🔌 ELECTRICAL

Cable Size Calculator (IS 732) – Online 3-Phase & LV Cable Sizing

Free online cable size calculator (IS 732 / BS 7671). Sizes single- and 3-phase LV cables by current-carrying capacity and 3% voltage drop.

📐 Standard: IS 732 / BS 7671
✅ Free to use
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📱 Mobile friendly
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Cable Size Calculator Calculator
Reference: IS 732 / BS 7671
🔌 ELECTRICAL
Free online cable size calculator (IS 732 / BS 7671). Sizes single- and 3-phase LV cables by current-carrying capacity and 3% voltage drop.
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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.

🧮 Worked Example

A 3-phase 15kW motor (PF 0.85, 415V) runs through a 40m cable. Full load current: I = 15,000 / (1.732 × 415 × 0.85) ≈ 24.6A.

From IS 732 tables, a 6mm² copper cable (clipped direct) carries 36A - well above 24.6A. Checking voltage drop: Vd = (1.732 × 40 × 24.6 × 0.0172) / 6 ≈ 2.93V (0.71%), comfortably within the 3% limit. If the run were 100m instead, Vd would rise to 7.3V (1.76%) - still compliant, but a 10mm² cable would bring it down to about 4.4V (1.06%) with margin for future load growth.

📊 Cable Sizing Reference (IS 732 / IEC 60364)

How a cable size is selected: under IS 732 (and IEC 60364) the conductor must pass two independent checks — it must carry the design current after derating, and it must keep voltage drop within the 3% limit over the cable run. The larger of the two sizes governs; this calculator applies both automatically.

Indicative Current-Carrying Capacity — PVC-Insulated Copper (IS 732)

Conductor (mm²)Capacity (A)Typical use
1.515Lighting circuits
2.520Socket / small power
427Power circuits, small AC
634Geyser, larger AC
1046Sub-mains, small motors
1661Sub-mains, motor feeders
2580Distribution feeders
3599Feeders
50119Main feeders
70151Main feeders
95182Incomers, large feeders

Indicative values for PVC-insulated copper conductors, two to three loaded cores in conduit at 40 °C ambient. Ratings vary with installation method and manufacturer — always confirm against the cable maker's data sheet and the actual reference method before finalising.

Derating Factors (apply before comparing with design current)

Ambient temp (°C)Factor (PVC)Grouped circuitsFactor
301.151 circuit1.00
351.082 circuits0.80
40 (base)1.003 circuits0.70
450.914 circuits0.65
500.826 circuits0.57

Effective capacity = tabulated rating × ambient factor × grouping factor. In much of India the design ambient is 40–45 °C, so a derating below the 40 °C base is common and must not be ignored — it is a frequent cause of under-sized feeders.

Copper vs Aluminium

Aluminium has about 1.6× the resistivity of copper, so an aluminium conductor must be roughly one to two sizes larger for the same current and voltage drop. Aluminium is lighter and cheaper for large feeders and risers; copper is preferred for final circuits, control wiring and where terminations and space are tight. This calculator handles both — switch the material to compare.

❓ 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.
What cable size do I need for a 5 HP (3.7 kW) motor in India? +
A 3.7 kW three-phase motor at 415 V draws roughly 7-8 A full load, but cables are sized for starting and derating, not just full-load current. A 2.5 mm² copper cable is typically adequate for a short run, stepping up to 4 mm² for longer runs or where voltage drop or grouping derating applies. Always confirm with this calculator using the actual length and installation conditions.
What is the difference between IS 732 and BS 7671 cable sizing? +
Both follow the IEC 60364 framework and use the same two checks — current-carrying capacity and voltage drop. IS 732 is the Indian wiring standard (3% voltage-drop limit is common practice) while BS 7671 is the UK standard (4% for general circuits). The capacity tables and derating factors are broadly similar; the main differences are the reference installation methods and permitted voltage-drop limits.
How does ambient temperature affect cable sizing? +
Cable ratings are quoted at a base ambient (often 40 °C in India). At higher ambients the conductor cannot dissipate as much heat, so its safe capacity falls — for PVC insulation the derating factor is about 0.91 at 45 °C and 0.82 at 50 °C. Ignoring this is a common reason feeders run hot; always apply the ambient and grouping factors before comparing capacity with the design current.

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