Cable Size Calculator (Electrical)
Cable cross-section in sq.mm from current, length, and phase
Current, distance और phase से cable size निकालें — sq.mm में
Electrical cable size in India is specified in square millimetres (sq.mm) of conductor cross-section, per BIS / IS 3961. Sizing a cable correctly means satisfying two constraints simultaneously: (1) current-carrying capacity (ampacity) — the cable must not overheat under full load, and (2) voltage drop — voltage at the load end must not fall below 97% of source voltage for lighting, or 95% for motors and heavy loads. Typical copper cable ampacity: 1.5 sq.mm handles 14 A, 2.5 sq.mm handles 20 A, 4 sq.mm handles 25 A, 6 sq.mm handles 32 A, 10 sq.mm handles 45 A, 16 sq.mm handles 63 A. Voltage drop is calculated using V = 2 × L × I × ρ / A for single phase, or V = √3 × L × I × ρ / A for three phase, where ρ is copper resistivity (0.0175 Ω·mm²/m). This calculator iterates through standard sizes and returns the smallest size satisfying both limits.
Calculator
3% lighting · 5% motor
Formula: V_drop = √3 × 30 m × 16 A × 0.0175 / 2.5 = 5.82 V. Three-phase uses √3 because return path is distributed across 3 lines. Values based on BIS / IS 3961 indoor air installation; derate 20–30% for conduit/buried.
ƒFormula
Ampacity: pick from table (BIS / IS 3961) | V_drop (1φ) = 2 × L × I × ρ / A | V_drop (3φ) = √3 × L × I × ρ / AA = conductor cross-section (sq.mm). L = cable length one-way (m). I = load current (A). ρ = resistivity: copper = 0.0175 Ω·mm²/m, aluminium = 0.028 Ω·mm²/m. For single phase multiply by 2 (current flows go+return); for 3-phase use √3 × L because return via neutral is distributed across 3 lines. Target V_drop ≤ 3% for lighting, ≤ 5% for motors.
Units: Cross-section: sq.mm · Length: metre · Current: ampere
Cable size sq.mm mein kaise nikaalen?
Indian electrical cables sq.mm mein specify hote hain — 1.5 sq.mm, 2.5, 4, 6, 10, 16, 25, 35 etc. Sahi size select karne ke liye do cheezein check karni padti hain: (1) ampacity — cable heat nahi karna chahiye, aur (2) voltage drop — load pe voltage 3-5% se zyada nahi girna chahiye. Thumb rule copper cable: 1.5 sq.mm → 14A, 2.5 → 20A, 4 → 25A, 6 → 32A, 10 → 45A, 16 → 63A. Lambi distance hai (50m+) toh voltage drop zyada critical ho jaata hai — current kam hone ke bawajood bada cable lagana padega.
Worked Examples
Sizing house AC circuit wiring
Problem: 1.5 ton split AC drawing 7 A at 230 V single-phase, 15 m run from MCB.
Solution: Ampacity: 7 A × 1.25 = 8.75 A — 1 sq.mm copper (rated 11 A) sufficient. Voltage drop: V = 2 × 15 × 8.75 × 0.0175 / 1 = 4.59 V = 2.0% — within limits. Use 1 sq.mm copper wire (or 1.5 sq.mm for extra margin — Indian standard practice for 20 A MCB circuits).
3-phase motor cable
Problem: 10 HP motor, 415 V 3-phase, 60 m distance from panel. FLA ≈ 13.5 A.
Solution: Design current: 13.5 × 1.25 = 16.9 A. Ampacity check: 2.5 sq.mm (20 A) fits. Voltage drop: V = √3 × 60 × 16.9 × 0.0175 / 2.5 = 12.3 V = 3.0% — acceptable but close. Use 4 sq.mm to allow ≤2% drop (safer for motor performance).
Long-distance submersible pump
Problem: 3 HP submersible pump, 230 V single-phase, 80 m cable down the borewell. FLA ≈ 14 A.
Solution: Design current: 14 × 1.25 = 17.5 A. Ampacity: 2.5 sq.mm (20 A) fits. Voltage drop: V = 2 × 80 × 17.5 × 0.0175 / 2.5 = 19.6 V = 8.5% — FAILS. Jump to 6 sq.mm: V_drop = 8.2 V = 3.6% — acceptable. Submersible cable must be 6 sq.mm copper.
Standard Copper Cable Ampacity (BIS / IS 3961 indoor installation)
| Size (sq.mm) | Copper ampacity (A) | Aluminium ampacity (A) | Typical use |
|---|---|---|---|
| 0.75 | 7 | — | Lighting drops, control wiring |
| 1.0 | 11 | 9 | Light circuits (6A breaker) |
| 1.5 | 14 | 11 | General lighting (10A) |
| 2.5 | 20 | 16 | Power sockets, AC (16A) |
| 4.0 | 25 | 20 | Water heater, small motor (20A) |
| 6.0 | 32 | 25 | Kitchen, 25A load |
| 10 | 45 | 36 | Sub-main distribution |
| 16 | 63 | 51 | Main single-phase supply |
| 25 | 85 | 68 | 3-phase 3 HP motor |
| 35 | 100 | 80 | 3-phase 5 HP |
| 50 | 125 | 100 | 3-phase 7.5 HP |
| 70 | 160 | 130 | 3-phase 10 HP |
| 95 | 190 | 155 | 3-phase 15 HP |
| 120 | 225 | 180 | Industrial main |
Values for PVC-insulated cable in air at 30°C. Derate by 20–30% for cable in conduit or buried. Aluminium handles ~80% of copper's current — use 1.5× cross-section for the same load.
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