Cable Derating: Why a Cable's Rated Capacity Is Not Its Real One

Why it matters on a project

This is one of the few electrical subjects where the failure is completely silent. An under-derated cable carries its load, operates its equipment and trips nothing. It simply runs hotter than intended and ages faster than it should.

It matters commercially because the correction — pulling in larger cable — is extremely disruptive once containment is full and ceilings are closed.

What reduces a cable's capacity

Where the problem concentrates

Derating is most severe exactly where cables naturally bunch: immediately outside the panel room, where every circuit in the building leaves from one place on the same containment.

That is the point to check, not a typical mid-route location. A design that works along the corridors can be inadequate in the first few metres.

The second concentration is anywhere cables pass through a hot environment — a plant room, a roof-level route in direct sun, an unventilated riser.

The fit-out problem

Later work adds circuits to containment that already exists. Each addition raises the grouping factor for every cable sharing that route, including cables installed years earlier and sized for a less crowded tray.

This is why containment capacity should be considered in terms of thermal grouping, not just physical space. A tray that still has room may already be electrically full.

Common mistakes

• Sizing from the table without applying factors. The reference conditions rarely exist on site.
• Using a nominal ambient temperature. Indian plant rooms and roof routes are frequently well above it.
• Checking a typical route instead of the worst one. The panel room exit governs.
• Adding circuits to an existing tray without re-assessing grouping. Existing cables are affected too.
• Running cables through or against thermal insulation. A sharp derate that is easy to create accidentally.
• Assuming the breaker protects against overheating. It protects against overcurrent, which is not the same thing.

Standards referenced

Electrical installation practice in IS 732; recommended current ratings in IS 3961; selection and erection of wiring systems in IEC 60364-5-52. Cable sizing, applicable derating factors and the design for a specific installation must be established by the project's electrical consultant.