How do you choose the right cable size?
Start from the design current of the load, then apply derating factors for ambient temperature, grouping with other cables, and installation method. Check that voltage drop over the full run stays within limits, usually 3% for lighting and 5% for power. The larger of the two results sets the size.
The four steps
First, establish the design current: the actual current the load will draw, including any diversity or future allowance you have agreed. Second, apply derating. A cable in free air carries far more than the same cable buried in a hot conduit alongside five others - grouping, ambient temperature and installation method all reduce the safe capacity, sometimes by half.
Third, check voltage drop. A cable can be perfectly adequate thermally and still be the wrong size, because over a long run the volt drop leaves the equipment at the far end undervolted. Motors are particularly unforgiving of this. Fourth, take whichever of the two results demands the larger cable - thermal capacity and volt drop are independent constraints and both must be satisfied.
The trade conventions used in Indian domestic wiring
These are the sizes commonly used in Indian residential work. They are a starting point for conversation, not a substitute for calculation - a long run, a hot roof void or a bank of cables in one conduit can all move the answer up a size.
| Size | Typical circuit |
|---|---|
| 1.0 sq mm | Lighting circuits |
| 1.5 sq mm | Lighting and ceiling fan circuits |
| 2.5 sq mm | General 6 A socket outlets |
| 4.0 sq mm | 16 A power sockets, small air conditioners |
| 6.0 sq mm | Geysers, larger air conditioners, sub-main runs |
| 10 sq mm and above | Sub-mains, distribution board feeds |
The three things people forget
Grouping. Six cables in one conduit do not each carry what one cable would. Heat has nowhere to go, and the derating factor is substantial.
Ambient temperature. A cable in a roof void in Gujarat in May is not operating at the 30 °C the tables assume. This is one reason electron-beam cross-linked cable rated to 105 °C is worth the difference in this climate rather than merely nice to have.
The neutral and earth. On circuits with significant harmonic content - LED lighting, variable speed drives, switch-mode supplies - the neutral can carry more current than any phase, and sizing it as if it will not is a common and consequential error.
Send us the numbers
Cable sizing depends on conditions we cannot see from a web page: the run length, how it is installed, what else shares the route, and what the ambient temperature actually reaches. Send us the load, the distance and the installation method and we will size it properly - it takes a few minutes and it is free.
Sources and further reading
Published by Electrolite Systems. Last updated
Last reviewed 3 September 2026. This guide explains how the decision is made; it is not a substitute for a design calculation for your specific installation.