UK Circuit Breaker Types — MCB, RCD, RCBO
MCB types B, C and D, RCD types A, AC and F, RCBO specifications and ratings from 6A to 63A per BS 7671.
MCB Trip Curves (BS EN 60898)
| Type | Magnetic Trip Range | Application | Examples |
|---|---|---|---|
| B | 3–5 x In | Domestic, commercial lighting | Sockets, lighting, immersion heater |
| C | 5–10 x In | Moderate inrush loads | Fluorescent banks, small motors, UPS |
| D | 10–20 x In | High inrush loads | Welders, transformers, X-ray, compressors |
Common MCB Ratings (Domestic)
| Rating (A) | Type | Typical Circuit | Cable Size |
|---|---|---|---|
| 6 | B | Lighting (single circuit) | 1.0 mm² or 1.5 mm² |
| 10 | B | Lighting (larger circuit) | 1.0 mm² or 1.5 mm² |
| 16 | B | Immersion heater, towel rail | 2.5 mm² |
| 20 | B | Radial socket circuit (small) | 2.5 mm² |
| 32 | B | Ring final circuit (sockets) | 2.5 mm² |
| 40 | B | Cooker circuit (small) | 6.0 mm² |
| 45 | B | Cooker circuit, shower (9.5 kW) | 6.0 mm² or 10 mm² |
| 50 | B | Large shower (10.8 kW+), cooker | 10 mm² |
RCD Types (BS EN 61008 / 61009)
| Type | Detects | Requirement | Application |
|---|---|---|---|
| AC | AC sinusoidal only | Basic (not for all loads) | Simple resistive loads only |
| A | AC + pulsating DC | Minimum for most circuits | Standard domestic circuits |
| F | AC + pulsating DC + high frequency | For VFD/inverter loads | EV chargers, VFD washing machines |
| B | AC + pulsating DC + smooth DC | 3-phase and DC applications | Some EV chargers, 3-phase supplies |
RCD Ratings
| Sensitivity (mA) | Purpose | Application |
|---|---|---|
| 30 | Personal protection (lethal shock) | All socket circuits, bathrooms, outdoor |
| 100 | Fire protection | Fire risk premises, sleeping accommodation |
| 300 | Fire protection | Fire protection on distribution boards |
Related Calculators
See the BS 7671 Circuit Guide for full domestic circuit specifications, or the UK Cable Sizes for cable current ratings.
How We Calculate This
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.