A cable’s printed current rating assumes ideal conditions. In real installations two things lower it: a hot environment and several cables bunched together. Multiply the base rating by the factors below to get the safe, derated current.
Ambient temperature (PVC-insulated, base 40 °C)
| Ambient | 35 °C | 40 °C | 45 °C | 50 °C | 55 °C |
|---|---|---|---|---|---|
| Factor | 1.08 | 1.00 | 0.91 | 0.82 | 0.71 |
Grouping (number of circuits together)
| Circuits | 1 | 2 | 3 | 4 | 6 |
|---|---|---|---|---|---|
| Factor | 1.00 | 0.80 | 0.70 | 0.65 | 0.57 |
Example: a cable rated 20 A, run with two other circuits (0.70) in a 45 °C space (0.91), gives 20 × 0.70 × 0.91 ≈ 12.7 A usable.
Indicative guidance for typical Indian domestic installations. Always confirm final cable sizes against the applicable IS standards, your local wiring rules and a licensed electrician for your specific installation.