Circuit Protection Selector
MCB, RCBO & Fuse Rating Calculator to BS 7671
Type B: Trips between 3x and 5x In. Ideal for domestic use.
Ensure your selected cable capacity (Iz) is greater than the -- device selected above.
Selecting the Correct Protective Device (MCB, RCBO, Fuse)
Choosing the correct overcurrent protective device is a fundamental step in electrical design. Under BS 7671, the device must protect the circuit conductors from both overloads and short circuits while preventing nuisance tripping during normal operation.
The Ib ≤ In ≤ Iz Rule
Every electrical designer must satisfy this core equation from Regulation 433.1.1:
- Ib (Design Current): The actual current demand of the load. The protective device must be larger than this to avoid tripping under normal use.
- In (Nominal Rating): The rated current of the MCB, RCBO, or Fuse.
- Iz (Cable Capacity): The current-carrying capacity of the cable after applying correction factors (Ca, Cg, Ci). The cable must be larger than the device to ensure the device trips before the cable melts.
MCB & RCBO Trip Curves (Types B, C, and D)
Standard circuit breakers are classified by their "Instantaneous Trip" characteristics. Selecting the wrong type can lead to breakers "popping" the moment a motor or LED array is turned on:
- Type B: Designed to trip between 3 and 5 times its rated current. Used for domestic lighting and socket circuits where inrush currents are low.
- Type C: Trips between 5 and 10 times its rated current. Highly recommended for commercial lighting (LED drivers), small motors, and industrial cooling fans.
- Type D: Trips between 10 and 20 times its rated current. Used for high-inrush equipment like large industrial transformers, X-ray machines, and heavy-duty motors.
Standard UK Device Ratings
Most manufacturers produce MCBs and RCBOs in standard increments. This tool automatically rounds up your design current to the nearest standard UK rating: 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, 125A.