How to Select Proper Cable Size for Motors – With Calculation

Choosing the correct cable size for motors is essential for efficiency, safety, and long-term reliability. An undersized cable can overheat and fail, while an oversized cable increases project costs unnecessarily. In this guide, we explain step-by-step how to calculate the proper cable size for motors.

Factors Affecting Motor Cable Size Selection

• Motor Power Rating (kW/HP)
• Full Load Current (FLC) based on voltage & efficiency
• Cable Length (distance between motor & panel)
• Voltage Drop Limit (usually 3-5%)
• Short Circuit Rating
• Ambient Temperature & Installation Method

Step-by-Step Cable Size Calculation

1. Find Motor Current: I = P / (√3 × V × PF × η)
2. Select Cable Based on Current: Refer to IEC/NEC standard cable charts.
3. Check Voltage Drop: ΔV = (√3 × I × L × R) / 1000
4. Verify Short Circuit Withstand: Ensure cable survives short circuit until breaker trips.
5. Final Selection: Choose next standard cable size above the calculated value.

Example

For a 15 kW, 415V, 3-phase motor, efficiency 90%, PF 0.85, length 50m:

I = 15000 / (1.732 × 415 × 0.85 × 0.9) ≈ 27 A → Suitable cable: 4 sq.mm Cu or 6 sq.mm Al (check voltage drop).

FAQs

1. What happens if motor cable is undersized?

It overheats, causes power loss, insulation failure, and may trip breakers frequently.

2. Which is better: copper or aluminum cable for motors?

Copper has higher conductivity and is more compact, but aluminum is cost-effective for long distances.

3. What is the allowable voltage drop for motor cables?

Usually 3% for motors, though IEC and NEC standards allow up to 5% for total system voltage drop.