Discuss What type and size of cable for a long underground run in the UK Electrical Forum area at ElectriciansForums.net
If the cable or duct is buried deep enough (say 0.4-0.5m below surface) then it is not important. However, if the field is ever used for agriculture then you really need around 1-1.2m depth.Also, is it bad to go direct from point A to point B if it involves crossing a field which may have LGVs crossing for event setup and take-down, should the trench go around the perimeter?
I think that may cost even more given the terrain, distance from the road and the 4 areas being spread over a wide range. Something to add to the thinking though, thanks!Have you considered a new metered supply closer to the site, may be worth getting in touch with the DNO for a quote.
Thanks for the detailed analysis. much appreciated.Realistically if you are looking at just a switch and maybe a small defrost heater then 1A / 230V is enough and you could use 2.5mm 2-core SWA with an RCD for supply disconnection (as it would fail on the typical min 6A OCPD for domestic style boards). If the house is PME supply and you are looking at outdoor stuff you might even want to make this a TT arrangement.
Using transformers for lower drop as @James suggests is possible, but I would suggest unnecessary complexity if you don't really need it.
Cable works out at about £1/m plus VAT.:
Superlec 6942X 2CORE 2.5MM SWA BS5467 STEEL WIRE ARMOURED CABLE - HARMONISED CORES
Please note: If you require advice on which cable to use, how and where to install it, it is recommended that you will need to contact the relevant qualified tradesperson for the job as the Superlec Direct Team are not qualified electricians.www.superlecdirect.com
You can either use SWA network cable and both can be buried directly in the ground but I would strongly advice using some twinwall duct for both power and fibre and you can use any outdoor style fibre that is strong enough to be pulled through. Mains SWA is safe enough beside even normal copper network cable as it has an earthed screen in the form of the armour. This is the sort of thing (but shop around, this was just my first result that was what I was trying to illustrate) so around £1.60 per meter plus VAT:
Flexi Duct CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coil
Flexi Duct CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coilwww.drainagepipe.co.uk
Typically you would want an access chamber around every 200m to allow cable pulling and potentially to allow a section to be repaired. It also allows you to have one run out and then split at the box to serve other end points.
BT/Openreach typically use this system for pavements or footpaths (not on a road where heavy concrete or cast iron cover and a concrete or brick box is then needed for the loading):
BT Quadbox Chambers
We stock the BT Quadbox Chamber Access System approved by British Telecom. Order online today, free delivery.www.drainagepipe.co.uk
However, you can see that a complete box (say 4 * stacked sections to get 60cm depth, and lid) you are looking at around £600 per box.
So for 600m run you are looking at around 4 grand for duct, 3 boxs, and SWA cable delivering 1A power (before you add fibre and network switch). However, if fibre is needed out there then most of that cost is going to apply anyway.
It's parkland so no agri involved, but always best to plan for the unexpected!If the cable or duct is buried deep enough (say 0.4-0.5m below surface) then it is not important. However, if the field is ever used for agriculture then you really need around 1-1.2m depth.
If this was a permanent system then you might want it routed around the edge of fields to allow for future changes in land use (e.g. adding building) without having to change this, however, that might not be an issue for the owner here.
Looking at the 2-core SWA cable, for 1.5mm the armour is lower R than the conductor!Realistically if you are looking at just a switch and maybe a small defrost heater then 1A / 230V is enough and you could use 2.5mm 2-core SWA with an RCD for supply disconnection (as it would fail on the typical min 6A OCPD for domestic style boards).
That was 5s as a sub-main. Probably it would be considered a final circuit where it is Table 41.2 for 0.4s disconnection and then a max Zs of 15.6 ohms for 3A fuse, so still OK.That would meet disconnection on a 3A FCU as Table 41.4 has Zs for a 3A BS 1362 fuse as 22 ohms. It still needs RCD protection, but it is always nicer to know you can safely disconnect on the OCPD!
@pc1966 Are you purposefully not taking volt drop into account? If so, may I ask please what your reasoning is behind this?That was 5s as a sub-main. Probably it would be considered a final circuit where it is Table 41.2 for 0.4s disconnection and then a max Zs of 15.6 ohms for 3A fuse, so still OK.
While they oly give 3A and 13A in that table, you can also get smaller fuses (1A & 2A) but for typical network switch with switch-mode PSU you run the risk of switch-on surge weakening or taking out too small a fuse.
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