Cable Derating
Cable derating is the correction of a cable's catalogue current rating for real installation conditions — ambient temperature, grouping with other cables, burial or tray stacking — so the working capacity is often substantially below the datasheet number.
Why it matters on an MEPF project
Catalogue ampacity assumes idealised conditions Indian sites never provide: a 45 °C shaft, twelve cables sharing a tray, or ducts in sun-baked ground all shave the real capacity. The classic failure is a feeder sized to the datasheet, installed in a crowded tray, spending its life warm — insulation aging fast, voltage drop worse than computed, and protection coordination quietly off.
Derating stacks multiplicatively: ambient factor × grouping factor × installation-method factor. Design must therefore be route-aware — the same load may need a fatter cable on one route than another. Voltage drop then applies as its own independent check on long runs; passing ampacity does not excuse failing drop.
How it's specified in practice
| Parameter | Typical / working position | Governing reference |
|---|---|---|
| Ambient temperature | Ratings quoted at reference ambient; hot shafts/roofs derate meaningfully | IS 732 practice / cable standards |
| Grouping | Each added circuit in a tray cuts everyone's capacity | derating tables |
| Installation method | Buried, ducted, tray, clipped — each with its own factors | derating tables |
| Voltage drop | Separate limit check on long runs | design practice (IS 732 context) |
Common mistakes
- Datasheet ampacity used raw; the tray is not a laboratory.
- Trays filled over years with no re-check of the original grouping math.
- Long runs passing current checks but failing voltage drop at the motor.
- Aluminium/copper substitutions without redoing the numbers.
Related on this site
One of 68 working definitions in the MEPF & solar engineering glossary. The buying side of the same decisions is in the buyer's guides; the numbers are in the calculators.
Frequently asked
How much does grouping really cost?
Enough that a heavily shared tray can halve effective capacity versus a lone clipped cable. The tables are unforgiving — which is why our cable calculator asks about route and grouping, not just amps.
My breaker never trips but the cable runs hot — why?
The cable was likely sized to catalogue and derates below the breaker's assumption. Warm-running cables age silently; thermography plus a resize is the fix, not tolerance.
Standards referenced: IS 732 (practice) · cable standards derating tables