Soil thermal resistivity and ambient temperature

ianlloyd said:

(How many installations just dig a trench, drop the cable in then just backfill with the soil that's been dug out with no bedding/cover?)

I would have thought quite a lot - especially at the domestic end of the market. As long as the soil doesn't contain sharp stones and isn't going to be compressed significantly, it usually works out fine (obviously quite different in flinty soil under a roadway).

Where does sand/pea gravel bedding/cover come into this

There does seem to be a tendency to ignore such things - e.g. indoors adding capping or putting a cable into a conduit seems not to require a change in reference method - and until not so long ago, an underground cable could be buried direct, or in ducting, for the same reference method. So my guess is that the standard figures do make an adequate allowance for usual sorts of air gaps immediately surrounding the cable.

   - Andy.

Yeah, I suppose I'm thinking more of the industrial end whereby I'd expect bedding/cover etc.

For all the installation methods, dont forget that the figures in the big book are informative annexes, not normative rules you must follow, so if you have more accurate data for the specific cable or installation conditions or loading profile, you are always welcome to use that information. This may allow one  to deduce a less wasteful cable size, and on big projects where it is worthwhile it is done.  But equally, for short runs and JFDI jobs where something just has to be done right now and work first time, and the cable price is not dominant, the effort to perform and document the accurate calcs, and to be confident to do so, is not really worth it, and the one size fits all approach is more sense.