🔌 Electrical

Armoured Cable Calculator (SWA) – Size, Current Rating & Voltage Drop

Free armoured cable (SWA) calculator: current-carrying capacity, voltage drop and short-circuit withstand per IS 1554 / IS 7098. No sign-up.

📐 Standard: IS 1554 / IS 7098
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SWA Cable Full Calculator Calculator
Reference: IS 1554 / IS 7098
🔌 Electrical
Free armoured cable (SWA) calculator: current-carrying capacity, voltage drop and short-circuit withstand per IS 1554 / IS 7098. No sign-up.
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ℹ️ About This Calculator

This comprehensive SWA cable calculator performs all three checks required for correct cable selection: current-carrying capacity (with derating), voltage drop (per km from cable data), and short-circuit withstand (adiabatic equation). All three must pass — the most onerous determines the final cable size. Per IS 1554 (PVC-insulated) and IS 7098 (XLPE-insulated) armoured cables.

IS 7098 Part 2 (XLPE-insulated steel wire armoured cables) is the preferred specification for new LT power distribution cables in India. XLPE cables carry approximately 25–30% more current than equivalent PVC cables. Short-circuit rating check is critical for cables near the transformer (high fault levels). All three checks must pass; typically voltage drop governs for long runs, current rating for short runs near the transformer.

📐 Three-Check Cable Sizing Method

IS 1554 / IS 7098

Check 1 – Current Carrying Capacity:
  I_cable ≥ I_design / (k_temp × k_depth × k_grouping)

Check 2 – Voltage Drop:
  ΔV = I_design × (r cos φ + x sin φ) × L × 2 / 1000 [single-phase]
  ΔV = I_design × (r cos φ + x sin φ) × L × √3 / 1000 [three-phase]
  r, x from IS 1554/IS 7098 cable data tables (Ω/km)
  ΔV% ≤ 5% total (IS 732 / NBC)

Check 3 – Short Circuit Rating:
  I_sc × √t ≤ A × K
  K = 143 for copper/XLPE; K = 115 for copper/PVC
  K = 94 for aluminium/XLPE; K = 76 for aluminium/PVC

🧮 Worked Example

A 3-phase feeder carries a design current of 180 A over a 120 m run, with a fault current of 15 kA cleared in 0.3 seconds. Check 1: with derating factors (0.9 depth × 0.85 grouping) a 95 mm² XLPE/SWA cable rated 210 A gives an effective capacity of 210 × 0.9 × 0.85 = 161 A, which is insufficient, so 120 mm² (240 A rated, 184 A effective) is chosen. Check 2: at r = 0.196 Ω/km, voltage drop = 180 × 0.196 × 120 × √3/1000 ≈ 7.3 V (1.8%), well within the 5% limit. Check 3: required copper/XLPE area = (15,000 × √0.3)/143 ≈ 57 mm², comfortably less than 120 mm². The 120 mm² cable passes all three checks.

❓ Frequently Asked Questions

Which check most often determines the cable size? +
For short cable runs (under 50 m) near the transformer: short-circuit withstand often governs – small cables cannot withstand the high fault current even for the short time before protection operates. For medium runs (50–200 m): current-carrying capacity usually governs. For long runs (over 200 m): voltage drop usually governs – the cable must be upsized to keep ΔV% within the 5% limit.
What is the protection operating time used in the short-circuit check? +
Use the relay/fuse operating time at the maximum fault current for that cable. For MCCBs and fuses close to the transformer: 0.1–0.4 seconds is typical. For cables with time-delayed protection (backup relay): 1–3 seconds. Shorter operating time allows a smaller cable to pass the short-circuit check. Always use the actual clearing time from the relay coordination study, not a guess.
Can I use a 3-core cable for a 3-phase circuit? +
For a 3-phase circuit with a separate earth conductor: use 3-core cable (phases only) + separate earth. For a 3-phase circuit with neutral and earth combined in the cable: use 3.5-core (3 phases + half-size neutral) or 4-core (3 phases + full neutral), with the armour serving as earth or a separate earth conductor. 4-core cables are standard for most Indian LV distribution (3 phases + neutral in one cable).
How much space should I allow in cable ducts? +
CPWD and NBC guidelines: do not fill ducts more than 40% of cross-sectional area (grouping derating assumed if more cables are added). For a 150 mm × 150 mm duct (22,500 mm²): maximum cable fill = 9,000 mm². Select cables based on their overall diameter and stack no more than the fill limit. Keep 25% spare capacity (empty ducts) for future cables. Separate HV, LV power, and signal cable ducts.
What are the standard SWA cable sizes available in India? +
Standard conductor sizes per IS 1554/IS 7098: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630 mm². Voltage rating: 1.1 kV (LT) most common; 3.3 kV and 6.6 kV for HT feeder cables. All SWA cables are available in copper and aluminium conductors. Aluminium conductors are used for 50 mm² and above where weight and cost savings are significant.

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