Armoured Cable Selection Guide: SWA vs STA vs AWA, Armour Coverage and Burial Rating
Three armour types, four core configurations and a dozen installation conditions — armoured cable specification goes wrong most often at the armour and glanding stage rather than at the conductor. This guide sets out the decision sequence we use when quoting buried distribution projects.
Step 1 — Fix Voltage Grade and Armour Family
Voltage grade narrows the armour choice immediately. At 0.6/1 kV, steel tape (YJV22) is the economic default for trench work and steel wire (YJV32) takes over where tensile load or impact risk is high. From 3.6/6 kV upwards, steel wire armour is standard for three-core, and aluminium wire armour (YJLV72) becomes mandatory for single-core circuits.
| Condition | Recommended armour |
|---|---|
| Direct burial, ordinary trench, three-core LV | Steel tape (YJV22) |
| Direct burial with vehicle loading, rocky backfill | Steel wire (YJV32) |
| Shaft, vertical drop, river or lake crossing | Steel wire, double armour on request |
| Single-core AC, any voltage | Aluminium wire (YJLV72) or non-magnetic armour |
| Corrosive soil, coastal or saline ground | Steel wire with PE or LSZH oversheath |
Step 2 — Confirm Armour Coverage and Fault Duty
Armour is also the earth-fault path, so its cross-section must pass the adiabatic check. Using the standard equation S = I√t / k with k = 143 for steel armour at XLPE temperatures, a 25 kA earth fault cleared in 0.2 s needs about 78 mm² of equivalent armour. Where the calculated armour area falls short, add a separate CPC or increase the armour wire diameter — the factory can supply 2.0 mm and 2.5 mm wires on request.
Step 3 — Size the Conductor for Derated Ampacity
Buried cable loses ampacity faster than most buyers expect. A 4-core 185 mm² copper XLPE cable rated near 390 A in free air falls to roughly 330 A in soil at 2.5 K·m/W, and to about 280 A when two circuits are laid side by side. Apply all four factors before fixing the size:
- Soil thermal resistivity — 2.5 K·m/W reference, dry sand 3.5 (factor 0.80–0.85).
- Soil temperature — 20 °C reference, 30 °C in tropical ground (factor 0.90).
- Grouping — two circuits touching, factor 0.85; four circuits, 0.70.
- Burial depth — 1.0 m reference.
Step 4 — Check Pulling and Route Constraints
Armoured cable is heavy: a 4×185 mm² SWA drum can approach 5 tonnes. Limit pulling tension to about 50 N/mm² of conductor for copper, use a swivel and pulling stocking, and keep bend radius at 15×OD multi-core / 20×OD single-core. On long runs, plan a joint bay rather than exceeding drum length.
Step 5 — Fix Sheath and Fire Requirements
Standard supply is PVC ST2 black. For tunnels, stations and plants where smoke density matters, specify LSZH oversheath (WDZ-YJV22); for corrosive ground use PE. Where flame propagation is a contractual requirement, add IEC 60332-3 and state it in the purchase order so the test appears on the certificate.
Selection Checklist
- Voltage grade and system earthing arrangement confirmed.
- Armour family matched to burial, tensile and single-core conditions.
- Armour cross-section checked against earth-fault current.
- All four derating factors documented in the calculation note.
- Sheath compound and flame test specified in writing.
- Drum lengths and joint bays planned against route length.
For construction details and designations, see the armoured electrical cable guide; for termination practice, see glanding and earthing.
