Breaker curves (B, C, D)
A 16 A breaker isn’t a 16 A breaker. The curve type — B, C, or D — determines how much above the rated current the breaker will tolerate before its magnetic trip kicks in. Pick the wrong curve and you either get nuisance trips on every motor startup, or you protect the load too slowly.
Two trip mechanisms in every MCB
A modern miniature circuit breaker (MCB) has two trip mechanisms in one housing:
- Thermal trip — a bimetallic strip that heats up under sustained overload and trips after seconds-to-minutes. Protects against gradual overload (too many loads on one circuit).
- Magnetic trip — an electromagnet that snaps instantly (under 100 ms) when current exceeds a multiple of the rated current. Protects against short circuits.
The thermal characteristic is roughly identical across curve types — what varies is the magnetic trip threshold, expressed as a multiple of the breaker’s rated current (In).
Curve B: 3-5× In
Magnetic trip range. 3 to 5 times the rated current. A 10 A B-curve trips magnetically between 30-50 A.
Use it for. Residential lighting, general-purpose socket circuits, computers and small electronics, resistive loads (heaters, water heaters, ovens). Most domestic circuits in the UK (BS 7671) and Germany (DIN/VDE) are B-curve by default.
Don’t use it for. Loads with high inrush current (large motors, big LED arrays with capacitor inrush, transformers). The B-curve trips on the inrush spike before the load even starts.
Curve C: 5-10× In
Magnetic trip range. 5 to 10 times the rated current. A 16 A C-curve trips magnetically between 80-160 A.
Use it for. Loads with moderate inrush. Fluorescent lighting banks, small refrigerator and HVAC motors, transformers, mid-size LED installations. Many EU appliance manufacturers spec C-curve breakers for their products.
Trade-off. A short circuit between 5× and 10× In may take noticeably longer to clear than on a B-curve — not seconds, but tens of ms longer. Usually fine; matters when grading with upstream protection.
Curve D: 10-20× In
Magnetic trip range. 10 to 20 times the rated current. A 32 A D-curve trips magnetically between 320-640 A.
Use it for. Heavy industrial motors, large transformers, capacitor banks, welders, sub-distribution feeds where downstream breakers handle the protection role and the D simply needs to allow inrush through.
Rarely in residential. Some EV chargers with large pre-charge capacitor banks call for D-curve dedicated feeds. Most residential is fine on B or C.
Where Curve A fits in
Curve A is used on RCDs and RCBOs, not MCBs. It refers to the residual-current type the device can detect:
- Type AC — detects sinusoidal AC residual currents only. Largely deprecated for new installations across the EU.
- Type A — detects AC and pulsating DC residual currents. The standard residential RCD type in DIN/VDE, BS 7671, and AS/NZS 3000. Suitable for most loads except those with smooth DC residual current.
- Type B — detects AC, pulsating DC, AND smooth DC residual current. Required for EV chargers with single-phase AC charging where the EV’s onboard charger can present DC fault currents (NEC and BS 7671 Amendment 2 both call this out), and for PV inverters with similar DC fault profiles.
- Type F — A-plus, for frequency-modulated loads (variable-speed drives).
On the WireSketch panel sheet, the “curve” column shows B/C/D for MCBs (overcurrent) and A/B for RCDs/RCBOs (residual current). The same letter means different things in the two contexts — an awkward but standard convention.
How WireSketch picks defaults
When the wand auto-fills the panel from your fixtures:
- Lighting circuits → B-curve at 6-10 A (region-dependent)
- Socket circuits → B-curve at 16 A (EU) / 20 A (US) / 32 A ring (UK)
- Heavy appliances (oven, dishwasher, EV charger) → C-curve at appropriate amperage for the load
- RCD main protection → Type A by default; the Knowledge Base entry notes Type B for EV/PV under BS 7671 A2 + recent VDE updates
All of these are starting points. The licensed electrician designing the actual installation will pick the right curve for the specific equipment and the specific upstream-downstream grading needs.
The curve type matters more than people think. A nuisance-tripping 10 A C-curve will frustrate the homeowner for years. A 6 A B-curve on a circuit feeding a single large LED chandelier may trip on cold-start inrush. Pay attention to the curve column on the panel sheet; don’t default everything to B without thinking about inrush.