Cable exposed to direct solar radiation will be subject to greater thermal loads and additional heating. This extra heating needs to be considered in the calculation of sustained current capacities and a potential increase in the resistance of the cable.

IEC 60287

Where cables are subject to solar radiation (or other infra-red radiation), the current capacity can be derived using methods given in IEC 60287.  This is accomplished by reducing the allowable temperature rise, Δθ of the rating equations (1.4.4.1, 4.4.4.1).  The amount of reduction in Δθ is given by:

$\sigma D_e^{*}H{T}_4^{*}$

σ         - absorption coefficient of solar radiation for the cable surface (table 4 of the standard)
H        - intensity of solar radiation W.m2
$T_4^{*}$      - external thermal resistance (adjusted for solar radiation) K.m.W-1

In the calculation of current rating, T4 is the thermal resistance of the surrounding medium.  Part 2 of the IEC 60287 standard gives guidance on how to calculated T4, both considering and not-considering solar radiation. 

Derating Factors

Standards based on derating factors (BS 7671, ERA 69-30 and IEC 60502 for example), do not consider the effect of direct solar radiation.  Typically the standards will refer the user to IEC 60287 in these instances.  While IEC 60287, is the preferred method to determine the sustained current rating, the calculations are complex and just using an additional derating factor for solar radiation is commonly employed. 

The French standard NF C 13-200 suggests a derating factor of 0.85 for cables exposed to solar radiation. Cable manufacturers such as Nexans recommend a factor of 0.8 (black sheath).  As a conservative factor, 0.8 would seem a suitable value to apply for cables exposed to direct solar radiation.

Our recommendation is to increase the ambient temperature for the calculation, such that the derating factor used includes an additional 0.8 for solar radiation.  For example, BS 7671 90 °C thermosetting cables, the derating factor for 25 °C ambient is 1.02. Multiplying by 0.8 gives 0.82 which is the factor for 50 °C ambient (adding 25 °C to the expected temperature, will allow for direct solar heating).

Using an increased temperature more accurately reflects the actual condition and will be reflected in resistance calculations as well as sustained current capacity.  From a review of the derating factors given in the standards, allowing an additional 25 °C is generally conservative but will result in safely designed cables.  For some cables or naturally high ambient temperature environments, the engineer may wish to consider to smaller temperature adjustment, so as not to oversize cables.

Avoidance

In addition to (or as an alternative) to allowing for direct solar radiation effects, consideration can be given to physical measures.  These could include relocating the cables to shaded areas, covering ladder or tray with ventilated cable covers or installing some other shading system.