Plastic ducting is the best method of protecting any cables you may wish to lay in a domestic situation. When laid properly, it will alert future excavations to the presence of a cable, and will allow replacement or additional cables to be installed quite easily.
For domestic applications, such as a power feed to a shed, garage, or garden feature, 40 or 50mm diameter black or grey plastic ducting is readily available from most builders’ merchants, and is relatively cheap, around 75p per linear metre.
There are larger diameter plastic and vitrified clay ducts available for public works and site installations. As in the photo opposite, the ducts can be placed as a road crossing, prior to the actual road construction on new sites, thereby eliminating the need for the utility companies to dig up the road at a later date.
Most of the utility companies supply ducting to their own specification free of charge to builders and developers, or utilise an ex-stock product, such as the vitrified clay pipes and reimburse the contractor.
With the exception of water pipes, there is no standard depth for a duct, regardless of what service it may be carrying. They can be, literally, just below the surface, but are more usually 300-450mm deep or more.