Calculator
⚡ MOTOR CABLE SIZE CALCULATOR ⚡
Professional Grade | ForumElectrical
| Cable Size (mm²) | Single Phase (A) | 3-Phase (A) | Power @ 400V (kW) |
|---|---|---|---|
| 1.5 | 13 | 16 | 11.1 |
| 2.5 | 17 | 20 | 13.8 |
| 4 | 24 | 29 | 20.1 |
| 6 | 31 | 39 | 27.0 |
| 10 | 43 | 53 | 36.7 |
| 16 | 57 | 70 | 48.4 |
| 25 | 77 | 95 | 65.7 |
| 35 | 100 | 125 | 86.6 |
| 50 | 130 | 160 | 110.8 |
| 70 | 170 | 210 | 145.4 |
| Motor Power (kW) | Motor Power (HP) | Phase Current @ 400V (A) | Recommended Cable Size (mm²) | Cable Amperage (A) |
|---|---|---|---|---|
| 1.5 | 2 HP | 2.6 | 1.5 | 16 |
| 3.7 | 5 HP | 6.4 | 2.5 | 20 |
| 5.5 | 7.5 HP | 9.5 | 4 | 29 |
| 7.5 | 10 HP | 13.0 | 4 | 29 |
| 11 | 15 HP | 19.0 | 6 | 39 |
| 15 | 20 HP | 26.0 | 10 | 53 |
| 22 | 30 HP | 38.0 | 10 | 53 |
| 30 | 40 HP | 52.0 | 16 | 70 |
| 37 | 50 HP | 64.0 | 25 | 95 |
| 45 | 60 HP | 78.0 | 25 | 95 |
| 55 | 75 HP | 95.0 | 35 | 125 |
| 75 | 100 HP | 130.0 | 50 | 160 |
| 90 | 125 HP | 156.0 | 70 | 210 |
| 110 | 150 HP | 190.0 | 95 | 270 |
| 150 | 200 HP | 260.0 | 150 | 350 |
| Phase Cable Size (mm²) | Minimum Earth Conductor (mm²) | Alternative if not available | Rating Category |
|---|---|---|---|
| 1.5 | 1.5 | 1.0 | Very Small |
| 2.5 | 2.5 | 1.5 | Small |
| 4 | 4 | 2.5 | Small |
| 6 | 6 | 4 | Medium |
| 10 | 10 | 6 | Medium |
| 16 | 16 | 10 | Large |
| 25 | 16 | 10 | Large |
| 35 | 16 | 10 | Very Large |
| 50 | 25 | 16 | Very Large |
| 70 | 35 | 25 | Extremely Large |
| 95 | 50 | 35 | Extremely Large |
| 120 | 70 | 50 | Extreme |
| 150 | 70 | 50 | Extreme |
| Cable Size (mm²) | Load Capacity (Watts) @ 230V 1-Phase | Load Capacity (Watts) @ 400V 3-Phase | Safe Amperage (A) | Continuous Rating |
|---|---|---|---|---|
| 1.5 | 2,990 | 11,055 | 13 | ✓ Good |
| 2.5 | 3,910 | 13,860 | 17 | ✓ Good |
| 4 | 5,520 | 20,130 | 24 | ✓ Excellent |
| 6 | 7,130 | 27,010 | 31 | ✓ Excellent |
| 10 | 9,890 | 36,770 | 43 | ✓ Excellent |
| 16 | 13,110 | 48,410 | 57 | ✓ Premium |
| 25 | 17,710 | 65,720 | 77 | ✓ Premium |
| 35 | 23,000 | 86,560 | 100 | ✓ Premium |
| 50 | 29,900 | 110,850 | 130 | ✓ Industrial |
| 70 | 39,100 | 145,420 | 170 | ✓ Industrial |
Where:
• P = Power in Watts
• V = Supply Voltage (400V typical)
• PF = Power Factor (0.85 typical)
• η = Motor Efficiency (0.90 typical)
• √3 = 1.732
Select cable where:
Icable ≥ Idesign
% Drop = (Vdrop / Vsupply) × 100
Where: ρ = 0.0175 (Copper), L = Distance (m), I = Current (A), A = Cable (mm²)
16 < S ≤ 35 → Se = 16
S > 35 → Se = S/2
- ✓ Follows IEC 60364 & IS 1866 International Standards
- ✓ De-rating factors applied for environmental conditions
- ✓ Safety factor of 1.25 included for cable protection
- ✓ Earth conductor sized per IEC 60364-5-54
- ✓ Acceptable voltage drop: 3-5% maximum
- ✓ Ideal voltage drop: 2-3%
- ✓ For motors: ≤ 5% at full load
- ✓ Always use 4-core cable (3 Phase + 1 Earth) for motors
- ✓ Ensure proper cable support and routing every 50cm
- ✓ Use appropriate cable glands and conduits with labels
- ✓ Install MCB/MCCB rated for cable amperage
- ✓ Add 10-20% margin for future load expansion
- ✓ Keep cables away from heat sources and direct sunlight
- ✓ Use proper earthing as per local electrical codes
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The selection of appropriate cable size for a three phase motors is one of the most important in electrical installation and design.
An undersized cable can lead to
• Excessive voltage drop,
• Motor overheating and
• Potential fire hazards
while an oversized cable results in unnecessary cost and installation challenges.
The motor cable size calculator is a professional tool designed to eliminate guesswork and provide accurate and standards-compliant cable recommendations based on the real world parameters and international electrical codes.
Improper cable sizing is responsible for approximately 30% of electrical installation failures in industrial settings.
The calculator helps prevent this by automating complex calculations based on IEC 60364 and IS 1866 standards.
