Cable Voltage Drop Calculator: Formula and Limits

A cable voltage drop calculator estimates how much voltage is lost along a cable when current flows through it. Enter the cable length, current, conductor resistance, and supply voltage to get the estimated drop in volts and as a percentage; this arithmetic is not a cable-sizing or safety approval.

Formula

For a two-wire DC or single-phase circuit, the calculator uses voltage drop = 2 × L × I × R. For a balanced three-phase circuit, it uses voltage drop = √3 × L × I × R. In both expressions, L is the one-way route length in kilometres, I is current in amperes, and R is the conductor’s resistance in ohms per kilometre. The factor 2 accounts for current travelling out and back on a two-wire circuit; √3 is the line-to-line factor for a balanced three-phase circuit.

• Voltage drop (V): the estimated difference between the source voltage and the load-end voltage.
• Percentage drop (%): drop divided by the entered supply voltage, multiplied by 100.
• Estimated load-end voltage (V): supply voltage minus the calculated drop.
• Conductor resistance (Ω/km): resistance per kilometre for the conductor and conditions represented by the value entered.

The selector supplies a multiplier of 1 for DC or single-phase and approximately 1.732 for three-phase. The formula treats resistance as the relevant impedance and assumes the entered current is the circuit current. For AC circuits with appreciable reactance or a non-unity power factor, this simplified estimate may not represent actual voltage drop accurately.

Worked Example

Using the calculator defaults: one-way length 30 m, current 20 A, resistance 3.3 Ω/km, supply voltage 230 V, and DC or single-phase selection:

1. Convert the one-way cable length to kilometres: 30 ÷ 1,000 = 0.03 km.
2. Include the out-and-back path: 2 × 0.03 km × 20 A × 3.3 Ω/km = 3.96 V drop.
3. Calculate the percentage: 100 × 3.96 V ÷ 230 V = 1.7217%, which rounds to 1.72%.
4. Subtract the drop from the supply: 230 V − 3.96 V = 226.04 V estimated at the load.

The calculator displays a 3.96 V drop, 1.72% drop, and estimated load-end voltage of 226.04 V. These values are mathematical estimates, not a declaration that the cable or installation is suitable.

What to Enter and When to Use It

Use the calculator for a preliminary estimate when a known load draws current through a known cable route. It can help compare the effect of different lengths, currents, or conductor resistance values. It does not identify the correct conductor size: to calculate a drop for a proposed cable, obtain that cable’s resistance from an appropriate technical source and enter it.

• Length: enter the one-way distance from source to load, not the combined length of both conductors. The calculator applies the circuit-path factor itself.
• Current: enter amperes, not watts or milliamperes. If you know only the load power, converting power to current may require voltage, power factor, and circuit details.
• Resistance: enter ohms per kilometre. If a table gives ohms per 1,000 feet, convert it before use; 1 km is about 3,280.84 ft.
• Supply voltage: enter the voltage used as the reference for the percentage calculation. For three-phase systems, be clear whether the value is line-to-line or line-to-neutral and use a compatible calculation.
• Circuit type: choose three-phase only for a balanced three-phase load under the assumptions of the formula. A single-phase load connected to a three-phase supply is still treated as single-phase for this estimate.

Metric inputs are metres, amperes, volts, and ohms per kilometre. US customary cable lengths are often given in feet and resistance in ohms per 1,000 feet; convert those figures consistently rather than mixing unit systems. The result is in volts regardless of which source units you began with, once the entered resistance and length are in the specified units.

Resistance, Temperature, and AC Effects

Resistance depends on conductor material, cross-sectional area, and temperature. A resistance figure from a table may be specified at a particular reference temperature; a conductor operating warmer can have higher resistance and therefore greater voltage drop. Stranded construction, connections, and installation conditions can also matter. Use data that matches the actual conductor as closely as possible, and do not treat a nominal value as a guaranteed in-service measurement.

For AC circuits, conductor impedance includes resistance and reactance. A more complete voltage-drop calculation may also require power factor, phase configuration, conductor spacing, and operating conditions. The simple calculator omits those details. It is most useful as a basic resistive estimate, especially for short runs or situations where the omitted effects are known to be small; it should not be mistaken for a detailed design calculation.

How to Interpret the Result and Common Errors

A larger result means more voltage is lost between the source and load under the entered assumptions. The percentage makes drops easier to compare across different supply voltages, while the estimated load-end voltage shows the arithmetic consequence of subtracting the drop from the entered supply. The calculator does not judge whether either result meets a project requirement; acceptable limits depend on the applicable installation rules, equipment, and design.

• Doubling the length: with the other inputs unchanged, the calculated drop doubles.
• Entering total loop length: this counts the return path twice because the formula already includes it. Enter one-way length.
• Using the wrong resistance units: Ω/km is not interchangeable with Ω/1,000 ft without conversion.
• Assuming supply voltage is always exact: a real source voltage can vary, so the estimated load voltage is only as representative as the input.
• Rounding too early: retain the stated resistance and intermediate precision until the final result. Real conductor data may itself be rounded.

If the computed drop is greater than the entered supply voltage, the resulting negative load-end value signals that the assumptions or inputs need review; it is not a realistic operating prediction. The expression also does not account for protection, heating, allowable current, installation method, or fault conditions.

Safety and Scope

Voltage-drop arithmetic alone cannot establish whether a cable is safe or properly sized. Cable selection must consider current-carrying capacity, insulation and conductor temperature ratings, installation conditions, fault protection, terminal limits, local electrical requirements, and the equipment being supplied. Do not select or install wiring based only on this calculator. For an actual electrical installation, have the design and cable choice confirmed by a qualified electrical professional under the rules that apply at the site.

Frequently Asked Questions

How do I calculate cable voltage drop?
For DC or single-phase, multiply 2 × one-way length in kilometres × current in amperes × resistance in ohms per kilometre. For balanced three-phase, use √3 instead of 2, subject to the formula’s assumptions.

Do I enter one-way cable length or the full circuit length?
Enter one-way distance. The calculator includes the return conductor through the factor of 2 for DC and single-phase circuits.

What resistance value should I use?
Use the conductor resistance for the relevant cable, expressed in Ω/km, and check the reference temperature and conditions attached to the data. The calculator cannot determine resistance from cable size alone.

Can I use this calculator for three-phase power?
It provides a basic balanced three-phase estimate using the √3 multiplier. Unbalanced loads, significant reactance, power factor, and other circuit details may require a more complete method.

Does a low voltage drop mean the cable is safe?
No. Voltage drop is only one consideration. It does not verify current capacity, protection, temperature limits, installation compliance, or fault performance.

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in len | One-way cable length | m | 30 | 0
in cur | Load current | A | 20 | 0
in res | Conductor resistance | ohm/km | 3.3 | 0
in volt | Supply voltage | V | 230 | 0
sel phase | Circuit type | DC or single-phase=1, Three-phase=1.732
out drop | Voltage drop | V | 2 * len / 1000 * cur * res * phase
out pct | Voltage drop | % | 100 * drop / volt | 2
out recv | Estimated load-end voltage | V | volt - drop | 2