A Complete Guide on How to Choose the Correct RCBO for a Domestic Circuit
Table of contents
If you’re wondering what size RCBO do I need for your home, or you’re unsure how to choose the correct RCBO for a domestic circuit, you’re in the right place. In this RCBO sizing guide, we’ll explain how to match breaker ratings to real-world domestic circuits using BS 7671.
Whether you’re a new electrician, a homeowner planning an upgrade, or replacing an old fuseboard, selecting the right RCBO isn’t guesswork — it’s a critical safety decision based on load, cable size, and BS 7671 regulations.
This guide gives you everything you need to know, including real examples, approved cable ratings, and guidance for both RCBO sizing and consumer unit upgrades.
What Is an RCBO, and Why Is It Important?
An RCBO (Residual Current Breaker with Over-Current) combines two essential functions:
- Overcurrent protection – Like an MCB, it protects against overload and short circuit.
- Residual current protection – Like an RCD, it trips on earth leakage faults.
This guide assumes you’re selecting RCBOs for domestic consumer units — either for new circuits or while replacing old fuseboards.
Choosing the Correct RCBO for a Domestic Circuit
When people ask what size RCBO do I need, the correct answer depends on three core elements:
- ✅ The design current of the load
- ✅ The cable’s current-carrying capacity
- ✅ The installation method (how the cable is run — e.g., buried, bunched, clipped)
Step-by-Step: RCBO Sizing Explained
Step 1: Calculate the Design Current (Ib)
Use the formula:
Ib = P / V
Example:
7000W shower / 230V = 30.43A
Step 2: Choose an RCBO with Headroom
Apply a 125% safety factor:
30.43A × 1.25 = 38.04A
→ You’d select a 40A RCBO
Step 3: Match the Cable Size and Install Method
Use Appendix 4 of BS 7671 to confirm that the cable used (e.g., 6mm², 10mm²) is capable of carrying that current based on how it’s installed.
Improper pairing = serious safety risk.
⚠️ Never “Just Use a Bigger Breaker”
One of the most dangerous myths in home electrics is:
⚠️ Absolutely not!
If your 2.5mm² radial socket circuit was designed for a 20A RCBO, and you replace it with a 32A RCBO:
- The cable could overheat before the breaker trips
- You’re breaking Regulation 433.1.1
- You risk a fire hazard and invalidating your insurance
Even if it’s “been working for years,” that’s no guarantee it’s been safe.
What If You’re Replacing an Old Fuseboard?
Before installing a new consumer unit, a qualified electrician must perform an EICR (Electrical Installation Condition Report). This will determine if:
- Circuits are still suitable for their original breaker size
- Ring continuity is intact (essential for 32A circuits)
- Any circuits need downgrading to 20A or 16A radials
RCBO Sizes & Common Domestic Applications
| RCBO Size | Used For |
|---|---|
| 6A | – Lighting circuits- Smoke detectors (BS 5839-6 compliant) |
| 10A | – Larger lighting setups- Intruder alarm panels and power feeds |
| 16A | – Immersion heaters- Electric underfloor heating (zones)- Socket radials- Reduced-load rings (post-EICR) |
| 20A | – Larger socket radials- Ovens & washing machines (on dedicated circuits)- Electric underfloor heating (whole zones)- Downgraded ring finals |
| 32A | – Ring final socket circuits (fully tested)- Cookers- Some small EV chargers (7.2kW) |
| 40A | – Electric showers, high-demand cookers- Larger EVs |
| 45A–50A | – Hot tubs, saunas, commercial cooking gear |
Frequently Asked Questions (RCBO Sizing & Use)
⚠️ Always confirm with proper circuit design and cable sizing.
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I found this site so helpful as I’m soon to help my daughter out with a small update. She needs a double socket next to the consumer unit under the stairs at the same time I thought I’d drill the outside wall and give her a double waterproof socket, and was wondering what rating to fit e.g. 16 or 20 amp breaker , but as it will never be a high demand it seems 16amp will be the best option. Any comments welcome. Thank you
Thanks for your message — just to clarify, we can’t advise on the exact breaker size, but the general principle for any circuit is:
Decide what the circuit will feed and the current it needs.
Select the correct cable and determine how it will be installed (clipped direct, in trunking, or surrounded by insulation) so you know its true current-carrying capacity.
Once the cable size and capacity are known, you choose the appropriate overcurrent device to protect that cable.
Only a qualified electrician on site can confirm the correct setup, but the above is the basic method used when designing any circuit.
Your quote “Some EVs (up to 7.2kW) can use 32A RCBOs. Higher-demand chargers (11kW+) may require 40A — confirm with charger specs and cable route.”
Please note 7.2kW charging is single phase ie 32 amp. 11kW charging is a 3 phase circuit ie 3.6kW per phase.
22kW charging is 7.2kW per phase ie 32 amp per phase. Last I heard, due to likely several hours of continuous charging cable current rating is downrated by 20% , suggesting 6mm conductors are recommended.
Agreed on continuous load considerations. That’s also why most EV consumer units are supplied with 40 A RCBOs even for 32 A chargers, and why 6 mm² is generally treated as the practical minimum, with final sizing based on route and voltage drop.