Motor Current Calculations
Before selecting a cable you must first calculate the phase current that the motor will use to draw from the three phase (3 phase) supply.
This is the fundamental starting point for all cable sizing selections.
The phase current depends on several factors including
• Motor power rating (kW),
• Supply voltage (380V, 400V, 415V or 440V),
• Power factor and
• Motor efficiency.
Phase Current Formula
IPhase = P / (√3 X V X PF X η)
Where
• P – Motor Power in Watts
• √3 – 1.732 (three-phase/ 3 phase constant)
• V – Supply Voltage (400V typical)
• PF – Power Factor (0.85 typical for motors)
• η – Motor Efficiency (0.90 typical)
Eg: An 11 kW motor at 400V with 0.85 power factor and 0.90 efficiency draws approximately 19 amperes per phase. This calculation is automatic in the calculator but understanding it helps you to verify results and troubleshoot issues.
Function of Derating Factors
Real world installation conditions significantly impact on the cable amperage capacity.
Derating factors account for the environmental conditions that reduce a cables ability to dissipate heat.
The calculator includes four common installation conditions.
Open Air (1.0)
No de-rating applied and cable is freely exposed to atmosphere.
In Conduit/Duct (0.8)
20% reduction in ampacity due to the restricted heat dissipation.
Underground (0.7)
30% reduction due to the soil thermal resistance.
In Water (0.6)
For submerged installations (or) moist conditions there is a 40% reduction.
Application Example
A cable rated for the 70 amperes in open air can only safely carry
70 x 0.8 = 56 amperes
when installed in a conduit.
The calculator automatically applies these reductions when you select the installation method.
Safety Factor & Cable Selection
The calculator applies a safety factor (default 1.25) to the design current to ensure that the selected cable has adequate capacity for
• Normal operation,
• Temporary overloads and
• Aging.
This is a standard practice in electrical engineering that prevents the cables from operating at their absolute maximum capacity continuously.
The design current is calculated as
Design Current = Phase Current x Safety Factor / De-rating Factor
The recommended cable size is then selected as the smallest standard cable whose ampacity generally exceeds the design current.
Voltage Drop Considerations
Beyond ampacity, voltage drop is equally important.
Voltage drop occurs due to the resistance of the cable and increases with distance from the power source.
For motors the maximum acceptable voltage drop is typically at 5% at full load.
The calculator uses the formula
% Voltage Drop = (2 X ρ X L X I/A)/V X 100
Where
• ρ – 0.0175 (copper resistivity)
• L – Cable distance in meters
• I – Current in amperes
• A – Cable cross-section in mm²
• V – Supply voltage
If voltage drop exceeds the acceptable limits you should select a larger cable size.
This calculator automatically calculates this and displays the voltage drop percentage assist you to optimize cable selection.
Earth Conductor Sizing
The earth (ground) conductor is equally crucial as the phase conductors.
According to IEC 60364-5-54 earth conductor size depends on the phase cable size
• For phase cables up to 16mm² the earth conductor must be the same size,
• For cables between 16-35mm² use 16mm² earth and
• For cables larger than 35mm² use half the phase cable size.
Never undersize the earth conductor.
It should safely conduct the fault currents to a protective devices within milliseconds.
An undersized earth conductor can cause dangerous voltage levels on the equipment frames.
Cable Types and their Applications
The calculator supports different types of cable types which suited for various applications.
VIR (Vinyl Insulated) Cable
Basic & economical choice for dry indoor applications.
PVC Cable
Standard choice offering good insulation, flame resistance, and cost-effectiveness.
XLPE Cable
Superior choice with the higher temperature rating and better insulation properties.
Armoured Cable
For mechanical protection in the harsh environments (or) underground installations.
Quick Reference for Common Motor Sizes
| Motor Power (kW) | Motor HP | Phase Current (A) | Recommended Cable (mm²) | Earth Conductor (mm²) |
|---|---|---|---|---|
| 5.5 | 7.5 HP | 9.5 | 4 mm² | 4 mm² |
| 11 | 15 HP | 19.0 | 6 mm² | 6 mm² |
| 15 | 20 HP | 26.0 | 10 mm² | 10 mm² |
| 22 | 30 HP | 38.0 | 10 mm² | 10 mm² |
| 45 | 60 HP | 78.0 | 25 mm² | 16 mm² |
| 90 | 125 HP | 156.0 | 70 mm² | 35 mm² |
| 150 | 200 HP | 260.0 | 150 mm² | 70 mm² |
Industry Standard
The maximum acceptable voltage drop for motor circuits is 5% though 3% is ideal.
Always check the calculated voltage drop & use to consider the upgrading cable size if it exceeds 4%.
How to Calculate Cable Size in sq mm formula?
Copper Cable (IEC method)
Cable Size (mm²) = I/J
Where
I – Load current (A)
J – Current density
Note
Copper ≈ 5 – 6 A/mm²
Aluminium ≈ 3 – 4 A/mm²
3 Phase Motor Current (to find I first)
I= P x 1000 / √3 x V x η x cosϕ
Where
Copper Cable size ≈ I/5
Aluminium Cable size ≈ I/3
Always check voltage drop, installation method and standards before final selection.
Conclusion
The Motor Cable Size Calculator is an indispensable tool for the electrical professionals, engineers and technicians.
If you are sizing cables for a small 5.5 kW pump (or) a large 150 kW industrial motor the calculator provides accurate recommendations considering all essential parameters:
• Motor power,
• Supply voltage,
• Power factor,
• Installation method,
• Safety factors and
• Voltage drop.
This calculator is based on IEC 60364 and IS 1866 standards and is for reference purposes. Always consult local electrical codes and qualified electrical engineers before finalizing cable sizes for actual installations.