the 'laws' of Ib <= In <= Iz and I2 <= 1.45 Iz re: direct buried cable

As an aside, this kind of thing is why Graham and I and a few others were discussing design qualifications a few weeks ago. The underlying problem with all this discussion is the underlying assumptions on which these "rules" are based. You have already listed some of them, ground temperature, fusing factor, whether 70C for Tmax is reasonable etc. Most of these numbers have a tolerance of perhaps 10%, yet they are assumed to be worst case, or best case to suit the situation. Actually none of these numbers will by themselves cause the design to fail, at least in an understandable timeframe. Design is much more complex than just taking these numbers from tables, and the job is done, and those of you who have used a design program will realise this. Ok, you decide the cable is rated at say 68A. but then you look at the fusing factor (1.45 say) and decide it is only suitable to use at 61A. Here's the bit you need to understand, you have looked at a continuous overload situation at 61A x 1.45, what is the actual temperature rise if you choose 63A instead? Also how fast is the circuit broken at this higher current, and what is the corresponding temperature rise at your assumed ground temperature? What if the ground is more likely to be at 10 degrees, because maximum current will only occur with a winter load?

Realistically now, is there any evidence that this design by BS7671 to the letter is in any way correct for any actual installation? Where are the failures? BTW if the DNO used these methods you are unlikely to have a supply at all at most UK properties. I am not saying it is wrong, but it is probably pessimistic for many cases, and good designers know this and make proper judgement of the design.

The answer to Habs second question is clearly that this would only cause a possible max conductor temperature of 72 degrees or so for a short period. This is not serious, and are you sure all the corrections are accurate and in the right direction? That is where the skill is necessary. However just like the OSG the BS7671 numbers are safe as far as possible without any necessary judgement.

As an aside, this kind of thing is why Graham and I and a few others were discussing design qualifications a few weeks ago. The underlying problem with all this discussion is the underlying assumptions on which these "rules" are based. You have already listed some of them, ground temperature, fusing factor, whether 70C for Tmax is reasonable etc. Most of these numbers have a tolerance of perhaps 10%, yet they are assumed to be worst case, or best case to suit the situation. Actually none of these numbers will by themselves cause the design to fail, at least in an understandable timeframe. Design is much more complex than just taking these numbers from tables, and the job is done, and those of you who have used a design program will realise this. Ok, you decide the cable is rated at say 68A. but then you look at the fusing factor (1.45 say) and decide it is only suitable to use at 61A. Here's the bit you need to understand, you have looked at a continuous overload situation at 61A x 1.45, what is the actual temperature rise if you choose 63A instead? Also how fast is the circuit broken at this higher current, and what is the corresponding temperature rise at your assumed ground temperature? What if the ground is more likely to be at 10 degrees, because maximum current will only occur with a winter load?

Realistically now, is there any evidence that this design by BS7671 to the letter is in any way correct for any actual installation? Where are the failures? BTW if the DNO used these methods you are unlikely to have a supply at all at most UK properties. I am not saying it is wrong, but it is probably pessimistic for many cases, and good designers know this and make proper judgement of the design.

The answer to Habs second question is clearly that this would only cause a possible max conductor temperature of 72 degrees or so for a short period. This is not serious, and are you sure all the corrections are accurate and in the right direction? That is where the skill is necessary. However just like the OSG the BS7671 numbers are safe as far as possible without any necessary judgement.

David, the conversation seemed to boil down onto whether the tabulated current for RefMethD included the 0.9 or it did not (have a read of Graham's contribution !) and whether it was required or not .. the maths was perhaps just a mechanism for exploring it (or perhaps not).

If I summarised your reply to my understanding of it, I would agree that experience and knowledge does indeed count for a lot. I am not sure anything specifically helpful is added by what you have said (which is unusual when I read most of your contributions).

I do realise after many years of 'listening' here there and everywhere, that designs and subsequent construction can be entirely suitable (and safe) despite not following Regs verbatim due to all sorts of affects.  Still, one might start from basics and go from there. One never knows what one might find.

Regards