Breaker Size Calculator UK

Breaker Size Formula:

\[ \text{Breaker Size (A)} = \frac{I}{\cos \phi} \times \text{Safety Factor} \]

Recommended Breaker Size:

Calculated Value:

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1. What is Breaker Size Calculation?

The breaker size calculation determines the appropriate circuit breaker rating for an electrical circuit based on the expected current load, power factor, and safety considerations. Proper breaker sizing is crucial for electrical safety and compliance with UK regulations.

2. How Does the Calculator Work?

The calculator uses the breaker size formula:

\[ \text{Breaker Size (A)} = \frac{I}{\cos \phi} \times \text{Safety Factor} \]

Where:

\( I \) — Current in amperes (A)
\( \cos \phi \) — Power factor (dimensionless, typically 0.8-1.0)
Safety Factor — Typically 1.25-1.5 to account for inrush currents and other variables

Explanation: The calculation accounts for the apparent current (considering power factor) and applies a safety margin to ensure reliable operation.

3. Importance of Correct Breaker Sizing

Details: Proper breaker sizing is critical for:

Preventing electrical fires by avoiding overloaded circuits
Ensuring equipment protection
Complying with UK wiring regulations (BS 7671)
Providing adequate fault protection

4. Using the Calculator

Tips:

Enter the expected current in amperes
For resistive loads (heaters, incandescent lights), use power factor = 1.0
For inductive loads (motors, transformers), typical power factor is 0.8
Standard safety factor is 1.25 for general circuits, 1.5 for motors

5. Frequently Asked Questions (FAQ)

Q1: What are standard UK breaker sizes?
A: Common UK breaker sizes are 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, and 125A.

Q2: How does power factor affect breaker size?
A: Lower power factor means higher apparent current for the same real power, requiring a larger breaker.

Q3: What safety factor should I use?
A: 1.25 is typical for general circuits, while 1.5 is recommended for motors due to startup currents.

Q4: Can I use a larger breaker than calculated?
A: Never exceed the cable's current rating. The breaker protects the cable, not the appliance.

Q5: Does this calculator comply with UK regulations?
A: This provides general guidance, but final design should be verified against BS 7671 by a qualified electrician.