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If anyone could give me a hand with the following...
When sizing cables with a direct burial installation method, how far away from each other do cables need to be so that no grouping factor applies? Table 4C2 in BS7671 gives a derating factor of 0.8 for up to 0.5m away when there are 4 or more circuits but doesn’t state any distances exceeding 0.5m.
Should i consider that after 0.5m there is no grouping factor for example???
The model of use a constant grouping factor and up to some distance and then switching it off suddenly is a bit of a simplification. It also does not consider the long term drying out of the ground around the cables altering the behaviour.
For BS 7671 compliance, you can assume no grouping factors beyond 0.5m however
That said, unless you want to pay a fortune for trenching costs (in your example, you already have a trench approx. 1.8m wide) then it's often far more cost effective to dig a narrower trench, accept some grouping factors at say 0.25m spacing and use selected backfill to get the ground thermal resistance down to get you back to the cable size you intended to use.
Narrower trench, less excavation disposal and a smaller amount of imported sand or stonedust bedding is cheaper from a civil point of view, and still shouldn't result in larger cables due to narrower spacing as you've compensated by reducing the soil thermal resistivity