Fuse / Breaker Sizing — IEC 60364

COORDINATION RULE

  I_load  ≤  In  ≤  Iz                    (IEC 60364-4-43)

  Variables :
    I_load    load current in normal operation (A)
    In        standard rated current of the protection device (A)
              → first standard value ≥ I_load
    Iz        continuous current capacity of the cable (A)
              → depends on cross-section and installation method

  Rationale : the first inequality (I_load ≤ In) ensures that the
  protection does not trip during normal operation. The second
  (In ≤ Iz) ensures the cable is not overloaded before the protection
  operates. If In > Iz, the cable could burn before the protection
  trips — fire hazard.


STANDARD RATINGS

  In (A) :  6 — 10 — 13 — 16 — 20 — 25 — 32 — 40 — 50 — 63 — 80 — 100
            → 125 — 160 — 200 — 250 — 315 — 400 — 500 — 630 — 800 — 1000

  Rationale : IEC standard values. Always pick the first one greater
  than or equal to I_load. For domestic circuit breakers, the common
  ratings are 10 A (lighting) / 16 A (sockets on 2.5 mm²) /
  20 A (sockets on 4 mm²) / 32 A (cooking).


DO NOT CONFUSE

  In   →  rated current (A, continuous, normal operation)
  Icu  →  ultimate breaking capacity (kA — max Isc the device can interrupt)
  Icw  →  short-time withstand current (kA · s)

  Rationale : three distinct quantities. In sizes the overload
  protection ; Icu must be ≥ prospective Ik3 at the installation point ;
  Icw concerns withdrawable breakers and selectivity with downstream
  protections.