Feeder Voltage Drop Calculator and Formula

A feeder voltage drop calculator estimates the voltage lost along a two-conductor circuit from its length, current, conductor resistance, and system voltage. Enter those values to get the estimated drop in volts and as a percentage; this arithmetic is useful for checking a design, but it does not select a safe conductor size or replace applicable electrical design requirements.

Formula

For a two-wire single-phase circuit using the same conductor size and material on both outgoing and return paths, the estimated voltage drop is:

V_drop = 2 × I × L × R ÷ 1000

• V_drop is the voltage lost, in volts (V).
• I is circuit current, in amperes (A).
• L is one-way length from the source to the load, in feet (ft).
• R is the conductor resistance in ohms per 1,000 feet (Ω/1,000 ft).

The factor 2 accounts for the outgoing and return conductors. The calculator then finds the percentage drop as 100 × V_drop ÷ V_system, where system voltage is in volts, and estimates load-end voltage as V_system − V_drop. These equations assume a resistive, two-wire path and use the resistance value you provide. They do not model every electrical installation.

Worked Example

Using the calculator defaults: system voltage 240 V, current 20 A, one-way length 100 ft, and resistance 1.24 Ω/1,000 ft:

1. Calculate the round-trip length factor: 2 × 100 ft = 200 ft.
2. Calculate the drop: 2 × 20 A × 100 ft × 1.24 Ω/1,000 ft ÷ 1,000 = 4.96 V, rounded to 4.96 V.
3. Calculate the percentage: 100 × 4.96 ÷ 240 = 2.0666…%, rounded to 2.07%.
4. Estimate load-end voltage: 240 − 4.96 = 235.04 V.

The result is an estimate under the assumptions above, not a guaranteed measurement at the load.

What to Enter and When to Use It

Use the calculator to estimate the drop for a known feeder and load, or to compare possible lengths, currents, and conductor resistance values. The resistance field is not wire gauge: look up the resistance for the conductor material, size, and relevant operating conditions from a reliable technical source. Copper and aluminum conductors of the same nominal size do not have the same resistance.

• System voltage: enter the voltage between the conductors feeding the load, not a voltage-to-ground value unless that is the actual circuit voltage.
• Current: use the current expected to flow in the feeder for the case being assessed. If the load varies, test the relevant operating conditions separately.
• Length: enter the one-way route length in feet. Do not enter the sum of outgoing and return lengths, because the formula already multiplies by 2.
• Resistance: enter ohms per 1,000 feet. A value in ohms per foot must be multiplied by 1,000 before entering; a value for a different length unit must be converted consistently.

The displayed load-end voltage is a simple subtraction from the entered system voltage. It can help explain why a load receives less voltage than the source provides, but actual voltage also depends on source variation, connections, upstream conductors, and other circuit conditions.

Single-Phase, Three-Phase, and Metric Calculations

The calculator’s formula is for a single-phase, two-conductor circuit with a matched outgoing and return path. It is not the general formula for every feeder. For a balanced three-phase circuit, a common simplified resistive estimate is V_drop = √3 × I × L × R, where L is one-way length and R is resistance per unit length of one phase conductor. With alternating current and appreciable reactance, the estimate may also need power factor and conductor reactance. Those cases require the correct circuit model and consistent units; do not apply the two-wire multiplier automatically.

For metric calculations, use length in metres and resistance in ohms per kilometre. The corresponding two-wire expression is V_drop = 2 × I × L × R ÷ 1,000, when L is in metres and R is in Ω/km. For example, 100 m at 1.24 Ω/km is not the same input as 100 ft at 1.24 Ω/1,000 ft. Convert both length and resistance units before calculating. The calculator itself accepts feet and Ω/1,000 ft only.

Limits, Design Context, and Common Mistakes

A voltage-drop calculation is only one part of feeder design. It does not establish that a conductor, protective device, insulation, installation method, or connection is suitable. Permissible voltage drop and required conductor sizing depend on the project, electrical system, governing rules, load characteristics, and installation conditions. Have a qualified electrical professional verify the complete design and applicable requirements.

• Doubling the length twice: enter one-way distance; the equation already includes the return conductor.
• Mixing resistance units: Ω/1,000 ft, Ω/ft, Ω/km, and Ω/m are different quantities. Convert before using the formula.
• Using nominal voltage as a measured guarantee: source voltage can vary, so the calculated endpoint is an estimate, not a prediction of every moment of operation.
• Ignoring temperature and AC effects: conductor resistance changes with temperature, and AC impedance may matter, particularly for longer circuits or significant reactance.
• Treating a low percentage as proof of safety: voltage drop alone does not verify ampacity, fault protection, or other design constraints.

Results are rounded to two decimal places for display. Rounding does not improve the accuracy of the inputs; use appropriate conductor data and design methods when precision matters.

Frequently Asked Questions

How do I calculate feeder voltage drop?
For a simple two-wire single-phase feeder, multiply twice the current, one-way length, and conductor resistance per unit length. Keep units consistent; with feet and Ω/1,000 ft, divide the product by 1,000.

Do I enter one-way or round-trip length?
Enter one-way length in this calculator. Its formula includes both outgoing and return conductors through the factor of 2.

Can I use this calculator for a three-phase feeder?
Not directly. A balanced three-phase estimate uses a different relationship, commonly involving √3, and may need impedance and power factor. Use a calculation suited to the circuit and its conditions.

What does a 2% voltage drop mean?
It means the estimated drop is 2% of the entered system voltage. It does not, by itself, show whether the feeder meets project requirements or is correctly sized.

Where do I find conductor resistance?
Use reliable technical data for the conductor material and size, taking account of the conditions relevant to the calculation. Do not infer resistance from gauge alone without confirming the applicable value.

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in v | System voltage | V | 240 | 0.001
in a | Load current | A | 20 | 0
in l | One-way feeder length | ft | 100 | 0
in r | Conductor resistance | ohm/1000 ft | 1.24 | 0.000001
out drop | Feeder voltage drop | V | 2 * a * l * r / 1000 | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out end | Estimated load-end voltage | V | v - drop | 2