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Laying Cables Under Patios or Garden Paths

We were asked a really good question the other day from a member of the public.

"Can I lay 12v garden lighting cable onto a hardcore base before my patio is laid?"

We felt the answer could be useful to lots of here it is!

Most patios, drives and pathways are laid on top of a hardcore base that is typically 100mm/ 4" to 150mm/6" in depth.  The weight of vehicles and general footfall typically dictates the depth.  Slabs, blocks, tarmac etc are then laid on top of that hard base structure.

Some hardcore bases are topped of with a sand layer for laying blocks whilst other hardcore bases are left 'bare' as slabs with a mortar-base can be laid direct.  A tarmac 'base-course' also sits straight onto a compressed hardcore base.

The hardcore material is made up of stone aggregates of different sizes, angles and textures that when compressed ('wacked down') 'lock' together to form a very hard base.  There are various hardcore types but the most often referred to is 'MOT Type 1'.  MOT is an old reference to when roads were specified by the UK's  'Ministry of Transport'. 

We would NOT recommend...

Laying 12v garden lighting cable straight onto a hardcore base (between the top of the compressed hardcore and the bottom of the slab, tarmac or blockwork) or in a trench run through a hardcore base for the following main reasons:

  • The hardcore aggregate has angular and sharp pieces of various sizes that help it to lock together when compressed.  Whilst it may have been 'wacked' down using machinery before cable laying, there is always the possibility that part of the 12v cable (PVC sheath insulator) gets pierced by any sharp point of aggregate when the finish layer is laid (slabs, tarmac etc).  This can 
      • create an immediate non-working electrical circuit if the piercing is aggressive or
      • reduce the life of the inner core cable with water ingress and result in failure at an unknown future date
    • Slabs are typically laid on either a full bed or mortar (our Landscaper's preferred method) or on 'dots and dabs' of mortar.  Either way, the cable will be subject to a mortar coating.  Mortar contains cement which is naturally corrosive and whilst we've never tested the long term effect of mortar (made with cement) on the outer sheathing of the cables we feel it's sensible to avoid the problem.  It'll probably last for years but we don't know!
    • Asphalt ('Tarmac') 'base-courses' are also made up of pretty large sized aggregates with sharp edges to help them 'lock together' and are covered with asphalt or bitumen.  Worse, they are typically compressed using a heavy roller after laying onto the hardcore base.
    In all of the above the cable(s) is also permanently fixed and if a failure should occur it will entail a lot of work to rectify.

    We would recommend...

    Laying the 12v cable in a run of plastic or other form/construction of conduit or ducting.  Almost anything can be used as the conduit will protect the cable from being pierced or being corroded. 

    This conduit or ducting can easily be laid onto/into compressed hardcore by simply easing a small channel along the ducting-run, so that it lays roughly flush to the top of the hardcore.  It doesn't need to be perfect, too neat or pretty.  It's going to be buried after all!

    Cheap plastic piping or small diameter drain pipes from the DIY stores, Screwfix, Toolstation, Plumbers merchants etc will be sufficient.  It also allows for the cable to be withdrawn and new cable inserted at a later date if needed. 

    Try and get a pipe that will allow the 12v Plug & Play end-connectors (or other connectors if interim exit points are needed) to clear the inner bore of the conduit/ducting.  

    TIP: An alternative is to use an old piece of 'hosepipe'.  In this case the hosepipe can be split along its length with a Stanley knife and the cable can be progressively eased into it.  The 12v Plug & Play connectors won't pass through as the typical inner bore of the hosepipe in the UK is about 13mm and a connector is about 20mm diameter.  Hence the need to split it and ease the cable along the length of the hosepipe.   

    This provides a solution that will protect adequately but is not the best solution for cables needing to be withdrawn/re-inserted. 

    However, it's a good protector for any 'sharps' from hardcore, tarmac or the corrosive properties of mortar.

    Finally, the 12v cabling when powered up is totally safe, so if through any reason the cable does get pierced, even from inside a conduit/duct then no harm to any individual will occur!

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