Ambient temperature correction factor (Ca) for cable ampacity — why the base current-carrying capacity table already assumes a specific temperature

The guide on the grouping factor (Cg) covers how the current-carrying capacity of a cable must be reduced when it runs bundled together with other loaded cables. This article covers a separate correction that is applied for an entirely different reason, and is easily overlooked or confused with grouping: the ambient temperature correction factor (Ca).

The base table already assumes a reference temperature

The tabulated current-carrying capacity values in IEC 60364-5-52 (and the corresponding NEN 1010 tables) are not universal constants — they are calculated for a specific reference ambient temperature:

30°C for cables installed in free air or in conduit/trunking in air.
20°C for cables buried directly in the ground (a separate reference, distinct from the air-installation value — see the guide on cables in the ground).

Those tabulated values represent the current a cable can carry continuously at that specific ambient temperature without its conductor exceeding its maximum permitted operating temperature (for example 70°C for PVC, 90°C for XLPE). If the actual installation environment is hotter than the table's reference temperature, the cable reaches its maximum conductor temperature at a lower current than the table value suggests — hence the correction factor Ca, always less than 1 above the reference temperature (and greater than 1 for a genuinely cooler-than-reference environment).

Where Ca applies in practice

Any location where the ambient temperature around the cable meaningfully differs from the table's reference value requires Ca to be applied, independently of whether the cable is grouped with others:

A single cable run through a hot plant room, boiler room, or near process equipment that raises the local air temperature well above 30°C.
A cable tray in a roof void that reaches a significantly elevated temperature in summer, or directly under a roof exposed to strong sun.
An outdoor cable tray or single cable run in direct sun, where the effective ambient temperature at the cable is higher than the shaded air temperature would suggest.
A cable installed in a location that is intermittently but predictably hot (for example next to a switchgear compartment with its own heat dissipation).

Why Ca and Cg are combined, not substituted for each other

Ca (ambient temperature) and Cg (grouping) correct for two entirely different physical effects — the surrounding air temperature versus mutual heating between adjacent loaded cables — and are multiplied together with any other applicable factors, not used as alternatives:

I_z = I_tabel × Ca × Cg × (other applicable factors)

A cable run in a hot plant room and bundled with several other loaded cables needs both factors applied; applying only one because "the other correction was already done" understates the actual derating needed.

Note: the exact Ca values for a given temperature and insulation type are given in tables in IEC 60364-5-52 / NEN 1010; this article covers the underlying principle — that a correction is separately needed whenever the actual ambient temperature differs from the table's reference value — not the numeric table itself.

Practical relevance

When sizing a cable for an installation, it is not enough to look up the ampacity for the chosen installation method and cross-section and apply the grouping factor if applicable — the actual ambient temperature at the cable's location must also be assessed against the table's reference temperature (30°C in air, 20°C buried), and Ca applied if that location runs meaningfully hotter, even for a single, non-grouped cable.

Common mistakes

Applying only the grouping factor (Cg) and forgetting Ca for a cable installed in a hot location, even when it is not bundled with other cables — the reference-temperature assumption behind the base table applies regardless of grouping.
Using the 30°C-in-air reference table for a cable buried directly in the ground, or vice versa — the two installation methods use different reference temperatures and different base tables; mixing them up defeats the whole calculation.
Not re-assessing Ca when a cable's environment changes — for example a permanently loaded standby cable running through a space that becomes significantly hotter after other equipment is added nearby.
Confusing the ambient temperature correction factor with the conductor's maximum permitted operating temperature — Ca corrects the allowed current for a hotter surrounding environment; it does not change the insulation's own maximum temperature rating (70°C PVC, 90°C XLPE), which remains a separate, fixed limit.