Derating Factors for Cables Grouped in Air

Derating factors for cables grouped in air, including IEC 60287-2-2 reduction factor methods for free-air installations.

Updated June 4, 2026

IEC 60287-2-2 gives methods for obtaining reduction, or derating, factors for cables installed in free air and protected from solar radiation. The methods are principally intended for multicore cables or single-core cables in trefoil, but may be extended to other arrangements where appropriate.

This article summarises the grouped-in-air derating method and the main symbols used in the calculation. For design work, use the relevant edition of IEC 60287-2-2 directly.

Symbols

D_e	External diameter of multicore cable, or one single-core cable in trefoil, mm
e	Clearance between adjacent cables in a group, measured between surfaces, mm
F_g	Group reduction factor
h	Heat dissipation coefficient used for thermal resistance calculation, W·m^-2·K^-5/4
h_g	Heat dissipation coefficient of the hottest cable or circuit in a group, W·m^-2·K^-5/4
h_i	Heat dissipation coefficient of an isolated cable or circuit, W·m^-2·K^-5/4
I_g	Rating of the hottest cable in a group, A
I_t	Rating of one cable or circuit assumed to be isolated, A
k₁	Surface temperature rise factor
T_4g	External thermal resistance of the hottest cable in a group, K·m/W
T_4i	External thermal resistance of one cable assumed to be isolated while carrying I_t, K·m/W
W	Power loss from isolated cable, or circuit in trefoil, carrying I_t, W/m
Θ_c	Conductor temperature used for calculating I_t, °C
Θ_a	Ambient temperature used for calculating I_t, °C

Reduction factors for cables grouped in air with existing ratings

Where the sustained current rating for an isolated cable or circuit is known, the rating of the cable installed within a group of similar cables is:

$I_{g} = F_{g} I_{t}$

The group reduction factor, F_g, is calculated from:

$F_{g} = \sqrt{\frac{1}{1 - k_{1} + k_{1} (\frac{T_{4 g}}{T_{4 i}})}}$

The surface temperature rise factor is:

$k_{1} = \frac{W T_{4 i}}{θ_{c} - θ_{a}}$

The term T_4g/T_4i is derived iteratively from the ratio h_i/h_g:

${(\frac{T_{4 g}}{T_{4 i}})}_{n + 1} = (\frac{h_{i}}{h_{g}}) {\frac{1 - k_{1}}{(\frac{T_{4 g}}{T_{4 i}})} + k_{1}}^{0.25}$

Starting with:

${(\frac{T_{4 g}}{T_{4 i}})}_{n + 1} = (\frac{h_{i}}{h_{g}})$

Values for h_i/h_g are obtained from the data section below.

Reduction factor for IEC 60287 calculated ratings

When IEC 60287 is used directly to calculate sustained current ratings, IEC 60287-2-2.1 gives methods for calculating thermal resistance. To apply group derating for cables, substitute the heat emission coefficient h with the group heat emission coefficient h_g:

$h_{g} = \frac{h}{(h_{i} / h_{g})}$

Again, values for h_i/h_g are taken from the data section below.

Clearance values to avoid rating reduction

The data for calculating reduction coefficients also gives the conditions that need to be achieved to avoid applying group derating.

IEC 60287-2-2 grouped cables in air reduction coefficient data table — Data for calculating reduction coefficients. Image reproduced from IEC 60287-2-2, Table 1.

Where cables are arranged in both horizontal and vertical planes, the sustained current capacity should be derived using values of h_i/h_g for the vertical plane.

Derating Factors for Cables Grouped in Air

Symbols

Reduction factors for cables grouped in air with existing ratings

Reduction factor for IEC 60287 calculated ratings

Clearance values to avoid rating reduction

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