditions and equipment requirements still matter.
The drop in the wiring before the lead also counts towards the voltage available at the equipment. A calculation for one extension lead does not assess the whole circuit. Motor starting, variable loads and generator voltage changes can also affect the result.
This beginner’s example is for a single-phase alternating-current circuit. Three-phase and direct-current systems need the correct coefficients and method for those circuits. Do not reuse a single-phase coefficient without checking.
The practical takeaway
Use the shortest suitable lead that reaches the job without strain, know the equipment’s current, and compare the conductor size and cable specification. Choosing a larger connector alone does not reduce the resistance of the cable.
For help selecting an assembly, tell MotorCables the supply, equipment, load and required length. Continue with our Arctic PVC and H07RN-F cable comparison, generator lead guide, event power guide or workshop power guide.
Sources and calculation notes
- IET Wiring Matters: voltage-drop formula and the effect on equipment voltage. This guide uses the general formula, not that article’s EV-specific design example or thresholds.
- Nexans TITANEX H07RN-F 3G4, reference 10055534: published single-phase coefficient and power-factor assumption.
