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House-Wire Size Calculator

Estimate the right copper wire size for any home or building circuit — and the matching APAR wire. Based on Indian practice (IS 732 / IS 694), 230 V single-phase and 415 V three-phase.

230 V · 1φ
415 V · 3φ
15 m
Advanced options ▾

⚠️ Guidance only. Assumes copper conductors in conduit at normal ambient temperature, a 1.25× design margin and your selected voltage-drop limit. Actual sizing depends on installation method, grouping, ambient temperature and local regulations. Always have the final design confirmed by a licensed electrician.

How do you calculate wire size?

The method

The “size” of a wire is its cross-sectional area — the area of copper at a cut end. A larger area carries more current with less resistance, much like a wider pipe carries more water with less pressure loss. Two laws of physics set the minimum size.

V = I × R  ·  R = ρ × L / A
Ohm’s law (voltage drop) and Pouillet’s law (resistance from material & geometry)

Combining them — and allowing for the current’s return path — gives the minimum area:

A = (2 × ρ × L × I) / Vdrop
× 1,000,000 for mm². For three-phase, the factor 2 becomes √3 ≈ 1.732.

In practice the calculator uses standard mV/A/m figures for each size (which bake in resistivity, the return path and real construction), applies a 1.25× design margin, finds the smallest size whose rating covers the load, checks voltage drop against your limit, and finally coordinates the size with the MCB — the two safety checks a bare chart skips.

A worked example

Step by step
A 2-ton air-conditioner on 230 V draws ~13–14 A. With the 1.25× margin the design current is ~17 A, and the smallest size that covers it is 2.5 sq mm (19 A). But the suggested MCB is 20 A — and a breaker must never exceed the wire it protects, so the calculator steps up to 4 sq mm (26 A). Over a 20 m run the drop is ~1.3% — well inside the 3% limit.

Reference chart

Copper, in conduit
Size (sq mm)Current rating (A)Voltage drop (mV/A/m)
1.01144
1.51429
2.51918
42611
6347.3
10464.4
16612.8

Frequently asked questions

Good to know
Why does the calculator sometimes recommend a larger size than a wire chart?+
Because it also coordinates the wire with its protective MCB and checks voltage drop over your run length — a breaker must never be rated above the wire it protects, so the size is stepped up when needed.
What is voltage drop and why does it matter?+
Every metre of wire loses a little voltage. Over long runs this can dim lights or under-power appliances; keeping it under your chosen limit (3% is recommended) ensures reliable performance.
Can I use this for aluminium wires?+
No — these figures are for copper. Aluminium has higher resistance and needs a larger size for the same load.
What MCB should I use with the recommended wire?+
The result shows a suggested MCB rating coordinated to the wire. Always confirm with your electrician and switchboard design.
Does the length of the wire run really matter?+
Yes — the longer the run, the greater the voltage drop, so a long circuit may need a size larger than current alone would suggest.