Wire Size Calculator (Copper, IS 3961)
Find the smallest copper conductor cross-section satisfying continuous-current and voltage-drop constraints per IS 3961, for any load and distance.
How this works
For each gauge in the IS 3961 catalog (1.0 → 95 mm²) we compute the round-trip voltage drop and check both the voltage-drop and current-capacity limits. The smallest gauge that passes both wins:
vDrop = 2 × loadA × distanceM × R/km / 1000
vDropPct = vDrop / voltage × 100
fits = (vDropPct ≤ tolerancePct)
AND (loadA ≤ currentCapacityA)The factor of 2 covers the phase + neutral round-trip on a single-phase circuit. Resistance values are nominal DC at 20 °C from IS 8130; real installations must derate for ambient temperature and grouping.
Worked example
16 A load over a 30 m one-way run at 3% tolerance on a 230 V supply:
- 1.5 mm²: drop =
2 × 16 × 30 × 12.10 / 1000 ≈ 11.6 V= 5.05% — too high. - 2.5 mm²: drop ≈
7.1 V= 3.09% — still over. - 4.0 mm²: drop ≈
4.4 V= 1.92% — fits voltage drop AND 25 A capacity covers 16 A load.
Result: 4 mm² copper.
Sources
- IS 3961 (Recommended current ratings for cables), IS 8130 (Conductors for insulated cables), IS 732 (Code of practice for electrical wiring installations)
FAQ
Why do you size the wire on both current AND voltage drop?
A conductor has two failure modes. The first is overheating — too much sustained current melts the insulation, which IS 3961 prevents by tabulating a continuous-current capacity for each cross-section. The second is voltage drop — the resistance along a long run drops the voltage at the load below acceptable levels (IS 732 limits this to 3 percent for residential circuits), which causes lights to dim and motors to overheat. The smallest conductor that passes both checks is the right size; either check alone is incomplete.
Where does the factor of 2 in the voltage-drop formula come from?
Single-phase circuits carry current in a closed loop — phase out, neutral back. The voltage drop happens in both legs, so the total drop is 2 × (current × distance × resistance per km / 1000). Three-phase balanced circuits use a factor of √3 ≈ 1.732 instead of 2; this calculator assumes single-phase, which covers nearly all residential branch circuits.
What is a typical voltage-drop tolerance?
3 percent is the IS 732 default for residential lighting and general-purpose outlets. 5 percent is sometimes accepted for power circuits feeding water pumps or air-conditioning units where minor sag does not matter. 1–2 percent is the conservative target for sensitive electronics and long sub-mains. Increasing the tolerance allows a smaller conductor and saves cost, but at the price of dimming, motor stalling, and shorter equipment life.
Why might the calculator return no result?
The catalog tops out at 95 mm². If the load current or run distance is so large that even 95 mm² cannot satisfy the voltage-drop tolerance, no gauge fits and the calculator returns nothing. This usually means you need a different supply strategy — a sub-distribution board closer to the load, three-phase distribution, or a higher voltage. As a workaround, raise the tolerance to 5 percent or shorten the run by relocating the panel.
Are the catalog ratings derated for ambient temperature?
The continuous-current ratings here assume single-core copper conductors run in conduit at 30 °C — the most conservative installation method tabulated in IS 3961. Real installations need derating for ambient temperature, conduit fill, and grouping with other loaded cables. Apply the IS 3961 grouping factor (typically 0.7–0.85 for two to four cables) and the temperature factor (0.94 at 35 °C, 0.87 at 40 °C) before final sizing on hot or congested runs.
Does this size aluminium cable too?
No, only copper. Aluminium has roughly 1.6× the resistance of copper at the same cross-section, so an aluminium conductor must be one or two standard sizes larger to match a copper sizing. Use this calculator for copper and step up two standard sizes (e.g. 4 mm² Cu → 6 or 10 mm² Al) when ordering aluminium. For larger feeders consult an aluminium-specific table from the cable manufacturer.