BS 7769-2.2:1997

Electric cables. Calculation of the current rating. Thermal resistance. A method for calculating reduction factors for groups of cables in free air, protected from solar radiation | Status: Confirmed, Current

1 Scope The method described in this International Standard is applicable to any type of cable and group running horizontally, provided that the cables are of equal diameter and emit equal losses. Information is provided on the reduction in permissible current when cables are mounted adjacent to each other. It is limited to the following cases:

1. a) a maximum of nine cables in a square formation, see Figure 1, and
2. b) a maximum of six circuits each comprised of three cables mounted in trefoil, with up to three circuits placed side by side or two circuits placed one above the other, see Figure 2.

Caution is advised where air flow around the cables may be restricted by proximity to neighbouring objects.

NOTE Further work is to be done to extend and refine the data and to include the effect of dielectric loss.

Information is provided for the following situations:

• — Where a rating for one cable or circuit assumed to be isolated exists, group reduction factors can be derived for the same type of cable, see 4.1.
• — Where previously calculated ratings are not available, the data provided can be used to calculate permissible currents for groups of cables, using the formulae in parts 1 and 2, see 4.2.
• — Where adequate clearances can be provided between cables to avoid a reduction in permissible current, see clause 5.