Calculate the correct SWA (Steel Wire Armoured) cable size with gland selection for UK electrical installations
Calculate the correct SWA cable size for your installation according to BS 7671
Steel Wire Armoured (SWA) cable is a robust power cable designed for outdoor and underground electrical installations. It is the most common cable type for supplying power to outbuildings, garden offices, EV chargers, and commercial installations in the UK.
SWA cables are manufactured to BS 5467 and consist of multiple layers, each serving a specific protective function:
| Feature | PVC (70°C) | XLPE (90°C) |
|---|---|---|
| Max operating temperature | 70°C | 90°C |
| Current capacity | Standard | Higher (10-20% more) |
| Cost | Lower | Higher |
| Terminal compatibility | Standard 70°C terminals | Must derate to 70°C at standard terminals |
| Common use | Domestic and light commercial | Industrial and high-load installations |
Key Point: SWA cable uses the steel wire armour as the circuit protective conductor (CPC/earth). The armour must be properly terminated using a cable gland to maintain earthing continuity. Both ends must be earthed per BS 7671 requirements.
SWA cable is the correct choice whenever mechanical protection is required or cables are installed outdoors or underground. Here are the most common applications:
Typical size: 6-10mm² 3-core SWA
Installation: Buried 450mm deep
Gland: CW type for outdoor use
7.4kW (32A): 6mm² 3-core SWA
22kW (32A 3-phase): 6mm² 4-core SWA
Note: Check run length for voltage drop
Light use: 4mm² SWA
Workshop: 10-16mm² SWA
Sub-main: Size for total demand
32A supply: 6mm² SWA
40A supply: 10mm² SWA
Protection: 30mA RCD required
Typical: 1.5-2.5mm² SWA
VD limit: 3% for lighting circuits
Depth: 450mm buried
Single phase: 25-70mm² SWA
Three phase: 16-240mm² SWA
Note: Size for maximum demand
Important: SWA cable must always be terminated with the correct cable gland to maintain earth continuity through the armour. Use CW glands for outdoor and buried installations, BW glands for indoor use only.
The installation method significantly affects the current-carrying capacity of SWA cable. BS 7671 assigns different ratings depending on how heat can dissipate from the cable.
The most common installation for domestic SWA runs. Cable is buried directly in the ground at a minimum depth of 450mm (600mm under roads/driveways).
Cable pulled through underground ducting. Lower ratings than direct burial because the air gap inside the duct reduces heat dissipation.
SWA cable clipped directly to a wall, ceiling, or structure using P-clips or cable cleats at regular intervals.
Cables laid on perforated cable tray, either in trefoil formation (touching in triangular arrangement) or flat-spaced (side by side with spacing).
Cable suspended in free air with maximum heat dissipation. Gives the highest current ratings but requires adequate mechanical support.
Correct gland selection is critical for SWA cable installations. The gland provides mechanical retention, environmental sealing, and electrical continuity for the armour (CPC/earth path). Using the wrong gland size can compromise earthing and IP protection.
| Gland Type | Application | IP Rating | Seals |
|---|---|---|---|
| BW | Indoor only | IP2X | No weather seal |
| CW | Outdoor / weather exposed | IP66 | Outer sheath seal |
| E1W | Harsh / corrosive environments | IP66/68 | Inner + outer seal (double) |
| Conductor (mm²) | 2-Core SWA | 3-Core SWA | 4-Core SWA |
|---|---|---|---|
| 1.5mm² | 20S | 20S | 20S |
| 2.5mm² | 20S | 20S | 20S |
| 4mm² | 20S | 20S | 20 |
| 6mm² | 20 | 20 | 20 |
| 10mm² | 20 | 20 | 25 |
| 16mm² | 25 | 25 | 25 |
| 25mm² | 25 | 32 | 32 |
| 35mm² | 32 | 32 | 32 |
| 50mm² | 32 | 32 | 40 |
| 70mm² | 32 | 32 | 40 |
| 95mm² | 32 | 40 | 50S |
| 120mm² | 40 | 50S | 50 |
| 150mm² | 40 | 50S | 50 |
| 185mm² | 50S | 50 | 63S |
| 240mm² | 50 | 63S | 63 |
| 300mm² | 63S | 63 | 75S |
Reference: BS 6121 / BS EN 62444 - Cable gland sizing for Cu/XLPE/SWA cables (BS 5467). "S" denotes small bore variant for smaller diameter cables.
All SWA cable installations in the UK must comply with BS 7671:2018+A2:2022 (18th Edition) IET Wiring Regulations. The following key requirements apply:
The steel wire armour of SWA cable is classified as an exposed conductive part and must be earthed at both ends (Regulation 411.3.1.1). The cable gland provides the primary mechanism for bonding the armour to the earthing system. The armour can serve as the CPC (circuit protective conductor), but its adequacy must be verified using the adiabatic equation (Regulation 543.1.3) with k=51 for steel armour.
The voltage drop between the origin of the installation and the point of utilisation must not exceed 5% for power circuits (11.5V on 230V) or 3% for lighting circuits (6.9V on 230V). For long buried SWA runs, voltage drop often determines the cable size rather than current-carrying capacity.
SWA cables buried in ground must comply with Regulation 522.8.10:
SWA cable current ratings are found in BS 7671 Appendix 4:
Circuits supplying equipment outdoors or in outbuildings typically require 30mA RCD protection (Regulation 411.3.3). For TT systems, RCD protection is mandatory. The armour-as-CPC arrangement works with standard RCD operation, but the higher impedance of steel versus copper must be considered for earth fault loop impedance (Zs) calculations.
Important: This calculator provides sizing guidance based on BS 7671 current rating tables. Final cable selection must be verified by a qualified electrician considering all installation-specific factors including earth fault loop impedance, prospective fault current, and the adiabatic equation for CPC adequacy.
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