Cable Sizing Calculation for Motor Starter

Cable Sizing Formula:

\[ A = \frac{I_{start} \times L \times 2 \times \rho}{V_d} \]

Starting Current (I_start):

Amps

Cable Length (L):

Meters

Resistivity (ρ):

Ω·m

Allowed Voltage Drop (V_d):

Volts

Required Cable Cross-Section Area (A):

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1. What is the Cable Sizing Formula?

The cable sizing formula calculates the minimum cross-sectional area required for motor starter cables considering starting current, cable length, material resistivity, and allowed voltage drop. Proper sizing ensures safe operation and prevents excessive voltage drops.

2. How Does the Calculator Work?

The calculator uses the cable sizing equation:

\[ A = \frac{I_{start} \times L \times 2 \times \rho}{V_d} \]

Where:

\( A \) — Cable cross-sectional area (mm²)
\( I_{start} \) — Motor starting current (Amps)
\( L \) — Cable length (Meters)
\( \rho \) — Resistivity of conductor material (Ω·m)
\( V_d \) — Allowed voltage drop (Volts)

Explanation: The formula accounts for the voltage drop during motor starting, which is typically the most demanding condition for cable sizing.

3. Importance of Proper Cable Sizing

Details: Correct cable sizing prevents overheating, ensures proper motor starting, maintains voltage stability, and complies with electrical codes. Undersized cables can cause safety hazards and equipment malfunction.

4. Using the Calculator

Tips:

Starting current is typically 5-7 times the motor's full load current
Default resistivity is for copper (0.0000000172 Ω·m)
Typical allowed voltage drop is 3-5% of nominal voltage
For aluminum conductors, use resistivity of 0.0000000282 Ω·m

5. Frequently Asked Questions (FAQ)

Q1: Why multiply length by 2 in the formula?
A: The factor of 2 accounts for both the outgoing and return paths in the circuit (total conductor length).

Q2: What's the typical resistivity value for copper?
A: Copper resistivity is approximately 0.0000000172 Ω·m at 20°C. For aluminum, it's about 0.0000000282 Ω·m.

Q3: How does temperature affect cable sizing?
A: Higher temperatures increase resistivity. For precise calculations, use resistivity values at operating temperature.

Q4: What's an acceptable voltage drop?
A: Typically 3-5% of nominal voltage during motor starting. Check local electrical codes for specific requirements.

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