How this calculator works
- Step 1Resistivity at temperature0.02063 Ω·mm²/mCopper at 70 °C, from its 20 °C value.
- Step 2Resistance per km8.252 Ω/kmResistivity divided by the 2.5 mm² cross-section.
- Step 3Drop and percentage6.6 V · 2.87%2 × 25 m × 16 A × resistance per metre.
Current flowing through a cable’s resistance loses some voltage before it reaches the load. The calculator works out that loss from the metal, the cross-section, the length, the current and the conductor temperature.
Resistance rises with temperature. The page uses the linear rule ρ = ρ₂₀ × (1 + α × (T − 20)), with copper and aluminium values from NIST’s wire handbooks.
Single-phase and three-phase
- Single-phase or two-wire DC2 × L × I × RCurrent goes out on one conductor and back on the other, so the loop is twice the length.
- Balanced three-phase√3 × L × I × RGives the line-to-line drop for a balanced load at unity power factor.
Worked example
- Resistivity at T
ρ = ρ₂₀ × (1 + α × (T − 20))- Voltage drop
Vd = 2 × L × I × ρ ÷ A- Percentage
Vd ÷ supply voltage × 100
- ρ₂₀Copper resistivity at 20 °C 1/58 = 0.017241 Ω·mm²/m
- αTemperature coefficient 0.00393 per °C
- TConductor temperature 70 °C
- Resistivity at temperature0.017241 × (1 + 0.00393 × 50)0.020629 Ω·mm²/m
- Resistance of one conductor0.020629 ÷ 2.5 × 1,0008.252 Ω/km
- Voltage drop2 × 25 × 16 × 0.0082526.6 V
- Voltage at the load230 − 6.6223.4 V
- Percentage drop6.6 ÷ 230 × 1002.87%
At a 3% target, the exact cross-section needed is 2.39 mm², so 2.5 mm² is the smallest standard size that meets it.
A three-phase example: 100 m of 16 mm² copper carrying 50 A at 400 V drops 11.17 V, or 2.79%.
What changes the drop
Drop grows in direct proportion to length and current, and falls in proportion to cross-section. Aluminium’s higher resistivity and a warmer conductor both raise it.
Choosing a size for a target drop
The suggested size is the smallest standard cross-section whose calculated drop is at or below your target.
The calculator solves the formula for cross-section, then rounds up to the next size in the IS 8130 list, from 1.5 mm² to 630 mm². If even 630 mm² is not enough, it says so.
Voltage drop is only one test. A cable must also carry the current safely for how it is installed, and its protection must clear faults. Those checks are outside this tool.
Which percentage limit applies
Notified ECBC rules set 2% for feeders and 3% for branch circuits, for large commercial buildings only.
The Energy Conservation Building Code, as notified by Tamil Nadu in 2022 (clause 7.2.1.3), caps voltage drop at 2% for feeders and 3% for branch circuits at design load.
Those rules apply to commercial buildings with a connected load of 100 kW or more, or a contract demand of 120 kVA or more. For other installations, ask your electrician which limit applies. The default target here is a planning choice, not a legal limit.
What is not included
- No reactanceInductive reactance matters for large cables and long runs; it is ignored.
- No power factor or unbalanceThe load is treated as resistive and, for three-phase, balanced.
- Nominal cross-sectionReal conductors may have up to the tabulated maximum resistance, a few percent more.
IS 8130 lists the maximum resistance allowed for each conductor size. For 2.5 mm² copper it is 7.41 Ω/km at 20 °C, against 6.897 Ω/km from resistivity. A conforming cable can therefore drop slightly more than shown.
Related electrical tools
Voltage drop is Ohm's law applied along a cable. Load and resistance come first.
To relate voltage, current and resistance, use the Ohm’s law calculator. For an appliance’s energy use and its cost on your bill, try the electricity calculator.
Common mistakes
Mistake: Entering the out-and-back length.
Instead: Enter the one-way route length; the formula adds the return path.
Mistake: Using the breaker rating as the load current.
Instead: Use the expected design load, or the rating only for a worst case.
Mistake: Using line-to-line voltage for a single-phase circuit.
Instead: Use phase-to-neutral voltage for single-phase and line-to-line for three-phase.
Mistake: Choosing a cable on voltage drop alone.
Instead: Ask a licensed electrician to check current rating and protection too.
Sources and checks
Material constants come from NIST’s copper and aluminium wire tables; sizes from IS 8130; the limit from a state ECBC notification.
Copper uses the International Annealed Copper Standard: 1/58 Ω·mm²/m and 0.00393 per °C at 20 °C. Aluminium uses the EC-H19 (61% IACS) values: 0.028264 Ω·mm²/m and 0.00403 per °C.
An independent script recomputes every golden value differently. It uses conductivity, a Kirchhoff loop for single-phase and complex phasors for three-phase, then finds the size by trying every standard size in turn.
Safety note
Electrical work can cause shock, fire and death. This calculator gives an estimate for planning only. Consult a licensed electrician before choosing cable, changing a circuit or doing any wiring.
Read how we test our calculators, or browse more everyday tools.