Work out circuit breaker sizing instantly with clear inputs, formula shown and shareable results.
The breaker rating must sit between the design current and the derated cable ampacity. The curve then decides the instantaneous trip threshold: type B trips at 3-5 times rating, type C at 5-10 and type D at 10-20, so high-inrush loads need C or D to avoid nuisance tripping.
Breaker selection
Ib <= In <= Iz; magnetic trip = In x curve multiple; inrush must stay below the trip threshold
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.
The fundamental coordination rule: design current no greater than device rating, device rating no greater than cable capacity.
For transformers, large motors and switch-mode power supply banks whose inrush would trip a type C. Note it also lengthens fault clearance time, which affects earth fault protection.