HesapHanem

Voltage Drop Calculator

Estimate the voltage drop and remaining load voltage for a copper or aluminum cable run.

How it is calculated

The drop is Ohm's law applied to the cable's resistance: Vdrop = 2 × L × I × ρ / A, where L is the one-way cable length in metres, I the current in amperes, A the conductor cross-section in mm², and ρ the resistivity in Ω·mm²/m: 0.0175 for copper and 0.0282 for aluminium. The factor 2 counts the outgoing and return conductor. Then drop % = Vdrop / supply × 100 and voltage at load = supply − Vdrop. This is a simple resistive model for single-phase or DC circuits: it ignores temperature, cable reactance, power factor and three-phase factors. Area and supply must be above zero.

Worked example

A copper cable runs 30 m one way, carries 16 A, has a 2.5 mm² conductor and is fed from 230 V. Voltage drop = 2 × 30 × 16 × 0.0175 / 2.5 = 16.8 / 2.5 = 6.72 V. As a share of supply: 6.72 / 230 × 100 = 2.92%. Voltage at the load = 230 − 6.72 = 223.28 V. Choosing aluminium with the same cable would raise the drop to 10.83 V (4.71%).

Try this example →

Common mistakes

  • Entering the total length of both conductors. The tool already doubles the one-way length, so typing the round-trip distance doubles the drop twice over.
  • Entering the area in the wrong unit. The field is mm² of conductor cross-section, not the cable diameter or AWG number.
  • Using the circuit breaker rating as the current. The drop depends on the real load current, so enter the actual or design current.
  • Applying the result to three-phase circuits. The formula here is for single-phase or DC, so a three-phase run needs a different factor.
  • Treating the percentage limit as universal. Allowed drop depends on the circuit type and the standard you follow, so check your local rules for the limit.

About this calculator

This calculator works out the voltage drop along a cable run from its length, current, cross-sectional area, and the conductor material (copper or aluminum), returning the drop in volts, the drop as a percentage of supply voltage, and the actual voltage reaching the load.

Electricians and electrical engineers use it to check that a cable run stays within code — most standards call for under 3% drop on a branch circuit and under 5% total — before the wire is pulled, since undersized conductors on a long run waste energy as heat and can starve equipment of the voltage it needs.

Was this helpful?

You can pick up to 3 reactions.

Comments (1)

  • Editorial team

    Cross-section is the easiest lever: because the drop is inversely proportional to area, the default 30 m copper run at 16 A on 230 V falls from 6.72 V at 2.5 mm² to 4.20 V (1.83 percent) at 4 mm². Use the actual load current, since motors draw more while starting. This is a single-phase or DC model, so it does not fit three-phase runs. Which cable lengths and loads do you check most often?

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators