Voltage Line Loss Calculator: Formula and Example

A voltage line loss calculator estimates the voltage drop between a source and a load from the conductor length, current, conductor resistance, and circuit type. Enter the one-way route length, load current, and conductor resistance; choose a single-phase or three-phase circuit to get voltage drop and the percentage of source voltage lost.

Formula

The calculator uses the standard approximate resistive voltage-drop formula:

• DC or single-phase, two-wire circuit: voltage drop = 2 × I × R × L.
• Balanced three-phase circuit: voltage drop = √3 × I × R × L.
• Percentage drop: 100 × voltage drop ÷ source voltage.
• Approximate load voltage: source voltage − voltage drop.

In the calculator, length is entered in metres and resistance in ohms per kilometre, so the formula divides their product by 1,000. I is line current in amperes, R is the resistance of one conductor in ohms per kilometre, and L is the one-way route length in metres. The circuit factor is 2 for DC or single-phase two-wire circuits and √3 (approximately 1.732) for balanced three-phase circuits. V is source voltage in volts.

The single-phase factor of 2 accounts for current travelling out to the load and returning on the other conductor. In a three-phase circuit, the √3 relationship applies to balanced line-to-line voltage calculations. The selection does not change the entered voltage; use a source voltage appropriate to the calculation, such as line-to-line voltage for a three-phase line-to-line comparison.

Worked Example

Using the defaults—30 m one-way length, 20 A, 0.5 Ω/km, 230 V, and DC or single-phase—the round-trip conductor length is represented by the factor 2:

1. Voltage drop = 2 × 20 A × 0.5 Ω/km × 30 m ÷ 1,000 = 0.60 V.
2. Percentage drop = 100 × 0.60 V ÷ 230 V = 0.2609%, which rounds to 0.26%.
3. Approximate load voltage = 230 V − 0.60 V = 229.40 V.

The first result is the estimated voltage lost along the circuit. The load-voltage figure is a simple subtraction, not a guarantee of the actual voltage under operating conditions.

When to Use a Voltage-Drop Calculator

Voltage drop is the reduction in voltage between a supply point and equipment drawing current. This estimate helps explain why a distant load may receive less voltage than the source provides, and lets you compare route lengths, currents, or conductor resistance. It is useful for understanding a circuit or checking an assumption; it is not a substitute for designing or approving wiring.

Use the calculator when the conductor resistance is known or can be obtained from a reliable conductor data table for the relevant material, size, and operating conditions. If a table gives resistance per kilometre, enter that value directly. If it gives resistance per metre, multiply it by 1,000 before entering it. For resistance specified in ohms per 1,000 feet, convert the length and resistance to compatible units first, or calculate using a formula expressed consistently in feet and ohms per foot.

Choosing the Right Inputs

• Length: Enter the one-way physical route length, not the total out-and-back length. The formula includes the return path for DC and single-phase circuits.
• Current: Use the current expected to flow in the circuit, in amperes. If current varies substantially, calculate with a representative operating value and understand that the result varies with it.
• Resistance: This must be for one conductor, not the combined outgoing and return path. It depends on conductor material, cross-sectional area, temperature, and other conditions; a generic value may not represent the installed circuit.
• Circuit type: Select DC or single-phase for a two-conductor circuit, and three-phase for a balanced three-phase estimate. An unbalanced three-phase circuit needs a more detailed analysis.
• Source voltage: Enter volts, not kilovolts. For percentage drop, source voltage must correspond to the voltage being assessed.

For US customary data, resistance is often stated in ohms per 1,000 feet and distance in feet. Keep both quantities in a consistent unit system. One kilometre is about 3,281 feet; one metre is about 3.281 feet. Do not combine a resistance-per-kilometre figure with a length in feet without conversion.

What the Result Does—and Does Not—Mean

A larger current, longer route, or higher conductor resistance increases the estimated drop in direct proportion. For example, doubling current while keeping other inputs fixed doubles the calculated voltage drop. Percentage drop also depends on source voltage: the same drop is a larger percentage on a lower-voltage circuit.

This calculator treats conductor resistance as a fixed value and assumes a steady current. Real AC circuits can also have reactance, power-factor effects, and temperature-dependent resistance. Motor starting, changing loads, supply variation, connections, and conductor arrangement may affect actual voltage. The three-phase expression is an approximation for a balanced circuit; it does not model every installation detail.

Voltage drop is not the same as power loss. The calculator reports a voltage difference. For a simple resistive model, power dissipated in a conductor depends on current squared times resistance, but a complete circuit-loss calculation must use the resistance of the entire current path and appropriate AC assumptions. A small voltage-drop percentage alone does not establish that a conductor or installation is safe or suitable.

Common Errors and Practical Limits

• Counting length twice: Enter one-way length. Doubling it manually and using the single-phase factor of 2 would count the return path twice.
• Using total circuit resistance as conductor resistance: If a source already provides the resistance of both conductors together, do not apply a formula that doubles one-conductor resistance.
• Assuming nominal resistance is exact: Conductor resistance rises with temperature, so a room-temperature table value can understate operating drop.
• Mixing AC and DC assumptions: The simple formula does not include AC reactance. Long runs, larger conductors, or low power factor may need a more complete calculation.
• Relying on rounded results: The display rounds to two decimal places. A displayed 0.00 V does not necessarily mean the actual drop is zero.

Use applicable electrical codes, equipment instructions, and a qualified electrical professional to determine conductor selection, protective devices, installation methods, and acceptable voltage drop. Those decisions depend on more than this arithmetic, including load characteristics, environment, and local requirements.

Frequently Asked Questions

How do you calculate voltage drop?
For a DC or single-phase two-wire circuit, multiply 2 by current, one-conductor resistance, and one-way length. For balanced three-phase, use √3 instead of 2. Keep units consistent; this calculator converts metres and ohms per kilometre internally.

Do I enter one-way or total wire length?
Enter one-way route length. The single-phase and DC formula includes the outgoing and return conductors with its factor of 2. For three-phase, use the one-way route length with the √3 factor.

Does the calculator work for three-phase power?
It provides an approximate estimate for a balanced three-phase circuit. Use the appropriate line current, one-conductor resistance, and line-to-line source voltage when assessing line-to-line drop. An unbalanced circuit or a calculation requiring AC reactance needs a more complete method.

Why is actual voltage drop different from the estimate?
The result can differ because actual resistance changes with conductor temperature, current may vary, and AC reactance, power factor, connections, and source-voltage changes are not fully represented by this simple model.

Is voltage drop the same as power loss?
No. Voltage drop is a difference in voltage between two points. Power loss is energy dissipated in the conductors over time; calculating it requires current and the resistance of the complete current path, with AC effects considered where relevant.

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in l | One-way conductor length | m | 30 | 0
in i | Load current | A | 20 | 0
in r | Conductor resistance | ohm/km | 0.5 | 0
in v | Source voltage | V | 230 | 0
sel c | Circuit type | DC or single phase=2, Three phase=1.732
out drop | Voltage drop | V | c * i * r * l / 1000 | 2
out percent | Voltage drop | % | 100 * drop / v | 2
out load | Approximate load voltage | V | v - drop | 2