Top Tips For Using Underground Ducting

If you are planning to run any type of service underground, it should be run inside a duct, both to protect it from damage and to alert others to its presence in the ground. By following these few helpful tips you can install your cable or pipe safely and also enable any future services to be added with a minimum of fuss.

Choosing the right material

Flexible ducting, which has a smooth inner wall and an external wall that is corrugated for extra strength, is usually used for longer runs. Single wall PVC rigid duct is generally used for shorter runs, and in the case of grey PVC, for telecoms services. Twinwall duct is used where additional strength is required, such as below paths and roads.

Choosing the right colour

The colour of the duct is crucial, as it alerts others as to what type of service is being run inside it. The most common colour classifications used by Local Authorities are as follows:

When installing high voltage electric cables we would advise you to contact your Local Authority for guidance, as specifications vary. For domestic electrics such as running power to a garage, shed or extension black is the colour to use.

Installing the Duct

It is good practice to use as few bends as possible and for your duct to follow an obvious route, such as alongside a patio or path. It is usual to dig the trench to a depth of 300mm to 450mm, although with water pipes this depth must be increased to between 750mm and 1350mm to avoid any potential frost damage. There should be no sharp stones on the bed of the trench – best practice is to lay the duct on a bed of 50mm of sharp sand. When backfilling the installation, use a little more sharp sand over the top of the duct to indicate its presence. Marker tape can also be used to indicate the presence of electric cables.

Drawing Pipes and Cables through The Duct

Black and blue Flexible duct coils are supplies with a draw cord already installed. The use of draw cords allows the cable or pipe to be pulled through the duct easily. We would always advise that you attach a second cord to the pipe or cable before you draw it through the duct and it leave it in place afterwards – this will allow you to introduce additional pipes or cables into the duct very easily if required at a later date.

Small rigid ducts such as BT duct do not come with a draw cord, although this can be supplied separately. To draw this through the duct, a good tip is to attach a small rag to the cord, just large enough to fill the duct but small enough to slide along it. At the far end, attach a vacuum cleaner hose and seal off any gaps between the duct and the nozzle with duct tape. Run the vacuum for a few minutes to dry the air in the duct, and then introduce the rag with the cord tied to it – you should see it disappear into the duct as the suction from the vacuum cleaner pulls it through.

Joining Lengths of Duct

Flexible duct comes complete with a joiner attached to one end and additional couplers can be purchased separately as required.

Rigid black PVC duct and grey BT duct comes with a blown end socket on one end – to join two lengths together simply place the plain end of one pipe into the socketed end of the next length, place a small piece of wood against the other end of the length and tap gently with a hammer or mallet.

Twinwall duct is plain ended and is joined using couplers that can be purchased separately as required.