SWA (Steel Wire Armoured) Cable Sizing

Steel wire armoured (SWA) cable is the standard choice for buried underground runs, external power supplies, and sub-main distribution in the UK and many countries that follow IEC-based wiring standards. Sizing it follows BS 7671 (the IET Wiring Regulations), which uses different tables, correction factors, and voltage-drop notation than the NEC approach described elsewhere on this site. This article focuses specifically on the BS 7671 method.

Current-carrying capacity and reference methods

BS 7671 Appendix 4 tabulates current-carrying capacities for cables in various installation conditions. Each set of conditions is called a reference method (previously installation method), identified by letter codes such as Method C (clipped direct), Method D (buried direct or in duct), and so on.

For SWA cable the most common reference methods are:

The same cable has a higher rating when buried (Method D) than when clipped in still air (Method C), because soil provides better heat dissipation than open air when the ground is not too dry.

For example, a 4 mm² two-core SWA copper cable might have a rating near 36 A when clipped direct (Method C) and around 44 A when buried (Method D1), based on typical BS 7671 Appendix 4 tables. Always use the actual table values for the conductor material, size, and installation method that apply to your circuit.

Correction factors

Three correction factors commonly apply to SWA circuits.

Ca: ambient temperature correction. BS 7671 Table 4B1 gives factors for ambient temperatures above or below the reference (30°C for air-installed cables, 20°C for buried cables). At a higher ambient temperature the conductor cannot reject heat as effectively, so its permitted current falls. At a lower ambient temperature the rating can be increased.

Cg: grouping correction. When multiple cables run together, they mutually raise each other's temperature. BS 7671 Table 4C1 gives grouping factors. Two cables touching reduce the rating to about 0.80 of the single-cable value; three cables touching reduce it to about 0.70.

Ci: thermal insulation correction. Applies when a cable is installed in thermally insulating material. For SWA buried or surface-run cables this factor is usually 1.0 (not applicable).

The derated current-carrying capacity is:

I_z = I_t × Ca × Cg × Ci

Where I_t is the tabulated rating for the chosen reference method. The cable is acceptable when I_z is greater than or equal to the design current I_b.

For a detailed explanation of how derating works and why it matters, Ampacity Explained covers the underlying principles, even though that article uses NEC notation.

Voltage-drop method: mV/A/m

BS 7671 uses a different notation for voltage drop than the circular-mil formula common in NEC practice. Cable manufacturers publish a voltage-drop figure in millivolts per amp per metre (mV/A/m), derived from the cable's resistance (and reactance for larger cables). Look this value up in the manufacturer's data sheet or standard cable tables for the conductor size and type.

The voltage-drop calculation is:

V_drop (mV) = (mV/A/m value) × I_b × L

Where I_b is the design current in amps and L is the one-way cable length in metres.

Convert to volts by dividing by 1,000, then compare to the permitted limit. BS 7671 Appendix 12 recommends that voltage drop from the origin of the installation to any point of use should not exceed:

Some designers and local network operators apply tighter limits. Always check the applicable specification.

The voltage-drop method connects directly to the same underlying physics as Voltage Drop Explained; the mV/A/m figure is essentially the cable resistance expressed in convenient units.

The armour as circuit protective conductor (CPC)

One distinctive feature of SWA cable is that the steel wire armour is commonly used as the circuit protective conductor (CPC, the earth conductor) for the circuit. This is explicitly permitted under BS 7671 where the armour cross-section is large enough to satisfy the disconnection time requirements of Chapter 41 and the adiabatic equation of Regulation 543.1.

When the armour is used as CPC, it must be terminated at both ends in appropriate earth clamps or glands that make solid metallic contact with the armour wires. A loose-fitting gland that grips only the outer sheath does not provide a reliable CPC connection. For circuits where a separate CPC is required (for example, when the fault-current capacity of the armour is marginal), a separate earth core or external conductor must be run.

Worked example: a 32 A circuit, 30 m run

A workshop sub-board is fed by a 32 A single-phase circuit run in 6 mm² two-core SWA copper cable, clipped direct (Reference Method C). Ambient temperature is 35°C. Only one cable on this route.

Step 1: Find tabulated rating. From BS 7671 Appendix 4 tables (typical value for 6 mm² two-core copper SWA, Method C): I_t ≈ 46 A.

Step 2: Apply correction factors. Ca for 35°C ambient with 70°C thermoplastic (PVC) insulation from Table 4B1: Ca ≈ 0.94. Cg = 1.0 (single cable, no grouping). Ci = 1.0 (not in insulation).

I_z = 46 × 0.94 × 1.0 × 1.0 = 43.2 A

I_z (43.2 A) is greater than I_b (32 A). The cable passes the current-carrying capacity check.

Step 3: Check voltage drop. For 6 mm² two-core copper SWA cable, a typical mV/A/m figure is around 7.3 mV/A/m (single-phase, resistance dominant at this size).

V_drop = 7.3 × 32 × 30 = 7,008 mV = 7.01 V

On a 230 V single-phase supply, 7.01 V is about 3.0%, right at the 3% guidance for lighting or slightly under the 5% limit for power. If this circuit is for a power load, 3.0% is acceptable. If a tighter limit applies, step up to 10 mm² and recheck.

Step 4: Verify armour as CPC. Check the armour cross-section against the adiabatic equation (Regulation 543.1.3) and the disconnection time requirements. If the armour passes, terminate both ends in proper SWA glands bonded to earth bars.

Calculating with a tool

A cable sizing calculator can help you verify current and voltage-drop figures quickly, particularly when comparing conductor sizes or materials. Keep in mind that the tool uses NEC circular-mil notation by default; for BS 7671 mV/A/m calculations, cross-check against the cable manufacturer's data sheet and the appropriate Appendix 4 table.

SWA cable sizing should be verified against the current edition of BS 7671 (IET Wiring Regulations) and any applicable network operator or project specification. The tabulated values used in this article are typical examples and may differ from the actual tables in your edition. A qualified electrician or electrical engineer should design and verify all permanent installations.