Fuse Size Calculator
Determine the appropriate fuse size for continuous and motor loads
Fuse Size Calculator
Compliant with NEC Guidelines
Results:
Recommended Time-Delay Fuse (A): 0.00
Recommended Non-Time-Delay Fuse (A): 0.00
Notes:
How to Use the Calculator
AC Continuous Loads
- Select "AC" as System Type and "Continuous Load" as Load Type.
- Enter the continuous operating current in Amps.
- Click "Calculate" to get the recommended fuse sizes.
- This applies to resistive loads, lighting, heating, etc.
AC Motor Loads (Inductive)
- Select "AC" as System Type and "Motor Load" as Load Type.
- Enter the Motor Full Load Current (FLC) from the motor nameplate.
- Click "Calculate" for the recommended fuse sizes (both Time-Delay and Non-Time-Delay).
DC Loads (All Types)
- Select "DC" as System Type. The Load Type field will hide.
- Enter the load current (continuous or motor FLC) in Amps.
- Click "Calculate" for the recommended fuse sizes (both Time-Delay and Non-Time-Delay).
- Always verify calculations with local electrical codes (e.g., NEC).
Fuse Sizing Fundamentals
Continuous Loads
For continuous loads (operating for 3 hours or more), the overcurrent protection device must be sized at 125% of the continuous load current for AC systems [^NEC_210.20(A)].
Motor Loads
- Motors have high inrush currents during starting.
- For AC motors, Time-Delay fuses are typically sized at 300% of FLC, while Non-Time-Delay fuses are at 175% [^NEC_430.52].
- For DC loads (both continuous and motor), a 150% factor is commonly applied for both fuse types.
- Fuses are sized higher than FLC to prevent nuisance tripping.
Key Sizing Rules:
AC Continuous Load: Fuse Size = Current × 1.25
AC Motor (Time-Delay Fuse): Fuse Size = Motor FLC × 3.00
AC Motor (Non-Time-Delay Fuse): Fuse Size = Motor FLC × 1.75
DC Loads (All Types): Fuse Size = Load Current × 1.50
Calculation Formulas
AC Continuous Load:
Fuse Size (A) = Current (A) × 1.25
AC Motor Load (Time-Delay Fuse):
Fuse Size (A) = Motor FLC (A) × 3.00
AC Motor Load (Non-Time-Delay Fuse):
Fuse Size (A) = Motor FLC (A) × 1.75
DC Loads (All Types):
Fuse Size (A) = Load Current (A) × 1.50
Example Calculation
AC Motor Load Example:
Motor FLC: 20 Amps
System Type: AC
Time-Delay Fuse Size = 20A × 3.00 = 60 Amps
Non-Time-Delay Fuse Size = 20A × 1.75 = 35 Amps
Recommended Time-Delay Fuse: 60A
Recommended Non-Time-Delay Fuse: 35A
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