B Curve MCBs are the most common devices used in domestic installations - but plenty of installers still ask where B Curve is the correct choice and where other curves provide better protection. This guide gives you a fast, practical breakdown.
Quick Answer: A B Curve MCB trips between 3–5× its rated current, making it ideal for domestic lighting and socket circuits with low inrush loads. Choose B Curve when you’re protecting standard resistive or lightly inductive circuits. Switch to C or D Curve where motors, compressors or higher inrush loads are present.
Browse all options here: B Curve MCBs.
Jump to:
What a B Curve MCB Actually Does
Curve type defines how quickly an MCB trips when exposed to short-duration inrush currents. For B Curve devices, the magnetic trip point is:
- 3–5 × rated current (In)
This makes them ideal for domestic circuits that don’t experience high inrush loads. Think lighting, sockets, heating controls or standard single-phase appliances.
B Curve MCBs are designed for circuits with:
- Low fault-inrush currents
- Resistive loads (heating, immersion, small appliances)
- Light inductive loads (fans, small pumps)
Where B Curve MCBs Are the Correct Choice
Most domestic boards are built around B Curve devices because their trip profile matches the load behaviour found in homes and light commercial settings.
Use a B Curve MCB on:
- Lighting circuits (LED, halogen, CFL)
- Socket rings and radials
- Small appliance circuits
- Heating controls
- Boiler supplies without high inrush
These circuits don't pull enough inrush current to warrant a C or D Curve device, so B Curve gives the correct level of protection under BS 7671.
Where B Curve MCBs Aren’t Suitable
B Curve devices can nuisance-trip on circuits that naturally produce higher inrush currents. In these cases, a C or D Curve device prevents false tripping while still meeting disconnection requirements.
Don’t use a B Curve MCB for:
- Motors and compressors (HVAC, pumps, refrigeration)
- Large inductive loads
- Workshop tools (saws, drills, lathes)
- EV chargers depending on model/load behaviour
- Heat pump outdoor units with high inrush
These applications can briefly draw several times their nominal current on startup, which exceeds a B Curve’s magnetic trip threshold.
Explore alternatives here: MCBs – All Types & Curves
Installer-Favourite B Curve MCBs
These units are commonly used for domestic and light commercial circuits across the UK.
- 16A 2-Pole 10kA MCB – B Curve
- 6A 6kA MCB – 1 Pole B Curve
- 16A 6kA MCB – 1 Pole B Curve
- 10A 1-Pole 10kA MCB – B Curve
- 40A 4-Pole 10kA MCB – B Curve
Browse all options: B Curve MCBs
Choosing the correct curve type often leads installers to review other parts of the protection setup - not just the MCB itself. If you’re speccing a board or upgrading an existing installation, it’s worth double-checking the wider circuit protection stack. That includes a suitable enclosure, any meter or change-over isolation, the right RCBO or RCD, and whether surge protection is required on the install. For full system planning, you can also browse our circuit protection range, surge protection devices, or three-phase boards to compare options quickly.
FAQs
Installers ask the same key questions when choosing between B, C and D Curve devices. Here are the quick, practical answers.
What does “B Curve” actually mean?
It refers to the magnetic trip range - a B Curve device trips between 3–5× its rated current to protect circuits with low inrush.
Are B Curve MCBs still the standard for domestic use?
Yes. Most UK homes and small commercial installs use B Curve devices for lighting, sockets and general circuits.
Should I upgrade a B Curve to a C Curve?
Only if the circuit has higher inrush loads or you’re getting nuisance-tripping. Otherwise, B Curve is the correct choice.
Do EV chargers need B Curve or C Curve?
It depends on the charger design and inrush behaviour - always check manufacturer guidance.
👉 Need install-ready protection for domestic or light commercial circuits? Shop all B Curve MCBs