Motor Overload Setting Calculator
Determine the optimal overload relay setting for your motor
Motor Overload Setting Calculator
Compliant with NEC Guidelines
Results:
Recommended Overload Setting (A): 0.00
How to Use the Calculator
Step-by-Step Guide
- Enter the motor's Full Load Amps (FLA) from its nameplate.
- Select the motor's Service Factor (SF) from the dropdown.
- (Optional) Input the ambient temperature of the installation site.
- Click "Calculate" to get the recommended overload setting.
- Click "Reset" to clear all fields and start over.
Understanding Inputs
- Full Load Amps (FLA): The current drawn by the motor at its rated horsepower and voltage.
- Service Factor (SF): A multiplier indicating how much overload a motor can safely handle.
- Ambient Temperature: The temperature of the air surrounding the motor and its controls.
Important Notes
- Always refer to the motor nameplate for accurate FLA and SF values.
- NEC guidelines are general; local codes may have specific requirements.
- Consider temperature-compensated relays for extreme ambient conditions.
Motor Overload Fundamentals
Motor Service Factor (SF)
The service factor (SF) of a motor defines how much overload it can handle safely beyond its rated full-load current. It is typically indicated on the motor nameplate.
- Example: A motor with a 1.15 SF can safely handle 15% more current than its full load amperage (FLA).
- Motors with higher SF values can tolerate slightly higher overload relay settings.
Ambient Temperature Adjustment
- The standard ambient temperature for overload relays is 40°C (104°F).
- Hot Environments (Above 40°C): Overload relays must be derated to avoid false tripping due to excessive heat. Solution: Use temperature-compensated relays or adjust the trip setting downward.
- Cold Environments (Below 40°C): Relays may not trip at the expected current level, increasing the risk of overheating. Solution: Choose relays with thermal compensation.
NEC Guidelines for Overload Relay Settings:
125% of full-load current for motors with SF = 1.15
115% of full-load current for motors with SF = 1.0
Calculation Formulas
For Motor with SF = 1.15:
Overload Setting (A) = FLA × 1.25
For Motor with SF = 1.0:
Overload Setting (A) = FLA × 1.15
Example Calculation
Motor with SF = 1.15:
Motor Full Load Amps (FLA): 50 A
Motor Service Factor (SF): 1.15
Recommended Overload Setting = 50 A × 1.25 = 62.5 A
Motor with SF = 1.0:
Motor Full Load Amps (FLA): 50 A
Motor Service Factor (SF): 1.0
Recommended Overload Setting = 50 A × 1.15 = 57.5 A
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