🔌 ELECTRICAL

Cable Size Calculator (IS 732 / BS 7671)

Select minimum cable size for LV circuits. Checks both current carrying capacity and 3% voltage drop per IS 732. Free online tool.

📐 Standard: IS 732 / BS 7671
✅ Free to use
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ℹ️ About This Calculator

Selecting the correct cable size is one of the most critical tasks in electrical design. An undersized cable overheats and becomes a fire hazard; an oversized cable wastes material cost. IS 732 (Code of Practice for Electrical Wiring Installations) requires that every cable be sized to satisfy two simultaneous criteria: current-carrying capacity and voltage drop.

IS 732 current capacity tables (Table 4, 5 etc.) give the rated current for different installation methods — clipped direct to surface, in conduit, or in trunking. Each installation method has a derating factor. For example, a 4mm² copper cable clipped direct carries 32A, but in a conduit (grouped with other cables) this may derate to 20A. Always apply the correct derating factors for ambient temperature, grouping, and soil thermal resistivity for buried cables.

📐 Cable Sizing Method (IS 732)

IS 732 / BS 7671

Step 1 — Full Load Current:
  3-Phase: I = P / (√3 × V × PF)
  1-Phase: I = P / (V × PF)

Step 2 — Select cable ≥ I (from IS 732 current capacity tables, derated for installation)

Step 3 — Check voltage drop:
  Vd = (2 × L × I × ρ) / A   [single phase]
  Vd = (√3 × L × I × ρ) / A  [three phase]
  Require: Vd/V ≤ 3%

Step 4 — If VD > 3%, select next larger cable size and recheck.

❓ Frequently Asked Questions

What is the minimum cable size for a 15A circuit in India? +
For a 15A single-phase circuit, IS 732 requires minimum 2.5mm² copper cable when clipped direct, or 4mm² if in conduit. Always verify the actual voltage drop for the specific cable length — longer runs may need a larger size.
Should I size the cable for current capacity or voltage drop? +
Both — simultaneously. IS 732 requires the cable to satisfy both the current carrying capacity (thermal limit) and the 3% voltage drop limit. Whichever requirement gives the larger cable size governs. For long runs, voltage drop typically governs; for short runs with high current, thermal capacity governs.
What derating factors apply to cables in conduit? +
IS 732 Table 22 gives grouping factors. For 2 cables in a conduit: 0.80. For 3 cables: 0.70. For 4 cables: 0.65. Ambient temperature above 30°C also requires derating per IS 732 Table 16. Both factors must be applied.
Copper vs aluminium cable — when should I use aluminium? +
Aluminium has about 60% of the conductivity of copper, so for the same current you need roughly 1.6× the cross-section. Aluminium is typically cost-effective for feeders above 35mm² and for buried underground cables. For branch circuits, socket outlets, and lighting — always use copper per IS 732.
What AWG is equivalent to 4mm² cable? +
4mm² is approximately equivalent to AWG 11 (or between AWG 11 and AWG 12). 2.5mm² ≈ AWG 13. 6mm² ≈ AWG 9. 10mm² ≈ AWG 7. The AWG system uses a different geometric progression so the equivalence is approximate.

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⚠️ Disclaimer: For preliminary engineering design only. Verify all results with a licensed engineer before use. Full disclaimer →

🔌 Cable Size Calculator
Reference: IS 732 / BS 7671