Circuitry and Related Parts of BS 7671: 2008
BS 7671: 2008 Appendix 4 includes revised information for grouping factors calculations. A new table of grouping factors for buried cables has been added (Table 4C2 reproduced below). Also new is a method for calculating the grouping factor for circuits where cables are of different sizes, as follows: F=
1 √n
where F is the group rating factor and n is the number of circuits. This formula will give a lower group factor than BS 7671 Tables 4C1 to 4C5 as these tables assume that cables in the group are of the same size. An overriding point to note before the grouping factor tables are given is the note 523.5 in BS 7671, which states that where a cable is known to carry 30% or less of its grouped rating, it can be ignored for the purposes of grouping. This is invaluable and should be utilized for BS 7671: 2008 calculations. The procedure for applying grouping factors is either to use the factor from BS 7671 Tables 4C1 to 4C2 or to use the following method: Compare the formula:
√
It = In2 + 0.48Ib2
( 1 C– C ) 2
2
g
g
(Equation 3)
with It =
Ib Cg
(Equation 4)
using the larger value. This method can only be used where the circuits within a group are not expected to be simultaneously overloaded. For circuits that are not fully loaded, the use of this (Equation 3) method produces a lower It than the grouping factor method using tables 4C1 to 4C5 from BS 7671. The procedures are summarized as:
37
C