Motor Breaker Size Calculator
Determine the correct circuit breaker or fuse size for your motor applications.
Motor Breaker Size Calculator
Compliant with NEC Standards
Results:
Calculated Breaker Amps: 0.00
Recommended Breaker Size: 0 A
How to Use the Calculator
Input Motor FLA
- Enter the Full Load Amps (FLA) of your motor.
- This value is usually found on the motor's nameplate.
Select Service Factor
- Choose the Service Factor (SF) from the dropdown.
- Common values are 1.0, 1.15, or 1.25.
Choose Breaker Type
- Select the type of overcurrent protective device.
- Options include Inverse Time Breaker, Non-Time Delay Fuse, or Time Delay Fuse.
Motor Protection Fundamentals
Overcurrent Protection
Motors require protection against:
- Overload (sustained excessive current)
- Short-circuit (fault current)
- Ground-fault (current to ground)
Breaker Types
- Inverse Time Breaker: Trips faster with higher current, suitable for motor protection.
Key Principle: The protective device must be sized to carry the motor's starting current without tripping, but still protect against sustained overloads and short circuits.
Calculation Formulas
Calculated Breaker Amps =
Motor FLA × Service Factor × Multiplier
Multiplier Values (NEC Guidelines):
- Inverse Time Breaker: 2.5 (250%)
Recommended Breaker Size =
Next Standard Size ≥ Calculated Breaker Amps
Example Calculation
For a motor with:
Motor FLA: 50 A
Service Factor: 1.15
Breaker Type: Inverse Time Breaker
Calculated Breaker Amps = 50 A × 1.15 × 2.5 = 143.75 A
Next Standard Breaker Size (from 143.75 A) = 150 A
Result: Recommended Breaker Size = 150 A
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