What cable size for 3 HP motor?

3 HP 3-phase motor draws ~4.5 A at 415 V. Design current with 1.25× starting margin = 5.6 A. Use 1.5 sq.mm copper (handles 14 A) for short runs; 2.5 sq.mm for runs over 30 m to control voltage drop.

What is voltage drop and why does it matter?

Voltage drop is the voltage lost in the cable itself due to conductor resistance. For motors, >5% drop reduces starting torque and can cause stalling. For lighting, >3% drop causes visible flicker. Always check voltage drop in addition to ampacity, especially for long runs.

Is copper or aluminium better for industrial cabling?

Copper. Lower resistance (same ampacity in smaller cross-section), more reliable crimped connections, no galvanic corrosion at joints. Aluminium is 40% cheaper for the same ampacity but needs larger cross-section and purpose-made connectors. For industrial and factory use, always prefer copper.

What is BIS / IS 3961?

Indian Standard IS 3961 (and 694) specifies ampacity and derating factors for PVC and XLPE insulated cables. Every ISI-marked cable in India follows these standards. When sizing, always use the ampacity table matching your installation type (air, conduit, buried, or tray).

How do I verify my cable installation is safe?

Use a Fluke or similar clamp meter to measure actual running current under load. Check that it stays below the cable's rated ampacity. Also measure voltage at both source and load — voltage drop should be under 5% for motors and under 3% for lighting.

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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

Quick Overview

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

Load current(करंट)

Cable length (one-way)(लंबाई)

Voltage(वोल्टेज)

Phase(फेज़)

Conductor(कंडक्टर)

Max voltage drop(voltage drop)

3% lighting · 5% motor

Required size

2.5sq.mm

Ampacity

20A

Voltage drop

5.8 V(1.4%)

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 or buried.

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ƒFormula

Ampacity: pick from table (BIS / IS 3961)    |    V_drop (1φ) = 2 × L × I × ρ / A    |    V_drop (3φ) = √3 × L × I × ρ / A

A = 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.

उदाहरण / Example: Example: 2 HP motor (approx 4 A at 415 V 3-phase), 40 meter distance. Ampacity requirement: 1.5 sq.mm (handles 14 A). Voltage drop check: V_drop = √3 × 40 × 4 × 0.0175 / 1.5 = 3.23 V = 0.78%, acceptable. 1.5 sq.mm copper cable sufficient.

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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Frequently Asked Questions: Cable Size Calculator (Electrical)

32 A load ke liye 6 sq.mm copper cable ideal hai (rated 32 A). Aluminium ho to 10 sq.mm chahiye. Long distance hai (30m+) toh voltage drop check zaroor karein, may need to go 1 size up.

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