Underground Electrical Cables. Is burying cable safe?

1) General Requirements and Considerations

All over the world, the wiring regulations will be different regarding what is allowed when it comes to burying electrical cables. I know the UK wiring regulations in depth regarding this matter and even then, there is some grey area. If you are considering burying electrical wiring underground then there are various issues that you will need to think about.

• Mechanical Protection
• Ground Disturbances
• Warnings
• Suitability of Cable
• Installation Method

2) Mechanical Protection

Underground cables will need some form of protection against mechanical damage. Mechanical damage could be anything such as a spade to a digger bucket or stones to rodents! We cannot bury a cable without considering what forms of damage could occur to the cable and take steps to prevent such damage from occurring so far as is reasonable. 
Such measure of mechanical protection could include Steel Wire Armored cabling, a type of wiring that is constructed such that mechanical damage is quite difficult to achieve with a spade for example. SWA cable (armored cable) is a direct burial cable and often does not require further protection. The wiring could be buried in suitably thick galvanised steel conduit or other form of shield from damage.

3) Ground Disturbances

If the cable is to be buried in the ground, then it must be done so at a depth that is not likely to see ground disturbances. Running a cable through a flower bed or garden border is fraught with danger! Garden flower beds are regularly turned over by spades and forks and as such, if the cable is not buried sufficiently deep then there is a fair chance of damage at some point.

4) Warnings

Where cables are buried, there should be some form of warning that electrical wiring is present before the cable depth is reached. It is recommended to provide warning tape or tiles at least 6 inches or 150mm above the depth of the buried cable to warn anyone disturbing the ground of the cables presence.

5) Suitability of Cable

Burying cables that are not suitably designed for wet and dirty conditions could lead to problems with the electrical installation further down the line. Not only must all the other factors be considered but the ability of the cable to survive its environment must be thought through as well.

6) Installation Method

If more than one cable is to be buried then we must consider the cables ability to carry electrical current safely and sufficiently. Groups of cables together cannot dissipate heat as effectively as single cables buried alone. There are temperature factors to consider as well as, ambient ground temperatures can increase or decrease the ability of cabling to carry electrical current.

7) What is the required burial depth of cables?

In the UK there is no set burial depth for underground wiring. This may vary across the world and you should check the local regulations in your area to be sure that electrical codes are followed. There is a common recommended depth of 600mm for underground electrical cable BUT, this does not always mean that 600mm is sufficient depth.

Each electrical project will have its own risk assessments, installation considerations and set of codes that need to be followed. 600mm may not be sufficient for certain areas where regular ground disturbances occur. 300mm may be more than sufficient in some instances if, for example, the cable is to be installed then a concrete surface placed over the top such as a driveway and a suitable risk assessment has been carried out.

Different depths are likely to apply to different projects. It is unlikely that a concrete driveway will ever be disturbed. In any event, your local building control department and electrical wiring codes should be consulted to ensure that your project is carried out safely.

8) Concrete Slab Over Wires

​Whilst a concrete slab installed over the top of ground wiring may offer some form of protection from mechanical damage to underground wire installations, we have to consider the physical damage that can occur to cables from stones, ground heave, temperature shifts and possible rodent damage. Underground installation of wiring should not only include protection from spades, shovels, digger buckets and attacks from the top surface but also the weight of the earth and coverings above. For these reasons it is always advisable to use additional protection for cables installed in this manner.

Armored cables like SWA are suitable as a direct-burial cable and are considered generally safe for underground purposes but care should be taken when looking at another type of outdoor wiring like rubber cabling for example. Whilst this cable is suited to most weather conditions and is very resistant to harsh locations, it would be easily damaged from sharp objects and stones in the ground in spite of any concrete slab covering above protecting from mechanical impacts. Underground conduit can offer some protection from these foreign objects but you should always ensure that your local and national electrical codes are followed whenever considering buried conduit, underground power lines or any type of outdoor electrical wire installation is to be carried out.

9) Use a Professional Electrician

A professional electrician should ensure that a safe installation is carried out whenever any cable is subject to direct burial use. They will likely use a suitable type of direct burial wire and use cable that is likely to withstand the harsh conditions such as wet location problems, freezing and hot conditions, penetration from buried ground objects and ensure that the cable has a sufficient armor layer.

10) ​Frequently Asked Questions

10.1) ​Can I bury a cable in a hose pipe?

​I have no idea where the idea originated that burying cable in hose pipe was acceptable. The hose pipe offers no mechanical protection to any normal type of wire and is likely to add confusion to anyone uncovering the hose pipe as to what it actually is. You would not normally assume that a hose pipe contains an electrical cable and as such, this could introduce additional danger to anyone uncovering the electrical wires. 

10.2) Is PVC conduit suitable for buried cables?

No. PVC conduit does not offer sufficient mechanical protection to wiring buried underground. Electrical systems should be designed such that under normal operation and installation conditions, they are not likely to suffer damage and introduce electrical shock risk. It’s very easy to put a spade through a PVC conduit and cable in one motion. 

10.3) Is galvanised metal conduit suitable for buried cables?

As with PVC conduits, metal conduits are often not suitable either unless the wall of the conduit is sufficiently thick to offer mechanical protection to the wiring contained inside. It is generally accepted that a thickness of 3 millimetres and over is sufficient mechanical protection. Each situation should be assessed on its own merits though, and 3mm may not be thick enough if there are extenuating circumstances or conditions that may mean impact is likely. 

10.4) What is the best way to install buried cables?

​In order that local codes and the national electrical code is followed, a licensed electrician should be consulted to carry out the installation of wiring such as buried cables. Electrical installations are NOT a DIY job and real danger can exist where safety requirements are not followed.