AC Voltage Drop Calculator: Formula and Limits

An AC voltage drop calculator estimates how much voltage is lost along a cable carrying alternating current, using the load current, cable length, conductor resistance, and—when needed—reactance and power factor. Enter the one-way cable length and the electrical values in consistent units; the result is an estimated drop in volts and as a percentage of the supply voltage. This calculation is useful for understanding a circuit, but it is not a substitute for selecting wiring or protective devices to applicable electrical rules.

Formula

This calculator uses the resistive approximation for a single-phase, two-conductor circuit:

Voltage drop, ΔV = 2 × I × (L ÷ 1000) × R

Percentage drop = 100 × ΔV ÷ V_s

Estimated load voltage = V_s − ΔV

• ΔV is the estimated voltage drop, in volts (V).
• I is the load current, in amperes (A).
• L is the one-way cable length, in metres (m). The factor of 2 accounts for the outgoing and returning conductors.
• R is the resistance of one conductor in ohms per kilometre (Ω/km); dividing length by 1,000 converts metres to kilometres.
• V_s is the supply voltage, in volts.

The formula assumes that both conductors have the same resistance and that the cable’s impedance can be treated as resistance. It is a convenient estimate, not a full AC circuit model. The calculator does not include reactance, power factor, temperature corrections, or a separate return-path length.

Worked Example

Using the calculator’s defaults—120 V supply, 10 A current, 30 m one-way cable length, and 3.3 Ω/km conductor resistance:

1. Convert the length: 30 m ÷ 1,000 = 0.03 km.
2. Account for both conductors: 2 × 10 A × 0.03 km × 3.3 Ω/km = 1.98 V.
3. Calculate the percentage: 100 × 1.98 V ÷ 120 V = 1.65%.
4. Estimate the load voltage: 120 V − 1.98 V = 118.02 V.

Rounded to the calculator’s two decimal places, the results are a 1.98 V drop, a 1.65% drop, and an estimated 118.02 V at the load.

When to Use an AC Voltage Drop Calculator

Use an estimate to compare the effect of cable length, load current, or conductor resistance on voltage available at a load. It can help explain why a long run or higher current produces a larger drop. It is also useful for checking arithmetic when a cable’s resistance is already known. For an initial comparison, keep the supply voltage and current the same while changing one cable property at a time.

Voltage drop is distinct from the current a cable can safely carry. This calculator does not establish conductor ampacity, insulation suitability, fault protection, or compliance with installation requirements. Those depend on factors such as installation method, ambient conditions, conductor material, circuit type, and local rules. Have a qualified electrical professional determine the actual wiring design.

AC Impedance, Power Factor, and Circuit Type

Alternating current flows through impedance, which includes resistance and reactance. In a sufficiently short or low-reactance circuit, resistance may provide a useful approximation. For longer runs, larger conductors, or circuits where reactance matters, a more complete calculation uses the cable’s resistance and reactance and the load’s power factor.

A common balanced, three-phase approximation is expressed as ΔV ≈ √3 × I × L × (R cos φ + X sin φ), when length and impedance units are consistent. Here, X is conductor reactance per unit length and φ is the angle associated with load power factor. This is not the formula used by the calculator above. Single-phase AC calculations also have versions that include resistance, reactance, and power factor, but the exact expression depends on the circuit and how cable values are specified. Do not apply the two-conductor resistive formula to a three-phase circuit without checking that the circuit model is appropriate.

Inputs, Units, and Common Mistakes

• Use one-way length: Enter the distance from supply to load, not the combined outgoing-and-returning conductor length. The formula includes the factor of 2.
• Match resistance units: This calculator expects Ω/km. If a data source gives Ω/m, multiply by 1,000 to convert to Ω/km. If it gives total conductor resistance for the run, do not treat that total as a per-kilometre value.
• Use the relevant current: Enter the circuit’s load current in amperes. A different operating current produces a different estimate.
• Keep voltage units consistent: Supply voltage and voltage drop are both expressed in volts. The percentage calculation compares those values.
• Do not confuse conductor and loop resistance: The formula doubles the resistance contribution for a two-wire path. If your supplied resistance already represents the complete loop, applying that factor again would overstate the result.

To use US customary measurements, first convert feet to metres (1 ft = 0.3048 m) and use the same resistance unit expected by the calculator. Alternatively, use a formula designed for feet and ohms per foot. Mixing feet with Ω/km without conversion can produce a result off by a large factor.

Understanding Results and Limitations

The voltage-drop result is an estimated difference between the source and load voltage under the entered conditions. The percentage expresses that difference relative to the entered supply voltage. A larger current, longer cable, or greater conductor resistance increases the result in direct proportion under this formula. Halving the current or length, for example, halves the calculated resistive drop if all other inputs stay fixed.

Real conductor resistance changes with temperature, and actual current may vary over time. Cable impedance can also be affected by frequency, conductor arrangement, and circuit configuration. The estimate therefore should not be treated as a precise measurement of voltage at a device. For important equipment or an installed circuit, use appropriate design data and have the installation assessed by a qualified professional.

Frequently Asked Questions

Does AC voltage drop use the same formula as DC?
For a simple resistive circuit, the same resistance-based relationship can be a useful approximation. AC cable calculations may also need reactance and power factor, so a DC-style estimate is not complete for every AC circuit.

Why does the formula multiply cable length by two?
In a single-phase two-conductor circuit, current travels to the load and returns to the source. If the length entered is the one-way distance and each conductor has equal resistance, the total path has twice that length.

What does a negative estimated load voltage mean?
It indicates that the inputs or assumptions are not appropriate for this simple calculation, or that the estimated drop exceeds the entered supply voltage. Check the units, length convention, current, and resistance; do not interpret the output as a realistic operating voltage.

Can I use this for a three-phase circuit?
Not directly. The calculator is set up for a single-phase, two-conductor resistive estimate. Three-phase voltage-drop calculations use a different circuit relationship and may need resistance, reactance, and power factor.

Does a low voltage drop prove that a cable is safe?
No. Voltage drop alone does not determine safe current capacity, protection, installation suitability, or compliance. Cable and circuit selection should be evaluated by a qualified electrical professional using the applicable requirements.

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in v | Supply voltage | V | 120 | 0.001
in i | Load current | A | 10 | 0
in l | One-way cable length | m | 30 | 0
in r | Conductor resistance | ohm/km | 3.3 | 0
out drop | AC voltage drop (resistive estimate) | V | 2 * i * (l / 1000) * r | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out end | Estimated voltage at load | V | v - drop | 2