🔌 Electrical

Wire Size Calculator

Calculate the correct wire/conductor size for any circuit. Accounts for current-carrying capacity, voltage drop, and derating factors per IS 732.

📐 Standard: IS 732 / NEC 310
✅ Free to use
📄 PDF export
📱 Mobile friendly

ℹ️ About This Calculator

Selecting the correct wire size is fundamental to electrical safety and efficiency. An undersized wire overheats and poses a fire risk; an oversized wire wastes material. This calculator determines the minimum conductor cross-section based on load current, circuit length, allowable voltage drop, and installation method derating per IS 732.

IS 732 (Code of Practice for Electrical Wiring Installations) is the primary Indian standard for sizing wires and cables in building electrical systems. Conductor current ratings are tabulated for different installation methods (clipped to surface, in conduit, underground). Derating factors k1 (ambient temperature), k2 (grouping), k3 (soil thermal resistivity for underground) must be applied. NBC 2016 Part 8 cross-references IS 732 for all wiring design in buildings.

📐 Wire Sizing Formula

IS 732 / NEC 310

Current Check:
  I_design = P / (V × PF × η) for single-phase
  I_design = P / (√3 × V × PF × η) for three-phase
  I_derated = I_tabulated × k1 × k2 × k3

Voltage Drop Check:
  ΔV% = (2 × I × L × ρ) / (A × V) × 100   [single-phase]
  ΔV% = (√3 × I × L × ρ) / (A × V) × 100  [three-phase]

Where: L = one-way length (m), ρ = resistivity (Ω·mm²/m), A = cross-section (mm²)
Limit: ΔV ≤ 3% for final circuits, ≤ 5% total (IS 732)

❓ Frequently Asked Questions

What is the maximum allowable voltage drop for a circuit? +
Per IS 732 and NBC 2016: 3% for the final distribution circuit (from distribution board to socket outlet or load), and no more than 5% from the supply origin (utility meter) to the farthest point. For EV chargers and sensitive equipment, limit to 2.5%.
Copper vs aluminium – which should I use? +
Copper is preferred for most wiring (better conductivity, smaller size, easier jointing). Aluminium is used for large mains and service conductors where cost savings justify the larger size (aluminium needs one or two sizes larger than copper for the same current). IS 732 covers both materials. Never use aluminium below 16 mm² (risk of loose joints and fire).
What derating factor applies for cables in a conduit? +
For 2 circuits (4 conductors) in a conduit: 0.80. For 3 circuits: 0.70. For 4 circuits: 0.65. For 5–6 circuits: 0.57. For cables clipped directly to a surface (no conduit): no grouping derating required when cables are touching each other. IS 732 Table 4 gives grouping factors.
How do I size a wire for motor loads? +
Motor wiring must carry full-load current (FLA) plus 25% overload margin: I_wire = 1.25 × FLA. The overload relay is set at 1.05–1.15 × FLA. The short-circuit protective device (MCCB) is set at 3–10 × FLA depending on motor type. All three must be coordinated – the wire rating must exceed the overload relay setting.
Can I use a smaller wire if I use a bigger MCB? +
No. The MCB (or fuse) protects against short-circuits, not sustained overloads. The wire must independently carry the design current with derating applied. If the MCB is larger than the wire rating, the wire can carry excessive current without the MCB tripping – this is a fire hazard. Always: wire rating ≥ MCB rating × 1.0 (or MCB rating ≤ wire rating).

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

🔌 Wire Size Calculator
Reference: IS 732 / NEC 310