Electrician 2 Voltage Drop Calculator Explained

An Electrician 2 voltage drop calculator estimates the voltage lost along a circuit conductor from its length, current, conductor resistance, and circuit type. Enter those electrical values to get the estimated drop in volts and as a percentage of source voltage; this arithmetic is a planning aid, not a wire-size or safety recommendation.

Formula

This calculator uses a simplified resistance-based voltage-drop formula:

• Voltage drop (V) = k × I × L × R ÷ 1,000
• Voltage-drop percentage (%) = 100 × voltage drop ÷ source voltage
• Estimated load voltage (V) = source voltage − voltage drop

I is load current in amperes (A); L is the one-way distance from source to load in feet; and R is conductor resistance in ohms per 1,000 feet. The factor k is 2 for a DC two-wire or single-phase circuit, accounting for the outgoing and return path, and approximately 1.732 for a balanced three-phase circuit. The 1,000 converts the resistance unit to the entered length.

The calculator’s resistance input is a value you must obtain for the relevant conductor and operating conditions. It does not infer conductor material, gauge, temperature, installation method, or frequency. If resistance is given in ohms per 1,000 metres, first convert it or use a formula with metric units; do not enter that value as though it were ohms per 1,000 feet.

Worked Example

Using the defaults—120 V, 10 A, a 100 ft one-way run, 1.93 ohms per 1,000 ft, and a DC two-wire circuit—the calculation is:

1. Use k = 2 because the circuit has outgoing and return conductors.
2. Voltage drop = 2 × 10 × 100 × 1.93 ÷ 1,000 = 3.86 V.
3. Drop percentage = 100 × 3.86 ÷ 120 = 3.2166…%, which rounds to 3.22%.
4. Estimated load voltage = 120 − 3.86 = 116.14 V.

These are estimates based on the resistance entered. The result is not a measurement of an installed circuit.

How to Use the Calculator

Enter the voltage at the circuit source, the expected load current, the one-way route length, and the conductor resistance in the stated units. Choose the circuit type that matches the calculation. The first result is the estimated drop in volts; the additional results show the drop relative to source voltage and the calculated voltage remaining at the load.

• For a 100 ft route, enter 100—not 200—because the formula’s factor accounts for the return conductor in a two-wire circuit.
• For a route measured in metres, convert length to feet (1 m = 3.28084 ft) if using the calculator as configured.
• Use current expected at the load, not a breaker rating or a guessed current, unless that value genuinely represents the operating current.
• Use a resistance figure that corresponds to the conductor and conditions being considered. The example’s resistance is only a default for demonstrating the calculation.

What the Result Means—and What It Does Not

Voltage drop is the difference between voltage at the source and the estimated voltage at the load while current flows. A larger current, longer route, or greater conductor resistance increases the calculated drop. The percentage helps compare a drop across circuits with different source voltages; it is not itself a pass/fail judgment.

The model assumes a steady current and uses resistance only. Real AC circuits can also be affected by conductor reactance, power factor, temperature, and the circuit’s arrangement. Motor starting, changing loads, connections, and supply conditions can make actual voltage vary. The calculated load voltage is therefore a simplified estimate, not a guarantee of the voltage available at every moment.

Common Mistakes and Limits

• Doubling the distance twice: The two-wire and single-phase factor already accounts for the round trip. Enter one-way length.
• Mixing resistance units: Ohms per 1,000 feet and ohms per 1,000 metres are different. A unit mismatch can make the answer substantially wrong.
• Using a nominal value as a measurement: Source voltage and load current can vary. For a specific installation, measured values and suitable test methods may be needed.
• Treating the output as conductor sizing: This calculator does not assess ampacity, overcurrent protection, short-circuit performance, installation conditions, or applicable requirements.
• Rounding too early: Keep intermediate values unrounded where possible, then round the displayed result. Small rounding differences do not make an estimate exact.

The expression can produce a negative estimated load voltage if inputs imply a drop greater than the source voltage. That signals an implausible or extreme result under this simple model, not a usable operating condition.

When to Get a More Complete Assessment

Use this calculation to understand the relationship between current, distance, resistance, and voltage loss. It can help compare assumptions, but it cannot decide whether a particular conductor or installation is acceptable. Wiring choices affect fire and shock risk and depend on details the calculator does not collect. For actual electrical work, have a qualified electrician evaluate the circuit, equipment, installation conditions, and locally applicable requirements. Do not select or install conductors based on this estimate alone.

Frequently Asked Questions

What does an Electrician 2 voltage drop calculator calculate?
It estimates voltage lost along a circuit from current, one-way length, conductor resistance, and a circuit-type factor. It also reports the loss as a percentage and estimates remaining load voltage.

Should I enter one-way length or total wire length?
Enter one-way route length. For DC two-wire and single-phase calculations, the factor of 2 represents the outgoing and return paths.

Can I use the result to choose a wire gauge?
No. The result covers simplified voltage drop only. Conductor selection also involves ampacity, protection, installation conditions, and other requirements that this calculator does not evaluate.

Why does the calculator ask for conductor resistance?
Resistance determines how much voltage is lost for a given current and distance. It varies with conductor properties and operating conditions, so the calculator cannot reliably choose it without additional information.

Does this formula work for three-phase circuits?
It uses the common 1.732 factor for a balanced three-phase estimate. Actual AC voltage drop may require accounting for reactance, power factor, conductor arrangement, and load conditions.

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in v | Source voltage | V | 120 | 0 | 1000000
in i | Load current | A | 10 | 0 | 1000000
in l | One-way conductor length | ft | 100 | 0 | 1000000
in r | Conductor resistance | ohm/1000 ft | 1.93 | 0 | 1000000
sel c | Circuit type | DC two-wire=2, AC single-phase=2, AC three-phase=1.732
out drop | Estimated voltage drop | V | 2 * i * l * r * c / 1000 | 2
out pct | Voltage drop | % | 100 * drop / v | 2
out loadv | Estimated load voltage | V | v - drop | 2