Voltage Drop Calculator for Transformer Feeders

Enter the load current, one-way cable length, resistance, reactance and power factor for the circuit, then check the load-end voltage against the equipment requirement.

Use the voltage drop calculator to estimate the voltage change along an AC feeder. Enter the load current, one-way cable length, resistance, reactance and power factor for the circuit, then check the load-end voltage against the equipment requirement.

AC feeder voltage drop calculator

Estimate voltage drop, percentage and approximate load-end voltage from the circuit inputs.

AC phaseSupply voltage (V, line-to-line)Load current (A)One-way cable length (m)Resistance R (ohm/km)Reactance X (ohm/km)Lagging power factor (0-1)

Formula: ΔV = √3 × I × L / 1000 × (R × PF + X × √(1 − PF²))

Estimate only. Enter one-way cable length and conductor impedance at the intended operating temperature. Three-phase mode assumes a balanced circuit and line-to-line supply voltage.

Enter the circuit conditions

Select single-phase two-wire or balanced three-phase AC. Enter supply voltage, current, one-way length in metres, resistance and reactance in ohms per kilometre, and lagging power factor. Use conductor impedance at the intended operating temperature. The three-phase voltage is line-to-line.

Approximate feeder voltage drop

Single phase: ΔV = 2 × I × L / 1000 × (R × PF + X × √(1 − PF²))

Balanced three phase: ΔV = √3 × I × L / 1000 × (R × PF + X × √(1 − PF²))

Illustrative feeder result using the stated one-way length and impedance.

Read the worked example

For a balanced 480 V feeder carrying 100 A over 50 m, take R = 0.20 ohm/km, X = 0.08 ohm/km and power factor = 0.90. The approximate line-to-line drop is 1.86 V, or 0.39%, leaving about 478.14 V at the load. These are illustrative input values; enter the selected cable's actual data.

Item or condition	What to compare	Input or result
Supply and load	480 V · 100 A · PF 0.90	Balanced three-phase AC
Cable example	50 m; R 0.20; X 0.08 ohm/km	One-way length
Calculated result	1.86 V; 0.39% drop	478.14 V at load

Separate the cable from the source

This calculation estimates the feeder component of voltage change. Add source variation, transformer regulation and starting conditions in the overall system study. Review conductor ampacity, protective-device coordination and installation conditions with the electrical designer.

Review Three-Phase Power

Keep line voltage and current quantities consistent in a balanced circuit.

Calculate Feeder Current

Convert a transformer rating and voltage into a current input.

Review Transformer Capacity

Include source and transformer conditions in the system review.

Review Three-Phase Transformers

Connect feeder voltage and load requirements to equipment selection.

Frequently asked questions

Which length should I enter?

Enter one-way cable length. The single-phase formula accounts for the outgoing and return path.

Can this calculator select cable size?

Use its result alongside ampacity, installation, thermal and fault-duty requirements in the electrical design.

Review feeder voltage and transformer capacity

Share the conductor data, circuit length, load conditions and source requirements for an electrical design review.