Cable Sizing Calculation for Motor

Cable Size Formula:

\[ A_w = \frac{I \times L \times K}{V_{drop} \times \sigma} \]

Current (I):

amps

Cable Length (L):

feet

Correction Factor (K):

Voltage Drop (V_drop):

volts

Conductivity (σ):

Ω⁻¹·mm²/m

Cable Size (A_w):

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1. What is Cable Sizing Calculation for Motor?

Definition: This calculator determines the appropriate cable cross-sectional area (A_w) for motor circuits based on current, cable length, correction factors, allowable voltage drop, and conductor conductivity.

Purpose: Ensures proper cable selection to prevent excessive voltage drop, overheating, and energy loss in motor installations.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ A_w = \frac{I \times L \times K}{V_{drop} \times \sigma} \]

Where:

\( A_w \) — Cable cross-sectional area (mm²)
\( I \) — Current (amps)
\( L \) — Cable length (feet)
\( K \) — Correction factor (typically 1.25 for motors)
\( V_{drop} \) — Allowable voltage drop (volts)
\( \sigma \) — Conductivity of copper (58 Ω⁻¹·mm²/m at 20°C)

Explanation: The formula calculates the minimum cable size needed to maintain voltage drop within acceptable limits while carrying the required current.

3. Importance of Proper Cable Sizing

Details: Correct cable sizing prevents voltage drop issues, reduces energy loss, minimizes heat generation, and ensures motor performance and longevity.

4. Using the Calculator

Tips: Enter the motor current, cable length, correction factor (default 1.25), allowable voltage drop (default 3V), and conductivity (default 58 for copper). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical voltage drop allowance?
A: For motors, 3% of supply voltage is common (3V for 100V, 7.2V for 240V systems).

Q2: When should I adjust the correction factor?
A: Increase K for high ambient temperatures, bundled cables, or frequent start/stop cycles.

Q3: What conductivity value should I use for aluminum cables?
A: Use approximately 35 Ω⁻¹·mm²/m for aluminum conductors.

Q4: How do I find the motor current?
A: Check motor nameplate or use: I = P / (V × η × PF), where P is power, V is voltage, η is efficiency, PF is power factor.

Q5: Should I round up the calculated cable size?
A: Yes, always select the next standard cable size larger than the calculated value.