Motor circuit breaker sizing calculator

A motor circuit breaker sizing calculator is a tool used to determine the correct size of a circuit breaker for a specific motor circuit. Here's a step-by-step guide to help you use a motor circuit breaker sizing calculator:

Inputs needed:

1. Motor nameplate information:
   • Motor horsepower (HP)
   • Motor voltage (V)
   • Motor current (A)
   • Motor power factor (PF)
2. Circuit characteristics:
   • Circuit voltage (V)
   • Circuit current (A)
   • Circuit length (ft)
   • Cable size (AWG or mm²)
3. Environmental factors:
   • Ambient temperature (°F or °C)
   • Altitude (ft)

Calculation steps:

1. Calculate the motor full-load current (Ifl) using the motor nameplate information:
   • Ifl = Motor HP x 746 / (Motor voltage x Motor power factor)
2. Calculate the circuit current (Ic) using the circuit characteristics:
   • Ic = Motor full-load current x (1 + 0.05 x Cable size)
3. Calculate the circuit voltage drop (Vd) using the circuit characteristics:
   • Vd = Circuit voltage x (0.02 x Cable size)
4. Calculate the circuit fault current (If) using the circuit characteristics:
   • If = Ic + (Vd / Circuit voltage)
5. Determine the required circuit breaker size (CB) using the motor circuit breaker sizing calculator:
   • CB = If x (1 + 0.05 x Cable size)

Example calculation:

Suppose we have a 10 HP motor with a nameplate voltage of 230V, power factor of 0.8, and a cable size of 14 AWG (2.5 mm²). The circuit voltage is 230V, and the circuit length is 100 ft.

1. Calculate the motor full-load current:
   • Ifl = 10 HP x 746 / (230V x 0.8) = 34.4 A
2. Calculate the circuit current:
   • Ic = 34.4 A x (1 + 0.05 x 14 AWG) = 37.2 A
3. Calculate the circuit voltage drop:
   • Vd = 230V x (0.02 x 14 AWG) = 6.52 V
4. Calculate the circuit fault current:
   • If = 37.2 A + (6.52 V / 230V) = 38.1 A
5. Determine the required circuit breaker size:
   • CB = 38.1 A x (1 + 0.05 x 14 AWG) = 40.5 A

In this example, the required circuit breaker size would be a 40-amp circuit breaker.

Important notes:

• The motor circuit breaker sizing calculator is a simplified tool and should be used as a guide only. It's essential to consult the motor nameplate and circuit characteristics to ensure accurate calculations.
• The calculator assumes a standard circuit configuration and may not account for unusual or complex circuit configurations.
• It's recommended to consult with a licensed electrician or electrical engineer for complex or critical applications.

You can find online motor circuit breaker sizing calculators or download software tools that can perform these calculations for you. Some popular options include:

• National Electric Code (NEC) calculator
• Motor circuit breaker sizing calculator by Square D
• Electrical calculator by Schneider Electric
• Motor circuit breaker sizing software by ETAP