SWA Cable Size Calculator — Steel Wire Armoured Sizing for Submains and Outbuildings

Steel wire armoured cable is a multicore cable with a layer of galvanised steel wires wound around the insulated cores, under an outer PVC sheath. The armour does two jobs: it protects the cable from mechanical damage — spades, rodents, ground movement, impacts — and it can act as an earthed metallic layer around the live conductors, which is what makes SWA suitable for burial.

SWA is available with PVC or XLPE insulation (XLPE runs at a higher conductor temperature and carries more current for the same size), in 2-core, 3-core, 4-core, and 5-core configurations. For a single-phase submain, 3-core is common (line, neutral, and a core used as CPC, or armour as CPC with the third core spare); for three-phase, 4-core or 5-core.

The calculator above includes SWA cable types with their tabulated ratings — select the armoured option and the installation method (buried, in duct, clipped, on tray) to size from the correct table.

The jobs where SWA is the standard answer:

The common thread is exposure: outdoors, underground, or anywhere twin and earth would be vulnerable. BS 7671 requires that a cable buried in the ground incorporates an earthed armour or metal sheath suitable for use as a protective conductor — or is installed in a conduit or duct that provides equivalent protection against mechanical damage. SWA is the simplest way to meet that.

Four things decide an SWA size, and on long runs it is rarely the first one:

• Current-carrying capacity — the tabulated rating for the cable type and installation method, derated where needed. Buried cables have their own ratings, affected by soil thermal resistivity and burial depth; cables in ducts are rated differently from direct-buried.
• Voltage drop — the usual governing factor for outbuilding submains. A 40-60 metre run at 40A+ eats into the 5% limit fast, and the submain should not use the whole allowance — the final circuits in the outbuilding need their share too.
• Earth fault loop impedance — a long submain adds impedance, and the protective device at the origin still has to disconnect in time. Check with the disconnection time calculator.
• Grouping — where several SWA cables share a trench or tray, grouping factors reduce each cable's capacity.

A useful habit for submains: size for voltage drop first, then confirm capacity. The calculator runs both checks together and tells you which one governed.

A garden workshop needs a submain for an assessed demand of 10kW — a small consumer unit feeding sockets, lighting, and a 3kW heater. The trench run is 40 metres:

This is the classic SWA sizing story: the load says 10mm², the distance says think bigger. The calculator shows the voltage drop at each size so the trade-off is explicit rather than guessed.

BS 7671 sets the principles for buried cables: the cable must incorporate an earthed armour or metal sheath suitable for use as a protective conductor (or be in a duct with equivalent mechanical protection), the route must be marked by cable covers or suitable marker tape, and the cable must be buried at a depth sufficient to avoid damage from any reasonably foreseeable disturbance of the ground.

BS 7671 deliberately does not state a fixed depth — "sufficient" depends on what happens above. Accepted practice in the trade:

Ducting is worth serious consideration even where direct burial is permitted: a duct with a draw cord makes the inevitable future upgrade — a bigger outbuilding load, an EV charger, three-phase — a pull-through rather than a re-dig. Note that cables in ducts have lower tabulated ratings than direct-buried cables of the same size, so re-check the capacity if you switch from direct burial to duct. The calculator covers both methods.

SWA is terminated with mechanical glands that clamp the armour, providing strain relief and electrical continuity from the armour to the gland plate. Getting the termination right matters for safety, not just neatness:

• The armour must be earthed. It is an exposed (or extraneous) metallic layer around live conductors — the gland and banjo/earth tag connect it to the earthing arrangement at the supply end as a minimum.
• Armour as CPC — BS 7671 recognises the earthed armour of a buried cable as suitable for use as a protective conductor. Whether it is adequate on its own depends on its conductance for the size and length of the run, verified against the adiabatic requirement — use the adiabatic equation calculator or run a dedicated CPC core where in doubt.
• Outdoor glands — use weatherproof glands with shrouds for external terminations, and maintain the enclosure's IP rating.
• Earthing at the outbuilding — decide deliberately whether the outbuilding takes the origin's earthing arrangement or a local TT electrode; on PME supplies this decision needs particular care for outdoor and water-associated equipment.