Cables exposed to direct solar radiation are subject to additional thermal loading. This extra heating needs to be considered when calculating sustained current capacity and cable resistance.
Solar radiation is most important for cables installed outdoors on tray, ladder, supports or other exposed routes where there is little or no shading.
IEC 60287 method
Where cables are subject to solar radiation, or other infrared radiation, current capacity can be derived using the methods in IEC 60287. The method reduces the allowable temperature rise, Δθ, used in the rating equations.
The reduction in Δθ is given by:
| σ | Absorption coefficient of solar radiation for the cable surface, from IEC 60287 |
| H | Intensity of solar radiation, W/m2 |
| T4* | External thermal resistance adjusted for solar radiation, K·m/W |
In current-rating calculations, T4 is the thermal resistance of the surrounding medium. IEC 60287 Part 2 gives guidance on calculating T4 both with and without solar radiation.
Using derating factors
Standards based on derating factors, such as BS 7671, ERA 69-30 and IEC 60502, do not normally include the direct effect of solar radiation. In these cases, they typically refer the user to IEC 60287.
Although IEC 60287 is the preferred method for determining sustained current rating, it is common in practice to apply an additional solar-radiation derating allowance. For example, NF C 13-200 suggests a derating factor of 0.85 for cables exposed to solar radiation, while manufacturers such as Nexans recommend a factor of 0.8 for cables with a black sheath. A factor of 0.8 is therefore a conservative value for cables exposed to direct solar radiation.
Ambient temperature adjustment
A practical approach is to increase the ambient temperature used in the calculation so that the derating factor includes an additional 0.8 allowance for solar radiation.
For example, for 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 approximately the factor for 50 °C ambient. Adding 25 °C to the expected ambient temperature therefore allows for direct solar heating in a conservative way.
Using an increased ambient temperature better reflects the actual condition and affects both resistance calculations and sustained current capacity. Reviewing derating factors from common standards suggests that an additional 25 °C is generally conservative, although for some cables or naturally high ambient-temperature environments, the engineer may choose a smaller adjustment to avoid excessive oversizing.
Avoiding solar radiation effects
In addition to allowing for solar radiation in the calculation, physical measures can reduce the effect. These may include relocating cables to shaded areas, covering ladder or tray with ventilated cable covers, or installing another form of shading system.
For thermal radiation between cable, busbar and enclosure surfaces, see Radiation in Cable Thermal Analysis.