Voltage Drop Calculator
Estimate voltage drop and percentage loss in copper or aluminium cables for single and three phase.
Voltage drop
2.75 V
Drop as % of supply
1.2%
Typically should stay under 3%
DC / single-phase: drop = 2 × L × I × ρ / A. Three-phase uses √3 instead of 2. Copper ρ = 0.0172, aluminium ρ = 0.0282 Ω·mm²/m.
Voltage Drop Calculator on True Calculator gives you an instant, accurate answer with no sign-up and no app install. Estimate voltage drop and percentage loss in copper or aluminium cables for single and three phase. Every result shows the formula and a worked example so you can verify the calculation yourself, and all values are computed in your own browser — your numbers never leave your device.
Popular uses: voltage drop calculator · cable voltage drop · wire size drop
How We Calculate
This calculator uses standard computing formulas and algorithms verified by our team. All calculations are performed instantly in your browser using JavaScript — no data is sent to any server.
Algorithms follow well-documented public specifications used by browsers and programming languages.
When to Use This Calculator
Use this calculator when planning wiring for a home extension, a shed, a borewell pump, or an inverter installation where the battery or mains sits far from the load. Long runs of cable on Indian construction sites and rural properties are where voltage drop shows up as dim bulbs, slow motors and overheating wires. Before buying cable, enter the run length, the load current of the appliances, and the available gauge sizes to see which one keeps the drop within limits. The same calculation helps electricians quote for jobs, owners compare copper against cheaper aluminium, and solar installers size the DC wiring between panels and inverter, where low-voltage circuits are especially sensitive to loss.
How to Use This Calculator
- Step 1: Measure the one-way cable length from the supply to the load in metres.
- Step 2: Enter the expected load current in amps and the conductor cross-section in mm².
- Step 3: Choose copper or aluminium and single- or three-phase, then enter the supply voltage.
- Step 4: Compare the drop percentage against the 3% guideline for lighting circuits.
Worked Example
A 230 V single-phase supply feeds a 10 A load through 20 m of 2.5 mm² copper wire. Enter length 20, current 10, area 2.5, copper, single phase and 230 V. The calculator computes 2 × 20 × 10 × 0.0172 ÷ 2.5 = 2.752 V, which is 1.2% of 230 V — comfortably inside the usual 3% limit. Switching to aluminium raises the drop to 4.512 V, or 1.96%, showing why copper is preferred for long runs.
Tips and Common Mistakes
- •Keep drops under 3% for lights and 5% overall from the meter for general power circuits.
- •Enter the one-way distance; the calculator adds the return path automatically.
- •When in doubt between cable sizes, pick the next larger mm² — it halves the drop.
- ✗Avoid entering the total of both conductors as the length — double counting inflates the drop.
- ✗Don't ignore the supply voltage field; without it you get the drop in volts but no percentage to judge against.
Frequently Asked Questions
What is voltage drop?
Voltage drop is the loss of potential along a cable caused by its resistance. For a 20 m copper run of 2.5 mm² wire carrying 10 A, the drop works out to about 2.75 V, or 1.2% of a 230 V supply.
Why does the calculator use 2 × length for single phase?
The current travels out along one wire and back along the other, so the total conductor length is twice the one-way distance. Three-phase systems use √3 × length instead because the phases share the return path.
How much voltage drop is acceptable?
Most installations aim to keep drop under 3% for lighting circuits and under 5% overall from the meter to the farthest point. Above that, motors may run weak and lights can flicker noticeably.
Does copper really drop less than aluminium?
Yes — copper has a resistivity of 0.0172 Ω·mm²/m versus 0.0282 Ω·mm²/m for aluminium, so a copper cable of the same size drops roughly 40% less voltage for the same current and length.
How do I reduce voltage drop?
Use a larger cross-section cable, shorten the run, or raise the supply voltage. Doubling the cross-section halves the drop, which is why the calculator takes the wire area in mm² as an input.
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