Online Voltage Drop Calculator and Formula

An online voltage drop calculator estimates how much voltage is lost along a conductor when you enter the load current, one-way cable length, conductor size, and material. This calculator returns an estimated drop in volts and as a percentage of supply voltage; use the result as an engineering check, not as a cable-sizing or safety approval.

Formula

For a simple two-wire DC circuit, or a single-phase circuit treated as a resistive line, the estimated drop is ΔV = 2 × I × L × ρ ÷ A. The factor 2 accounts for current traveling out to the load and returning to the source. The calculator reports that drop, its share of supply voltage, and an estimated load-end voltage.

• ΔV is voltage drop in volts (V).
• I is load current in amperes (A).
• L is the one-way route length in metres (m).
• ρ is conductor resistivity in ohm-square-millimetres per metre (Ω·mm²/m). The selector uses approximate reference values: 0.0175 for copper and 0.0282 for aluminum.
• A is conductor cross-sectional area in square millimetres (mm²).
• V is the supply voltage, used to calculate the percentage and estimated voltage at the load.

The percentage is 100 × ΔV ÷ V, and estimated load voltage is V − ΔV. These equations assume the entered supply voltage is the voltage at the source and that the conductor’s resistance can be represented by the selected resistivity.

Worked Example

Using the calculator’s defaults: 10 A, a one-way length of 30 m, 2.5 mm² conductor, a 230 V supply, and copper resistivity of 0.0175 Ω·mm²/m:

1. Account for the outgoing and return conductors: 2 × 30 m = 60 m of total conductor path.
2. Calculate drop: 2 × 10 × 30 × 0.0175 ÷ 2.5 = 4.20 V.
3. Calculate percentage: 100 × 4.20 ÷ 230 = 1.83%.
4. Estimate voltage at the load: 230 − 4.20 = 225.80 V.

The displayed result is an estimate, not a measurement. Actual voltage at a load can vary with supply conditions, conductor temperature, connections, and the electrical characteristics of the circuit.

When to Use an Online Voltage Drop Calculator

A calculator is useful for a preliminary check when planning a circuit or comparing how length, current, conductor area, and material affect voltage loss. It can make trade-offs easier to see: increasing length or current raises the estimated drop, while increasing conductor area lowers it. For a fixed set of assumptions, doubling the length or current doubles the calculated drop; doubling area halves it.

It can also help explain why a distant load may receive less voltage than equipment near the source. For example, a long run supplying a motor or other substantial load can have a greater drop than a short run carrying the same current. Whether that drop is acceptable depends on applicable electrical rules, equipment requirements, circuit type, and installation details. The calculator does not decide those matters.

Choosing Inputs and Units

Use the current expected in the conductor, the actual one-way route length, and the conductor’s cross-sectional area. The calculator uses metric inputs. For US customary measurements, convert before entering values: 1 ft = 0.3048 m, 1 in² = 645.16 mm², and 1,000 circular mils equal approximately 0.5067 mm². Wire sizes described by AWG are not entered directly as an area; consult a reliable wire-size table for the corresponding cross-sectional area, and verify what the table represents.

For a two-conductor circuit, enter the distance from source to load—not the combined outgoing-and-return length—because the formula already multiplies by 2. For a three-phase circuit, do not automatically use this single-phase formula: balanced three-phase voltage drop uses a different relationship and may require power factor and conductor reactance. A suitable calculator or qualified calculation method should match the circuit.

• Choose copper or aluminum to match the actual conductor.
• Enter the conductor’s nominal area, not its diameter, insulation thickness, or outside cable diameter.
• Use consistent units; this calculator expects amperes, metres, square millimetres, and volts.
• Use the voltage at the source for the percentage calculation, rather than a nominal value if a measured or specified source voltage is the relevant input.

Assumptions, Limits, and Common Errors

Conductor resistance rises as the conductor warms, so an estimate using a fixed resistivity may understate drop in a hot operating condition. The calculator also ignores or simplifies effects such as conductor reactance, power factor, connection resistance, parallel conductors, harmonics, and the details of installation. AC voltage drop can depend on these factors, particularly on longer runs or with inductive loads. The selected resistivity values are approximations, not a substitute for a conductor manufacturer’s data or a full circuit calculation.

A frequent mistake is entering the round-trip cable length and then counting the return path twice. Another is confusing voltage drop with percentage drop: volts measure the absolute loss, while the percentage compares that loss with the entered supply voltage. Rounding intermediate values can also shift the final display slightly; use unrounded inputs where possible and round only the reported result.

Most importantly, a low calculated voltage drop does not prove that a conductor, cable, or protective device is safe or suitable. Ampacity, insulation rating, installation method, ambient temperature, fault protection, and local requirements are separate considerations. Do not choose wire size or protective equipment from this result alone. Have a qualified electrical professional verify the design and applicable requirements.

Understanding the Result

The voltage-drop output is the estimated difference between source voltage and load-end voltage. The percentage output expresses the same difference relative to the supply, making it easier to compare circuits with different nominal voltages. The estimated load voltage is simply the entered supply voltage minus the estimated drop; it is not a guarantee of the voltage an instrument will measure.

If a result seems unexpectedly large, check the one-way length, area, current, and material first. If it seems implausibly small, confirm that the return path is included by the formula, that the cable size was entered as area, and that the circuit type matches the calculation. A professional calculation may need additional inputs and a more complete AC model.

Frequently Asked Questions

Do I enter one-way or round-trip cable length?
Enter one-way length. The formula multiplies by 2 to include the outgoing and return conductors for the assumed two-wire circuit.

Does this calculator work for three-phase power?
Not as written. It uses a two-wire DC or simplified single-phase relationship. Three-phase calculations use different equations and may require power factor and reactance.

Why does a longer cable increase voltage drop?
A longer conductor has more resistance. With current and conductor area unchanged, this formula estimates drop as directly proportional to one-way length.

Can I use the result to choose a safe wire size?
No. Voltage drop is only one design consideration. Conductor ampacity, installation conditions, protection, equipment needs, and applicable rules must also be checked by a qualified professional.

Why might measured voltage differ from the estimate?
The actual conductor temperature, source voltage, connections, load behaviour, power factor, and AC reactance can affect the measured value. The formula simplifies or omits several of these effects.

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in i | Load current | A | 10 | 0
in l | One-way conductor length | m | 30 | 0
in a | Conductor area | mm² | 2.5 | 0
in v | Supply voltage | V | 230 | 0
sel r | Conductor resistivity | Copper=0.0175, Aluminum=0.0282
out drop | Voltage drop | V | 2 * i * l * r / a | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out end | Estimated voltage at load | V | v - drop | 2