Cables installed in troughs can be difficult to rate because there are relatively few defined sizing methods for this installation arrangement. One common approach is the empirical method in IEC 60287-2-1, Calculation of the current rating: Calculation of thermal resistance.
The method estimates the temperature rise of the air in the trough above ambient temperature. The cable current rating is then calculated as though the cable were installed in free air, but using the increased air temperature.
Trough temperature rise
The required temperature adjustment, Δθtr, is given by:
| Δθtr | Temperature rise above ambient, °C |
| WTOT | Total power dissipated in the trough per metre length, W/m |
| p | Part of the trough perimeter effective for heat dissipation, m |
Effective heat dissipation perimeter
When calculating p, include only the parts of the trough perimeter that are effective for heat dissipation. Any side exposed to sunlight should not be included. In practice, this is typically the length of the base and the two sides of the trough.
Because the calculated trough temperature rise affects the ambient temperature used for rating, it can have a direct impact on the permissible current-carrying capacity of the installed cables.