Voltage Drop Calculator

Voltage Drop Calculator — fast, accurate results online. Enter your values and get instant answers.

V
A
m
mm²

Results

Calculated
Voltage drop
—
In V
Drop percentage
—
Of supply voltage
Voltage at load
—
In V
Power lost in cable
—
In W

What voltage drop is and when it matters

Every cable has some resistance, so when current flows through it a portion of the source voltage is lost along the way. That loss is the voltage drop. The load receives less voltage than the supply provides, which can make motors run hot, lights dim and electronics reset. Long runs, thin conductors and high currents all make it worse.

Use this calculator when sizing a feeder, extension cord, garden lighting circuit, workshop sub-panel or solar run, and you want to know whether the conductor is thick enough. Enter the supply voltage, load current, one-way length, conductor area and material, and choose DC or single-phase versus three-phase. The tool returns the drop in volts, the percentage of supply, the voltage left at the load and the power wasted as heat in the cable.

Formula and variables

Vdrop = k × ρ × L × I / A

  • k is 2 for DC or single-phase (current goes out and returns) and √3 ≈ 1.732 for balanced three-phase line-to-line.
  • ρ is resistivity at about 20 °C: 0.0172 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminum.
  • L is the one-way cable length in metres.
  • I is the load current in amperes.
  • A is the conductor cross-section in mm².

The percentage is Vdrop / Vsupply × 100, and the cable loss is Vdrop × I for single-phase, or √3 × Vdrop × I for three-phase. The model uses resistance only, so it ignores reactance and temperature above 20 °C.

Worked example

A 120 V single-phase circuit feeds a 20 A load 30 m away through 4 mm² copper.

  • Loop resistance = 2 × 0.0172 × 30 / 4 = 0.258 Ω.
  • Vdrop = 0.258 × 20 = 5.16 V.
  • Percentage = 5.16 / 120 = 4.30%.
  • Voltage at load = 120 − 5.16 = 114.84 V.
  • Cable loss = 5.16 × 20 = 103.2 W.

These are the figures the calculator displays. Doubling the conductor to 8 mm² halves the drop to 2.58 V (2.15%), which shows why upsizing is the standard cure.

Common mistakes and how to interpret the result

  • Entering the round-trip length. The factor of 2 is already included for single-phase, so use the one-way distance.
  • Confusing AWG with mm². Convert first: 14 AWG is about 2.08 mm², 12 AWG about 3.31 mm² and 10 AWG about 5.26 mm².
  • Treating one number as a code limit. A common design guideline is about 3% for a branch circuit and 5% overall; this is guidance, and your local code and equipment ratings govern.
  • Ignoring temperature. Copper resistance rises roughly 0.4% per degree Celsius, so a hot conduit run drops more than the 20 °C figure.

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Frequently Asked Questions

How much voltage drop is acceptable?
Many designers aim for no more than about 3% on a branch circuit and about 5% from the service to the farthest load, but sensitive electronics or long low-voltage runs may need tighter limits. Check the requirements of your equipment and local rules.
Why does low-voltage wiring suffer more?
The same volts lost are a larger share of a small supply. Losing 1 V is under 1% of 120 V but over 8% of 12 V, which is why 12 V runs need much thicker wire.
Does aluminum need a larger conductor?
Yes. Its resistivity is about 64% higher than copper, so for equal drop it needs roughly 1.6 times the cross-section.
Does the calculator handle AC reactance?
No. It is a resistance-only estimate, which is accurate for DC and for short or small-conductor AC runs. Large conductors or long AC runs also have inductive reactance that adds to the drop.