Cable Voltage Loss Calculator: Formula and Limits

A cable voltage loss calculator estimates the voltage drop along a conductor from its length, current, material, size, and circuit type. Enter those details to get an approximate drop in volts and as a percentage of the supply; use the result as a screening calculation, not as proof that a cable is safe or correctly sized.

Formula

This calculator uses the simplified resistance-only formula for a circuit with equal outgoing and return conductors:

• Voltage drop (V) = k × I × R × L ÷ 1000
• Drop percentage (%) = 100 × voltage drop ÷ supply voltage
• Approximate load voltage (V) = supply voltage − voltage drop

k is 2 for DC or single-phase circuits, accounting for the outgoing and return paths, and approximately 1.732 (√3) for a balanced three-phase circuit using line-to-line voltage. I is circuit current in amperes (A); R is the resistance of one conductor in ohms per kilometre (Ω/km); and L is the one-way route length in metres (m). Supply voltage is in volts (V). The formula assumes the entered resistance is for the conductor at its operating temperature and does not include reactance or connection effects.

Worked Example

Using the defaults in the calculator: supply voltage is 230 V, current is 10 A, one-way cable length is 20 m, resistance is 18.1 Ω/km, and the circuit is DC or single-phase.

1. Convert the one-way length to kilometres: 20 ÷ 1000 = 0.020 km.
2. Calculate the two-wire loop resistance: 18.1 × 0.020 = 0.362 Ω.
3. Calculate voltage drop: 2 × 10 × 0.362 = 7.24 V, displayed as 7.24 V.
4. Calculate the percentage: 100 × 7.24 ÷ 230 = 3.1478…%, displayed as 3.15%.
5. Estimate voltage at the load: 230 − 7.24 = 222.76 V.

What to Enter and How to Read the Result

Enter the supply voltage measured or specified for the circuit, the expected current, and the route length in one direction from source to load. Use the conductor’s resistance per kilometre, not the resistance of the complete outgoing-and-return loop: the circuit factor in the formula accounts for that loop in DC and single-phase calculations. Choose three-phase only for a balanced three-phase circuit, and enter line-to-line supply voltage and the relevant line current.

The first result is the estimated voltage lost along the cable. The percentage expresses that loss relative to the entered supply voltage, while approximate load voltage subtracts the estimated drop from the supply. These outputs help compare candidate cable runs or identify when a more detailed calculation is warranted. Whether a particular percentage is acceptable depends on the equipment, installation rules, circuit conditions, and applicable local requirements; the calculator does not set a universal pass/fail threshold.

The resistance field is important. The default, 18.1 Ω/km, is a plausible example value for a conductor but is not a universal value for a cable size or material. Obtain resistance from reliable cable data for the actual conductor, and account for conductor temperature where needed. Resistance generally rises as a metal conductor warms, so using a cool-conductor value can understate the drop during operation.

Why Cable Length, Material, and Current Matter

Voltage drop occurs because a real conductor has resistance. Current flowing through that resistance produces a voltage difference between the source and the load. For a fixed conductor and circuit arrangement, the simplified formula makes the relationships clear: doubling current or length doubles the estimated drop; doubling resistance does the same. A larger conductor generally has lower resistance, while conductor material and temperature also affect resistance.

For DC and single-phase circuits, current travels to the load and returns to the source, so the relevant path is longer than the one-way route. The factor of 2 represents this in the simplified calculation. In a balanced three-phase system, the relationship between line current and line-to-line voltage produces the √3 factor instead. Do not apply the three-phase factor to a single-phase circuit, or use the single-phase loop factor for a balanced three-phase calculation.

Limits and Common Calculation Mistakes

This tool is an estimate, not a complete cable-design method. It uses conductor resistance only. In alternating-current circuits, cable impedance can include reactance as well as resistance; the effect can matter for longer runs, larger conductors, or particular loads. Motor starting current, unbalanced three-phase loading, harmonics, power factor, joints, and supply impedance may also affect actual voltage at equipment. For those cases, use a method suited to the installation and its load conditions.

• Entering total loop length: The formula already accounts for the return path in DC and single-phase mode. Enter one-way distance only.
• Mixing units: This calculator expects metres and Ω/km. If data is given in feet or Ω/1000 ft, convert it before entering it; 1 m is about 3.28084 ft, and 1 Ω/km is 0.3048 Ω/1000 ft.
• Using cable length instead of route length: Measure the conductor run along its actual route, not merely the straight-line distance between endpoints.
• Choosing the wrong circuit type: The multiplier depends on DC/single-phase versus balanced three-phase, not simply on whether the supply is described as AC.
• Treating the displayed decimals as exact: Results are rounded, and the source resistance and current may themselves be estimates. Extra decimal places would not remove uncertainty in those inputs.

Using the Estimate for Cable Decisions

A voltage-drop estimate can help you compare options, but it cannot determine a safe conductor size by itself. Cable selection also depends on current-carrying capacity, insulation and ambient temperature, installation method, grouping, protective devices, fault conditions, and applicable electrical codes. A cable that produces a modest voltage drop might still be unsuitable for other reasons; conversely, acceptable voltage drop does not establish compliance.

For a real installation, verify the cable data and design assumptions, and have sizing and protection checked by a qualified electrical professional. This is especially important for fixed wiring, high currents, long runs, three-phase equipment, or loads sensitive to voltage variation. A voltage-drop calculator is useful for understanding one part of the problem, not for replacing an installation assessment.

Frequently Asked Questions

Does cable voltage drop depend on one-way or round-trip length?
Enter one-way route length in this calculator. Its factor of 2 includes the return conductor for DC and single-phase circuits. If a formula or tool asks for total loop length instead, do not apply that factor again.

Why does the calculator ask for resistance?
Resistance captures the effect of conductor material, size, and temperature in one input. Use a value matching the actual cable and operating conditions; a generic example resistance cannot identify a cable’s correct size.

Can I use this for a three-phase circuit?
It provides a simplified estimate for balanced three-phase circuits when you enter line-to-line voltage, line current, and the correct conductor resistance. Unbalanced loads or significant AC reactance require a more complete method.

Is a 3% voltage drop always acceptable?
No single percentage is universally acceptable for every circuit. The appropriate limit depends on the equipment, installation, and governing requirements, so verify the relevant criteria rather than treating this calculator’s percentage as a compliance decision.

Why might measured load voltage differ from the estimate?
Actual current, conductor temperature, cable impedance, connections, supply variation, and load behavior can differ from the assumptions. Measurements also depend on where and when voltage is checked.

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in v | Supply voltage | V | 230 | 0.001
in i | Current | A | 10 | 0
in l | One-way cable length | m | 20 | 0
in r | Conductor resistance | ohm/km | 18.1 | 0
sel c | Circuit | DC or single-phase=2, Three-phase=1.732
out drop | Voltage drop | V | c * i * (r * l / 1000) | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out end | Approximate load voltage | V | v - drop | 2