Probably need a few more details .. like does the DNO provide a PE terminal - that'll dictate whether the 250m cable is TT or can be TN, and whether the 100A switchfuse will do for ADS (but double check Zs as the armour will be "small" compared with doubled cores, especially for that length) or whether you'll need an RCD at the supply end as well.

Unless the DNO are willing to guarantee a non-PME supply or there's a buried grid you can bond to, the stable end is likely to to have to be TT - but whether the 250m cable is part of the same earthing system will be a design choice. The TT system the TN system (if any) will have to be kept strictly and reliably separated - and nothing exposed and connected to differing earthing systems within reach of each other.

Horses mean section 705 of BS 7671 will apply (Agricultural and horticultural premises) - there's more in there than I can reasonably type here, but there are lots of additional requirements to deal with everything from the brute force and the "conductive liquids" that horses can bring, to the fire hazard from the straw. "Long wheel base" 4-legged animals are also particularly prone to differences in potential across the ground surface - so take care with the positioning/depth of electrodes.

   - Andy.

“70mm 4 core swa running underground 250 meters” have you got a quote for the price of the cable?

Why would stables need a 100 amp supply?

I assume its about £35 to 40 per metre, over that distance aluminium may be worth considering, it will be 1/3 the price for the same cross-sectional area.

There is a good reason for the rule of thumb that much more than 1 volt per metre is not an economic cable run - so 11kv lines don't run much more than 11km, long vehicles use 24V batteries and not 12V etc. Oh, and like this, by the time mains has to go more than a couple of hundred metres, for all but very small loads it gets very expensive very quickly. This is also why pop festivals and fairgrounds have so many gensets dotted about, rather than one big one. (or temporary transformers and HV distro, but in the UK that is tricky.)

Mike.

mapj1 said:

There is a good reason for the rule of thumb that much more than 1 volt per metre is not an economic cable run

So would it be cheaper to get a new transformer installed?

Sparkingchip said:

Why would stables need a 100 amp supply?

Is this a barn or stables?

would it be cheaper to get a new transformer installed?
Probably not in the UK, because of the controls over private site HV work and ongoing cost of ownership, but if the only cost was buying and installing a transformer and a thinner cable, probably. It's only an approximate  rule of thumb !! This example is borderline, the economics at twice the distance would become far more clear cut.

Note that 3 phase (or split phase) goes further, as you have drop in the phase wires, but a large chunk of the voltage drop in the neutral is removed from the  equation. Its also why lights on a larger sports stadia are often 400V units wired between 2 phases, rather than 230V phase to neutral.

Mike

35 mm conductors will fit into the cage clamp terminals of the 100-amp switches and RCDs, a 32-amp supply should be sufficient for stables, the cable is still going to be expensive, but not as expensive as four core 70 mm.

So 35 mm seems a good starting point for designing the installation, limiting the maximum demand to 32-amps.

It would not surprise me if 240 metres of 70 mm four core SWA costs over ten thousand pounds, 35 mm two core is still going to cost several thousand pounds.

If you email the cable manufacturers they will almost certainly calculate the size of the cable for you and recommend the size to use.

It is up to you to design the circuit and installation, I cannot do it for you as I do not know what electrical equipment and loads are to be supplied, so do not just use this information without verifying it for yourself and taking guidance from the cable manufacturer.

https://www.elandcables.com/cable-calculator

If the barn has a steel frame then this may provide a low resistance to earth.

If at all possible I would go for either a three phase supply, or split phase, as has already been said that will reduce the phase current, and also reduce voltage drop in the neutral.

If the above can not be achieved, then you can probably apply engineering judgement and allow a lot more voltage drop than the norm.

If the long cable from the transformer to the load belonged to the DNO, they might allow as much as 10% voltage drop in the service cable, and no one worries about that !

250 volts at the transformer, and 226 at the load is common on rural supplies. If going down this route, consider the load profile, A long hour 100 amp load, with 10% losses in the cable is approaching £1 an hour, or perhaps £6,000 a year. Ten years losses would pay for the larger cable. If on the other hand the load is usually only 10 amps or so, with rare peaks approaching 100 amps, then the losses are relatively unimportant.

Thanks for the response AJ it is greatly appreciated, the DNO has supplied a TT system and there is no PE terminal, my thoughts were to earth stake at the kiosk to have this protecting the kiosk and SWA cable. I was thinking of installing a 100mA RCD at the switch fuse to comply with ADS as the fuse will have overload protection and the RCD will have earth fault protection on the SWA cable. At the barn/stable  the SWA gland will be made off to an insulation sheet  and then a separate local earth stake will protect the distribution from this point with all 30mA RCD protected circuits. The Barn/Stable is metal framed and this will be supplementary bonded.