A common question is how to size an armoured single-core cable buried in the ground to BS7671. This is not possible, but there are alternatives.  This post will examine one alternative, namely ERA 69-30.

 You cannot size single-core unarmoured buried cables to BS7671 (although the standard allowed these cables to be used) because Appendix four does not contain the necessary data. The appendix contains data for multicore buried cables but not single-core. Alternatively, you can use ERA 69-30, which has installation methods and sizing data for buried single-core cables.  

The following video shows how this works: