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3 Phase Motor Breaker Size Calculator

Reviewed by Calculator Editorial Team

Properly sizing a breaker for a 3-phase motor is crucial for electrical safety and equipment protection. This calculator helps determine the appropriate breaker size based on motor specifications and safety factors.

Introduction

When selecting a breaker for a 3-phase motor, several factors must be considered to ensure safe operation and compliance with electrical codes. The breaker size must accommodate the motor's full-load current, account for inrush current, and consider protection against short circuits and ground faults.

This calculator provides a straightforward method to determine the appropriate breaker size based on the motor's horsepower, voltage, and efficiency. It follows industry standards and safety factors to ensure reliable results.

How to Use This Calculator

  1. Enter the motor's horsepower (HP) in the first field.
  2. Select the motor's voltage from the dropdown menu.
  3. Choose the motor's efficiency from the dropdown menu.
  4. Click the "Calculate" button to determine the recommended breaker size.
  5. Review the result and any additional information provided.

Important Notes

Always consult with a qualified electrician before making any electrical modifications. This calculator provides estimates based on standard assumptions and may not account for all specific conditions of your installation.

Formula

The calculation follows these steps:

  1. Calculate the motor's full-load current using the formula:
    I = (HP × 746) / (√3 × V × PF × E)
    Where:
    • I = Full-load current (amps)
    • HP = Horsepower (horsepower)
    • V = Voltage (volts)
    • PF = Power factor (assumed to be 0.85 for induction motors)
    • E = Efficiency (as a decimal)
  2. Apply a safety factor (typically 1.25) to account for inrush current and protection against short circuits.
  3. Round up to the nearest standard breaker size.

Worked Example

Let's calculate the breaker size for a 10 HP, 460V motor with 90% efficiency:

  1. Calculate the full-load current:
    I = (10 × 746) / (√3 × 460 × 0.85 × 0.9) ≈ 11.8 amps
  2. Apply the safety factor:
    11.8 × 1.25 ≈ 14.75 amps
  3. Round up to the nearest standard breaker size: 15 amps.

The calculator would recommend a 15-amp breaker for this motor.

Frequently Asked Questions

Why is the breaker size larger than the motor's full-load current?
The breaker size is increased to account for inrush current during startup and to provide protection against short circuits and ground faults.
What happens if I use a breaker that's too small?
A breaker that's too small can cause overheating, damage to the motor, and potential fire hazards. Always use a breaker that's properly sized for the motor.
Can I use this calculator for single-phase motors?
No, this calculator is specifically designed for 3-phase motors. Different formulas apply to single-phase motors.
What safety factors should I consider?
The calculator uses a standard safety factor of 1.25. However, you may need to adjust this based on specific conditions or local electrical codes.
How often should I check my motor's breaker size?
It's a good practice to review your motor's breaker size periodically, especially after any electrical modifications or if you notice unusual operation.