I'm planning to build a timber garden room for use as an amateur radio shack. It will have a TT mains supply from the house consumer unit. I can't attach the SWA to a fence as the boundary belongs to the neighbour so it will have to go underground.
The trench will be about 5 metres long under a gravel walkway and will cross a sewer pipe that is 600mm below the surface with an inspection cover nearby. How deep does the trench need to be?
Mike
Guidance Note 1 Section 5.7 [7th Ed (17th+A3, 2015, pages 83-85) or 8th Ed (18th, 2018, page 85-86)] recommends generally absolute minimum of 0.5 m to meet the “sufficient depth" requirement of Reg 522.8.10.
The reasoning here, is that at a depth of greater than 450 mm, the effects of ground freezing in the UK are less, putting less mechanical strain on the cable over time, plus as others have said, it's below spade, fork or “first scoop” depth.
There are, just to note, some specific Part 7 requirements or recommendations for depth of cables:
For footways and carriageways of highways (including bridleways, but perhaps I'd also be inclined to use for other public-access areas such as public footpaths) NJUG publication Guidelines on the Positioning and Colour Coding of Underground Utilities’ Apparatus Vol 1 for covers not only depth, but also the colour of the sheath/duct of cables. It may also be sensible to use these also for driveways, access routes, etc. LV is a minimum of 450 mm beneath a footway or verge, and 600 mm beneath a carriageway, and telecoms 250-350 beneath a footway/verge, and 450-600 mm beneath a carriageway – but minimum depths for street lighting and street furniture may differ.
Also worth noting that most armoured cables are not suitable for long-term or permanent submersion. So, a submersible cable would be required if you are laying below the water table. The soil type you lay armoured cable in should consider drainage etc. For example, if laying in soil with high clay content, a suitable method is to lay the cable on 100 mm sand over over a 150 mm to 300 mm mix of sand and aggregate (such as quarter or half down), and cover with sand to 100 mm above cable before back-filling - give or take use of tiles and/or, as has been mentioned, cable marker tape at a suitable depth so the tape is exposed before the cable is struck.
Guidance Note 1 Section 5.7 [7th Ed (17th+A3, 2015, pages 83-85) or 8th Ed (18th, 2018, page 85-86)] recommends generally absolute minimum of 0.5 m to meet the “sufficient depth" requirement of Reg 522.8.10.
The reasoning here, is that at a depth of greater than 450 mm, the effects of ground freezing in the UK are less, putting less mechanical strain on the cable over time, plus as others have said, it's below spade, fork or “first scoop” depth.
There are, just to note, some specific Part 7 requirements or recommendations for depth of cables:
For footways and carriageways of highways (including bridleways, but perhaps I'd also be inclined to use for other public-access areas such as public footpaths) NJUG publication Guidelines on the Positioning and Colour Coding of Underground Utilities’ Apparatus Vol 1 for covers not only depth, but also the colour of the sheath/duct of cables. It may also be sensible to use these also for driveways, access routes, etc. LV is a minimum of 450 mm beneath a footway or verge, and 600 mm beneath a carriageway, and telecoms 250-350 beneath a footway/verge, and 450-600 mm beneath a carriageway – but minimum depths for street lighting and street furniture may differ.
Also worth noting that most armoured cables are not suitable for long-term or permanent submersion. So, a submersible cable would be required if you are laying below the water table. The soil type you lay armoured cable in should consider drainage etc. For example, if laying in soil with high clay content, a suitable method is to lay the cable on 100 mm sand over over a 150 mm to 300 mm mix of sand and aggregate (such as quarter or half down), and cover with sand to 100 mm above cable before back-filling - give or take use of tiles and/or, as has been mentioned, cable marker tape at a suitable depth so the tape is exposed before the cable is struck.
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