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

> Cable derating is the reduction applied to a cable's tabulated current-carrying capacity to account for the conditions it is actually installed in — how many cables are bunched together, the ambient temperature, and the installation method. The tabulated figure assumes reference conditions that rarely exist on a real project.

## 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

| Condition | Effect | Where it bites |
| --- | --- | --- |
| Grouping | Cables heat each other | Busy trays leaving a panel room |
| Ambient temperature | Less temperature headroom available | Plant rooms, roof routes, shafts |
| Installation method | Determines how heat escapes | Conduit and sealed trunking worst |
| Contact with thermal insulation | Heat cannot dissipate | Cables buried in insulated build-ups |
| Buried runs | Soil thermal resistivity and depth | External and site distribution |

## 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.

## Frequently asked questions

### Why do cables need derating at all?

Because a cable's rating is limited by the temperature its insulation can tolerate, and that depends on how well heat escapes. The tabulated figure assumes a single cable in defined reference conditions. Put several cables together on a tray and each one's heat warms its neighbours; run them through a hot plant room or an unventilated shaft and the starting temperature is already higher. In both cases the cable reaches its temperature limit at a lower current than the table suggests.

### What are the main derating factors?

Grouping — how many circuits are bunched together and how closely. Ambient temperature — the actual air or ground temperature where the cable runs, which in Indian conditions is frequently well above the reference temperature the tables assume. Installation method — in conduit, on a perforated tray, clipped direct, buried, or in a sealed trunking, each of which affects heat dissipation differently. Thermal insulation in contact with a cable derates it sharply.

### What actually happens if derating is ignored?

The cable runs hotter than its insulation is rated for. That does not usually trip anything, because the protective device is sized on current, not temperature. Instead the insulation ages faster than designed, which shortens the cable's life and can eventually lead to failure. It is a slow, invisible degradation rather than an immediate fault, which is exactly why it gets overlooked.

### Does adding circuits to an existing tray matter?

Yes, and it is a common real-world problem. A tray designed for a certain number of circuits gets more added during a later fit-out, which increases the grouping factor for every cable already there — including ones nobody touched. The existing cables were sized for the old arrangement and are now under-rated for the new one.

### Who is responsible for applying it?

The designer sizing the installation. Derating factors and ratings are set out in standards such as IS 3961 and IEC 60364-5-52, applied within the wiring rules adopted for the project. Cable sizing and the applicable factors for a specific installation must be determined by the project's electrical consultant, since they depend on the actual route, grouping and conditions.

## Sources

- [IS 732](https://standards.bis.gov.in/website/published-standards/department-wise) — Code of practice for electrical wiring installations
- [IS 3961](https://standards.bis.gov.in/website/published-standards/department-wise) — Recommended current ratings for cables
- [IEC 60364-5-52](https://webstore.iec.ch/en/) — Selection and erection of wiring systems

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Source: https://hagerstone.com/glossary/cable-derating