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SWA Cable Sizing Calculator

Size steel wire armoured (SWA) cables for underground and outdoor installations. Calculate current capacity and voltage drop per IS 1554 and BS 5467.

📐 Standard: IS 1554 / BS 5467
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SWA Cable Sizing Calculator Calculator
Reference: IS 1554 / BS 5467
🔌 Electrical
Size steel wire armoured (SWA) cables for underground and outdoor installations. Calculate current capacity and voltage drop per IS 1554 and BS 5467.
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ℹ️ About This Calculator

Steel Wire Armoured (SWA) cables are used for underground, direct-buried, and exposed outdoor installations where mechanical protection is required. Sizing SWA cables involves checking both current-carrying capacity (with soil thermal resistivity and grouping derating) and voltage drop over the cable route. This calculator follows IS 1554 (for PVC-insulated armoured cables) and BS 5467 (for XLPE armoured cables).

SWA cables are standard for LT power distribution underground in Indian buildings and infrastructure. IS 1554 Part 1 (PVC insulated) and IS 7098 Part 2 (XLPE insulated) cover armoured cables. XLPE insulated cables (IS 7098) can carry more current than PVC cables of the same size due to higher permissible conductor temperature (90°C vs 70°C). All buried cables must be at minimum 600 mm depth per IS 1255; major roads require 900 mm depth with protective slabs.

📐 SWA Cable Sizing Procedure

IS 1554 / BS 5467

Current Capacity (Ampacity):
  I_rated = I_table × k_soil × k_depth × k_grouping
  k_soil: 1.0 at ρ=1.2 K·m/W; 0.95 at ρ=1.5; 0.88 at ρ=2.0
  k_depth: 1.0 at 0.5 m depth; 0.98 at 0.8 m; 0.96 at 1.0 m
  k_grouping: (per IS 1554 Appendix)

Voltage Drop (per km):
  ΔV = I × (R × cosφ + X × sinφ) × L / 1000 (line-to-line, 3-phase)
  R, X from IS 1554 cable data tables

Short-Circuit Rating:
  I_sc = A × K / √t  [K=143 for copper XLPE; K=94 for aluminium PVC]

🧮 Worked Example

Example: A 4-core XLPE/SWA cable feeds a sub-station load of 150 A at 415 V, direct-buried at 0.8 m depth in soil with resistivity 1.5 K·m/W, run 120 m to the panel. Table ampacity for the chosen 70 mm² cable is 178 A; applying k_soil = 0.95 and k_depth = 0.98 gives derated capacity = 178 × 0.95 × 0.98 ≈ 165.7 A, above the 150 A demand. Checking voltage drop: ΔV = 150 × (0.52×0.85 + 0.09×0.53) × 120/1000 ≈ 8.6 V, about 2.1% of 415 V - well inside the usual 5% limit, so the 70 mm² XLPE/SWA cable is confirmed.

❓ Frequently Asked Questions

What is the minimum depth for direct buried SWA cable? +
IS 1255 (Installation and Maintenance of Power Cables) specifies: LV cables (up to 1.1 kV) – 600 mm minimum depth in unmade ground; 750 mm under roads and paved areas. HV cables (3.3–33 kV) – 900 mm minimum. Reinforced concrete cable markers every 50 m along the route. Cable route drawings must be retained. Route markers at road crossings and direction changes.
Should I use PVC or XLPE insulated armoured cable? +
XLPE (IS 7098) is recommended for new installations: higher current rating (90°C conductor temperature vs 70°C for PVC), better short-circuit performance, lower dielectric losses, longer life (40+ years vs 25 years for PVC). PVC (IS 1554) is cheaper for small sizes and acceptable for short runs in benign environments. For any cable run over 50 m or in hot climates, XLPE pays back the small premium through energy savings.
Does the armour in SWA cable provide earth continuity? +
The steel wire armour (SWA) can be used as a protective earth conductor, but IS 3043 (Earthing) requires the armour to have adequate cross-section for the prospective fault current. Typically, SWA is used as an additional earth path alongside a dedicated earth core or separate earth conductor. Never rely solely on SWA armour as the only earth conductor for high fault level circuits.
What is the difference between SWA and STA cables? +
SWA (Steel Wire Armour): round steel wires applied helically over insulation; flexible, good for direct burial. STA (Steel Tape Armour): flat steel tapes applied with overlap; better crush resistance but less flexible, often used in ducts or where mechanical crushing is the primary risk. For most underground LV distribution, SWA is standard. STA is used for HV cables and where cable must resist very high compressive loads.
How do I terminate SWA cable? +
SWA termination requires: cable gland (armoured gland to grip and earth the armour), inner cable sheath sealed with heat-shrink or cold-shrink termination, individual core terminations with crimped lugs. The gland must be IP-rated for the installation environment. Earth tag from the gland must connect to the equipment earth terminal. For outdoor/underground: use IP68 waterproof glands. For switchgear: IP55 glands are typical.

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