The practical rule for sizing a miniature circuit breaker: Ib ≤ In ≤ Iz, cable derating, and picking the trip type for the load.
Sizing a miniature circuit breaker (MCB) is not "pick one bigger than the load". The breaker exists to protect the cable, so the cable sets the upper limit and the load sets the lower one. Get it backwards and the cable melts before the breaker trips.
Ib is the design load current, In is the breaker rating, and Iz is the cable's permitted current-carrying capacity. The breaker must sit between them: at or above the load so it does not nuisance-trip, and at or below the cable so the cable is the protected element. A 16 A load on a 2.5 mm² cable (≈24 A at 40 °C) takes a 16 A or 20 A MCB — never a 32 A one.
Iz falls with ambient temperature, grouping of cables, and enclosure type. Three circuits bunched in a hot trunking can lose 20–30 % of their rating; the MCB must track the derated value, not the catalogue one. When in doubt, upsize the cable rather than the breaker.
The letter is the magnetic trip threshold relative to In:
| Type | Magnetic trip | Use |
|---|---|---|
| B | 3–5× In | Resistive, low-inrush (lighting, sockets) |
| C | 5–10× In | General purpose, small motors (default) |
| D | 10–20× In | High inrush (transformers, welders, big motors) |
Type C is the panel default; choose D where the load has a heavy start surge so the breaker does not trip on every start, and B for clean resistive circuits.
ELEHPD miniature circuit breakers in Types B, C and D, 6 kA and 10 kA, with the distribution boards, RCBOs and busbar to match.
Ib ≤ In ≤ Iz: the breaker rating must sit at or above the design load current so it does not nuisance-trip, and at or below the cable's permitted current so the cable is what gets protected.
Iz falls with ambient temperature, cable grouping and enclosure type — bunched circuits in hot trunking can lose 20-30%. The MCB must track the derated value, and when in doubt you upsize the cable rather than the breaker.
C for general panels, D for heavy inrush loads like transformers and large motors, B for clean resistive circuits. Also check the breaking capacity (kA) against the fault current at that point.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.