SWA Armoured Cable Voltage Drop Calculator
Calculate voltage drop for Steel Wire Armoured cable - outbuildings, buried runs and external installations
🔩 SWA HAS HIGHER mV/A/m THAN STANDARD CABLE
Steel armour increases resistance. Use SWA-specific values from manufacturer data - typically 5-15% higher than equivalent T&E cable.
SWA Cable Quick Facts
- SWA has slightly higher resistance than equivalent T&E due to steel armour
- Use manufacturer mV/A/m values - they differ from standard copper tables
- Armour can be used as CPC but requires proper gland termination
- Direct burial depth minimum 450mm (600mm under roads)
- Typical outbuilding runs: 4mm² for lighting, 6mm² for 32A, 10mm² for workshop
Values from BS 7671 Table 4D1B (PVC copper at 70°C)
📐 SWA vs Standard Cable Comparison
Standard 6mm² T&E
mV/A/m = 7.3
30m at 32A = 7.01V (3.05%)
6mm² 3-core SWA
mV/A/m ≈ 6.4 (XLPE) to 7.9 (PVC)
30m at 32A = 6.14V-7.58V
Always check manufacturer data for exact mV/A/m values - they vary by insulation type and manufacturer.
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Common SWA Installation Scenarios
Garage supply (25m) - 32A
32A with 6mm² SWA (6.4 mV/A/m). At 25m: (6.4 × 32 × 25) ÷ 1000 = 5.12V (2.23%) ✓
Garden office (40m) - 32A
32A with 6mm² at 40m: (6.4 × 32 × 40) ÷ 1000 = 8.19V (3.56%) ⚠ consider 10mm²
Workshop with welder (50m) - 63A
63A with 16mm² SWA (2.4 mV/A/m). At 50m: (2.4 × 63 × 50) ÷ 1000 = 7.56V (3.29%) ✓
Outbuilding lighting only (30m)
6A with 4mm² SWA (9.5 mV/A/m). At 30m: (9.5 × 6 × 30) ÷ 1000 = 1.71V (0.74%) ✓
Annex full supply (35m) - 63A
63A with 16mm² at 35m: (2.4 × 63 × 35) ÷ 1000 = 5.29V (2.30%) ✓
Related Outbuilding Calculations
For specific outbuilding types: submain to outbuilding, workshop circuits, annex/granny flat. Use our cable sizing calculator for current capacity.
SWA Cable Construction
Steel Wire Armoured cable has XLPE or PVC insulated conductors, bedding layer, steel wire armour (for mechanical protection), and outer PVC sheath. The armour provides crush and impact resistance for direct burial or external mounting. Single-phase uses 3-core (L, N, E), three-phase uses 4-core or 5-core. The steel armour adds to conductor resistance, giving higher mV/A/m values than equivalent non-armoured cable.
SWA Installation Costs (2024)
Typical UK costs for SWA cable work.
| Work Type | Materials | Labour | Total |
|---|---|---|---|
| SWA to garage (<25m) | £200-400 | £350-550 | £550-950 |
| SWA to garden office (25-40m) | £350-600 | £450-700 | £800-1300 |
| SWA to workshop with CU (40m+) | £500-900 | £600-1000 | £1100-1900 |
| Trench digging (per metre) | £5-15 | £20-40 | £25-55 |
| SWA gland termination (each) | £15-30 | £30-60 | £45-90 |
Prices as of 2024. Consumer unit and internal wiring at outbuilding are additional.
SWA Installation Requirements
Direct burial requires minimum 450mm depth (600mm under roads/driveways). Cable should be laid on sand bed with sand cover and warning tape above. Route markers at each end and direction changes. Glands must be IP-rated for location. Underground joints are best avoided but must use resin-filled kits if necessary. Document the route for future reference.
Using Armour as Earth
SWA armour can serve as the circuit protective conductor (CPC) if correctly terminated with brass glands and earth tags. The gland creates electrical continuity between armour and metal enclosure. Calculate armour CSA using BS 7671 tables - it must meet adiabatic requirements for fault current. For longer runs, a separate CPC conductor in the cable may be preferred.
SWA vs Alternatives
SWA is standard for UK external/buried runs. Alternatives include: armoured data cable (for comms), SY cable (flexible armoured, not for burial), surface-mounted conduit with standard cable. For very long runs, aluminium SWA offers cost savings but needs larger sizes. Overhead cables avoid trenching but have planning and aesthetic issues.
Frequently Asked Questions
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