Voltage Drop Calculator: DC and Single-Phase AC

A voltage drop calculator estimates how much voltage is lost along a cable from its length, current, conductor material, and size. Enter those values and the supply voltage to get the estimated drop in volts and as a percentage; this is a planning calculation, not a cable-sizing or safety approval.

Formula

For a two-conductor DC or single-phase AC circuit, the estimated drop is ΔV = I × ρ × (2L / A). For a balanced three-phase circuit, this calculator uses ΔV = √3 × I × ρ × (L / A), which is equivalent to the single-phase expression multiplied by approximately 0.866. The selected system factor applies that change.

• ΔV is voltage drop in volts (V).
• I is load current in amperes (A).
• L is the one-way cable length in metres (m).
• A is conductor cross-sectional area in square millimetres (mm²).
• ρ is the approximate conductor resistivity: 0.0175 Ω·mm²/m for copper in this calculator, multiplied by 1.64 for aluminum.
• System factor is 1 for DC or single-phase and 0.8660254 for the stated three-phase approximation.

The percentage drop is ΔV ÷ supply voltage × 100. Estimated load voltage is supply voltage minus the calculated drop. The input voltage is used for the percentage and load-voltage results; it does not affect the voltage-drop calculation itself.

Worked Example

Using the defaults in the calculator—120 V, 10 A, 30 m one-way, 2.5 mm² copper, and DC or single-phase—the two-way current path is 60 m.

1. Approximate cable resistance is 0.0175 × 60 ÷ 2.5 = 0.42 Ω.
2. Voltage drop is 10 × 0.42 = 4.20 V.
3. Percentage drop is 100 × 4.20 ÷ 120 = 3.50%.
4. Estimated voltage at the load is 120 − 4.20 = 115.80 V.

What Voltage Drop Means

Conductors have electrical resistance. When current flows, some supply voltage is expended overcoming that resistance, so the voltage measured at a load can be lower than the voltage at the source. For a fixed cable, more current or a longer route generally means more drop; a larger conductor area or a lower-resistance material generally means less.

Voltage drop is not the same thing as power consumption or a fault. It describes a difference in voltage between two points under the stated load. The calculation treats the cable as having a steady, uniform resistance and assumes that the entered current flows through the circuit.

How to Enter the Values

• Supply voltage: Use the nominal voltage at the source, in volts. It is needed to express the drop as a percentage and estimate load voltage.
• Load current: Enter the current in amperes, not milliamperes. For example, 500 mA is 0.5 A.
• Length: Enter the distance one way from supply to load. The formula accounts for the return conductor in DC and single-phase circuits. Do not enter the round-trip distance unless you adjust the calculation separately.
• Conductor area: Enter the cross-sectional area in mm². A cable’s area is not its diameter. For AWG sizes, look up the corresponding conductor area rather than entering the gauge number.
• Material and circuit: Select copper or aluminum, and the applicable circuit type. Do not use the three-phase option for a single-phase circuit.

The calculator’s default is an example, not a recommendation for a real installation. It does not account for every cable construction, operating temperature, connector, installation condition, or applicable electrical requirement.

AC Accuracy and Important Limits

The simple formula is most useful as an estimate for DC and for resistive loads where conductor resistance dominates. AC conductors also have impedance, which includes reactance. In an AC circuit, the voltage change depends on the load’s power factor, conductor characteristics, and—in a three-phase circuit—the phase relationship. The calculator does not ask for frequency, power factor, cable geometry, or temperature, so it cannot produce a precise AC impedance calculation.

Conductor resistivity changes with temperature. A cable that is warm under load generally has greater resistance than the room-temperature approximation used here. The formula also omits resistance at terminals and connections, as well as effects such as harmonics and changing loads. Long runs, motor loads, sensitive equipment, or a result close to a design limit call for a more complete calculation using appropriate cable data and system details.

A three-phase result uses a standard balanced-load approximation. If phases are significantly unbalanced, the simple three-phase expression may not represent the actual drop. The calculator also assumes the same conductor size and material throughout the run.

Reading Results and Avoiding Mistakes

Voltage drop is reported in volts, while percentage drop makes it easier to compare circuits operating at different nominal voltages. The estimated load voltage is only a subtraction from the entered supply voltage; it is not a measurement or guarantee. Results are rounded to two decimal places, so small differences can arise when comparing them with calculations made at higher precision.

• Mixing metres with feet or mm² with AWG without conversion changes the answer substantially. This calculator expects metres and mm².
• Entering total circuit length where the input asks for one-way length doubles the distance counted for a two-wire DC or single-phase run.
• Using a device’s rated current when its actual operating current is different can misstate the drop. Use a relevant operating load for an estimate.
• Assuming a small voltage drop proves an installation safe is incorrect. This calculation does not select a cable, fuse, breaker, or protective device.

For a rough conversion, 1 m is about 3.281 ft, 1 mm² is about 1,973 circular mils, and 1 V is about 1,000 mV. Convert all values to the calculator’s units before entering them.

When to Use a More Complete Check

Use this calculator to understand the basic relationship between cable length, current, material, and voltage loss, or to compare preliminary scenarios. For an actual installation, cable selection and acceptable voltage drop depend on the equipment, wiring method, ambient conditions, applicable codes, and manufacturer specifications. Confirm the design with a qualified electrical professional; do not use this estimate by itself to choose wiring or protective equipment.

Frequently Asked Questions

Does cable length mean one way or round trip?
The input means one-way distance. The formula counts both outgoing and return conductors for DC and single-phase circuits. For three-phase, it uses the standard balanced-load phase relationship rather than simply doubling the route.

Can I use this calculator for AC?
It gives a resistance-based estimate for single-phase AC and a simplified balanced three-phase estimate. It does not model full AC impedance or power factor, so it may not be accurate enough for a detailed design.

Why does voltage drop increase with current?
For a fixed conductor and route, the approximate relationship is ΔV = I × R. Increasing current increases the voltage needed to overcome the conductor’s resistance.

Can I enter AWG instead of mm²?
No. The conductor input is an area in mm², not an AWG gauge number. Convert the gauge to its conductor area using a reliable cable reference first.

Does the calculator tell me what cable size to install?
No. It calculates an estimated drop for a size you enter. Cable selection also depends on current-carrying capacity, installation conditions, protection, equipment requirements, and applicable rules.

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in v | Supply voltage | V | 120 | 0.001
in i | Load current | A | 10 | 0
in d | One-way cable length | m | 30 | 0
in s | Conductor area | mm² | 2.5 | 0.001
sel mat | Conductor | Copper=1, Aluminum=1.64
sel sys | Circuit type | DC or single-phase=1, Three-phase=0.8660254
out drop | Voltage drop | V | i * (0.0175 * mat) * (d / s) * 2 * sys | 2
out pct | Drop as percentage of supply | % | 100 * drop / v | 2
out loadv | Estimated voltage at load | V | v - drop | 2