What is armoured cable (SWA)?

Glossary · Alarms, lighting and home installs

How armoured cable works

A normal household cable, such as twin and earth, is designed to sit inside walls and ceilings. It is not meant to be dug into the ground or left outdoors. Armoured cable is. It has insulated conductors at the centre, a layer of steel wires wrapped around them and an outer sheath of plastic. The steel wires are the armour.

The armour does two jobs. It protects the cable from knocks, such as a spade or a fork. It can also act as the cable’s earth connection. The IET says a cable buried in the ground is generally required to include an earthed armour or metallic sheath, or both, suitable for use as a protective conductor.^{[2]IET, Buried conduits and ducts (Wiring Matters)}

Cable makers produce armoured cable in several constructions. Prysmian, for example, says its XLPE insulated, PVC sheathed SWA cables are made to BS 5467 and can be installed in outdoor applications.^{[3]Prysmian, Power cables} NICEIC says SWA is the cable generally used to supply a shed, as it is heavy-duty, weather-resistant and designed for outdoor use and burial underground.^{[1]NICEIC, Wiring a shed: a guide for homeowners}

Where it is used and the rules around it

The table is drawn from NICEIC,^{[1]NICEIC, Wiring a shed: a guide for homeowners} the IET^{[2]IET, Buried conduits and ducts (Wiring Matters)} and our own garden sockets page.^{[4]Martin Day Electricians, Garden electrical sockets and outdoor wiring in Leeds}

Burying a cable has its own rules. The IET says the route must be marked by cable covers or a suitable marker tape, and that the cable must be deep enough to avoid damage from foreseeable disturbance of the ground. It lists a general minimum depth of 0.5 m, with more for some locations, and says depths should be increased in soft ground.^{[2]IET, Buried conduits and ducts (Wiring Matters)} The right depth for a given garden is an engineering judgement for the installer.

Supplying an outbuilding is regulated work. NICEIC says a new circuit from the house to an outdoor building is notifiable under Part P of the Building Regulations in England and Wales. It must either be notified to Local Authority Building Control or be done by an electrician registered on a competent person scheme.^{[1]NICEIC, Wiring a shed: a guide for homeowners} NICEIC says that work is not a DIY job and should be done by a registered electrician.^{[1]NICEIC, Wiring a shed: a guide for homeowners}

Example

Rosie in Adel wants to turn her garden shed into an office. Her electrician checks that her existing electrics can take the extra load, then plans a trench from the house to the shed. NICEIC says the electrician connects the cable either to the consumer unit or to a switched fused connection unit, and may fit a small dedicated consumer unit in the shed if the demand is higher.^{[1]NICEIC, Wiring a shed: a guide for homeowners}

The electrician uses armoured cable for the run, marks the route and tests and certifies the new circuit. They also notify the work under Part P, or do it as a registered competent person.^{[1]NICEIC, Wiring a shed: a guide for homeowners} Rosie leaves the digging, the connection and the testing to the electrician. For a larger office setup our home office wiring service covers the fuller data and lighting circuit.^{[4]Martin Day Electricians, Garden electrical sockets and outdoor wiring in Leeds}

What people get wrong

Many people think armoured cable is just a tougher cable that anyone can fit. The armour does an electrical job as well as a mechanical one, so how it is terminated and earthed matters, and that is work for a registered electrician.

The IET says buried cable generally needs an earthed armour or metallic sheath suitable for use as a protective conductor.^{[2]IET, Buried conduits and ducts (Wiring Matters)} The other common mistake is forgetting what is already underground. Electrical Safety First says to avoid digging in the garden close to electric cables, gas pipes or water services.^{[5]Electrical Safety First, Electrical safety advice for outdoor lighting} If you do not know where the cable runs, ask the electrician who installed it, or whoever holds the records of the route. Our garden electrical sockets service covers protected outdoor circuits.

The bottom line

Armoured cable is a heavy-duty cable with a steel wire layer, used for the underground and outdoor runs that ordinary house cable cannot handle. It is the cable generally used to power sheds and garden outbuildings, and buried cable normally needs an earthed armour or equivalent protection. The work is notifiable, it is not a DIY job, and a registered electrician should plan, lay, connect and certify it.