To my understanding, if the supply circuit fell within the scope of BS 7671:2018, as per note 5 of Table 4C1 in Appendix 4, I’d assume that the applicable grouping factor for a circuit consisting of 3 single core cables per phase, installed direct in ground (hypothetical example), would be 0.70.

Also depends on the layout - e.g. the spacing between individual cables in the ground - as that, as well as the differing thermal behaviour of soil compare with air can make quite a difference. BS 7671 seems to side-step the issue, there being no method number for single core cables buried direct in the ground and no method D listed in tables 4D3A, 4E3A, 4H3A, 4J3A etc. So while 4C1 might be thought to apply, it's difficult to see in BS 7671 terms what it would apply to. So I suspect buried single core cables is a method that BS 7671 simply doesn't cover and, as others have already mentioned, you'll have to fall back onto other approaches.

   - Andy.

To my understanding, if the supply circuit fell within the scope of BS 7671:2018, as per note 5 of Table 4C1 in Appendix 4, I’d assume that the applicable grouping factor for a circuit consisting of 3 single core cables per phase, installed direct in ground (hypothetical example), would be 0.70.

Also depends on the layout - e.g. the spacing between individual cables in the ground - as that, as well as the differing thermal behaviour of soil compare with air can make quite a difference. BS 7671 seems to side-step the issue, there being no method number for single core cables buried direct in the ground and no method D listed in tables 4D3A, 4E3A, 4H3A, 4J3A etc. So while 4C1 might be thought to apply, it's difficult to see in BS 7671 terms what it would apply to. So I suspect buried single core cables is a method that BS 7671 simply doesn't cover and, as others have already mentioned, you'll have to fall back onto other approaches.

   - Andy.