C Curve MCBs are designed to handle the surge currents that motors, compressors and workshop tools produce on startup - something B Curve devices can’t always cope with. If you’re getting nuisance-tripping or protecting inductive loads, C Curve is almost always the better choice.
Quick Answer: C Curve MCBs trip at 5–10× their rated current, making them ideal for high-inrush equipment such as motors, compressors, pumps, HVAC units and workshop tools. Use a C Curve whenever startup current is high enough to cause false trips on a B Curve device.
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What C Curve MCBs Are Designed For
The key difference is the magnetic trip point. While B Curve devices trip between 3–5× rated current, a C Curve MCB won’t trip until the load hits:
- 5–10 × rated current (In)
This higher threshold prevents nuisance-tripping during startup surges - common on circuits with motors, inductive equipment, solenoids or any machinery with moving parts.
C Curve is the right choice where you need:
- Reliable protection under high inrush
- Reduced false-trips on motor loads
- Correct disconnection times even with inductive behaviour
- Compatibility with HVAC, pumps and machinery
Examples of High-Inrush Loads
These loads naturally pull several times their nominal current for a fraction of a second on startup. That’s exactly what C Curve devices are built to handle.
- Heat pump compressors
- Air-conditioning units
- Workshop tools – table saws, drills, grinders
- Large extractor fans
- Circulation and booster pumps
- Industrial motors and conveyor machinery
- Hydraulic pumps and solenoids
If you’re getting unexplained B Curve trips on these circuits, a C Curve device normally resolves it.
B Curve vs C Curve - The Practical Difference
Both protect the circuit from overload and short-circuit faults. The difference is entirely in how they behave during those first milliseconds of inrush.
| Curve Type | Magnetic Trip Point | Best For |
|---|---|---|
| B Curve | 3–5× In | Domestic lighting, sockets, low-inrush circuits |
| C Curve | 5–10× In | Motors, compressors, pumps, HVAC, tools |
If your circuit has anything that spins, pumps, compresses or drags a load, C Curve is almost always the correct fit under BS 7671.
Installer-Favourite C Curve MCBs
These units are widely used for HVAC, pumps, tools and three-phase machinery where startup surges are unavoidable.
- 100A 4-Pole 10kA MCB – C Curve
- 40A 6kA MCB – 1 Pole C Curve
- 16A 6kA MCB – 1 Pole C Curve
- 20A 2-Pole 10kA MCB – C Curve
- 10A 1-Pole 10kA MCB – C Curve
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FAQs
Installers often have the same questions when deciding between B Curve and C Curve devices. These quick answers cover the key points.
When should I choose a C Curve over a B Curve?
Any time the circuit has a motor, compressor or other high-inrush load that can cause false trips on a B Curve device.
Do C Curve MCBs still meet domestic requirements?
Yes - as long as disconnection times and fault levels remain within BS 7671 limits. They’re common on heat pumps, AC units and large fans.
Will a C Curve stop nuisance-tripping on tools?
In most cases, yes. Tools with high startup currents typically require C Curve protection.
Are C Curve MCBs suitable for EV chargers?
Depends on the charger design. Some high-inrush models benefit from C Curve protection, but always follow manufacturer guidance.
👉 Need reliable protection for motor-driven or inductive loads? Shop all C Curve MCBs