Originally Posted by: achrn 
Originally Posted by: paul.skyrme The following are just a few things for you to consider
But with teh exception of solar gain and wind loading, none of those things become an issue that needs considering for an above-ground installation and not for below-ground, and a buried cable route brings its own problems that an above-ground route doesn't have (like, knowing exactly where it is).
I think a response that says basically just trust your electrician is far from helpful. It may come as news to you, but not every tradesman everywhere is a beacon of helpfulness who always has the best interests of his client at heart. Some are lazy, some are incompetent, some just want to do it the way they always do it.
A question 'my elecrtrician says I have to bury it, is there actually any legislation / regulation / rules that say that' seems a perfectly reasonable question, and does not (in my opinion) deserve a lecture about how idiots should leave electrics to the professionals (which is most definitely the tone here).
Actually, almost exactly the opposite.
However, I do see two issues. Dubious “electrician”, and a client that wants to dictate design considerations on the electrician, but seems unwilling to take on the liability for their design decisions under EAWR, by providing a design for the “electrician” to quote against.
There are many so called “electricians” these days that I would not trust to change a fuse in a 13A plug, let alone design a 3ph sub main to metallic outbuildings.
Unfortunately a course on BS7671 does not make a person an electrical designer, nor an electrician.
The reason for the list was to try and enlighten the OP as to things that they could ensure that the "electrician" is considering.
Poor design and installation work is not confined to small "one person" electrical companies it spreads right through to large national organisations also, I have even seen downright dangerous designs come from international design consultancies. Especially when it comes to earthing.
The electrical installation landscape is changing, and due to the due process that has to be undertaken when updating, BS7671, it is always behind the curve.
Unfortunately there is a large percentage of "electricians" who do not value CPD and consider it an unnecessary expense.
In the domestic market you have many operators who do work cheap and without the correct considerations of their work on the existing installation, correct design, correct verification of safety & provision of correct documentation, but the householders are happy because their hot tub works, they don't know. or care if it's potentially lethal, because the job was cheap, less than half the price of an "electrician" who wanted to do it correctly, and no VAT because they paid cash...
In a business undertaking, hopefully the person ordering the work will be pushing for certification, but as far as the rest goes, assessment of the effect on the existing installation, correct consideration of the hazards posed by the incoming supply, how the modification will affect things, connected loads and how these have changed, earthing and the influence of external effects on that, etc. etc., the client in business will likely be unaware of. Plus they have budget constraints which mean the client wants it cheap, and will try to pass liability to the “electrician”. Unfortunately, there are many “electricians” who are happy to take on this liability cheap without realising it exists. Then there are those who are just downright dangerous, like the two who are currently on trial following the death of a child in a public house.
The issue with large contractors is ensuring that the actual work done is compliant with the design etc. because the persons doing the work do not have to be qualified, even if the company holds registration with the biggest electrical scheme provider.
Unfortunately, I am going to say that reading BS 7671 is not adequate to make a person competent to undertake the design, not question in detail the proposal.
For example, whilst a 25mm sq. 4c SWA may carry the current, based on the tables in BS7671, what are the installation conditions along the length, thus giving the installed current carrying capacity. If the OP has selected this size, have they selected the correct BS for the cable to refer to the correct table, there are 2 with different operating temperatures. The higher rating is unlikely to be compatible with smaller switchgear (small in this case may be up to several hundred amps).
The thing that really rung alarm bells for me was the OP stating “to power some storage containers and a large storage shed, I want to run 90m of 25mm 4 core SWA as a long term measure”.
If the OP is confident in their selection of cable, and feels that they could defend their design against EAWR, then I would suggest they complete it and hand it over for quote, and take responsibility, however I don’t think that without additional competence, a read of BS7671 would be an adequate defence under Reg 29 if it ent wrong.
The electrical contractor may not be creative enough to think outside the box with regard to installation methods also. So I suspect that they may not be the correct person to do the design either.
My final point here is that unfortunately, products and the electrical landscape are changing so quickly bare compliance with BS7671 alone is unfortunately often inadequate due to external influences on the installation that are either not advised to the electrical installation designer by the client, or, the client is unaware of and the “electrician” is not competent to identify.