ElectroHub

MCB & RCCB Selection Calculator

One chain of inequalities decides the rating. The breaker must be large enough not to trip on the design current, and small enough that it protects the cable before the cable overheats. A breaker larger than the cable can carry makes the cable the fuse.

MCB & RCCB selection

One rule decides the rating: Ib ≤ In ≤ Iz. The breaker must be large enough not to trip on the design current, and small enough that it protects the cable. Fitting a breaker bigger than the cable can carry turns the cable into the fuse.

Breaker rating
Tripping curve
RCCB sensitivity
  • Coordination check: I_b 18.0 A ≤ I_n 20 A ≤ I_z 26.0 A. All three in order, which is the whole rule.
  • Curve C trips at 5–10× rating, which lets a motor's starting surge through without nuisance tripping.
  • A 30 mA RCCB is the personal-protection rating — required on socket circuits, and the only sensitivity that protects a person rather than the wiring.
Common trap: When no standard rating fits, the fix is a bigger cable — never a bigger breaker. Raising the breaker to make it "work" leaves the cable unprotected against exactly the overload the breaker was there to stop.

The formula

I_b ≤ I_n ≤ I_z

I_b
design current — what the circuit actually draws
I_n
breaker nominal rating, from the standard series 6, 10, 16, 20, 25, 32 A…
I_z
cable current capacity after derating

Worked example

A socket circuit drawing 18 A, wired in 4 mm² copper with a derated capacity of 26 A.

  1. Standard ratings at or above 18 A: 20, 25, 32 A…
  2. Of those, the ones at or below 26 A: 20 and 25 A
  3. Take the smallest: 20 A
  4. Check: 18 ≤ 20 ≤ 26 ✓

20 A, curve C, with a 30 mA RCCB — the sensitivity that protects a person rather than the wiring.

Where you'll use it

Every final circuit in a distribution board. Curve B trips at 3–5× rating for lighting and resistive loads; curve C at 5–10× for motors; curve D at 10–20× for transformers. When nothing fits, the fix is a bigger cable — never a bigger breaker.

The laws behind it

Parts this applies to

Resistor (fixed)Varistor / MOVTVS / Transient SuppressorIGBTSCR (Thyristor)Linear Regulator (78xx / LDO)Switches (toggle/push/slide/rotary/DIP)Solid-State Relay (SSR)

Related calculators

Cable Size CalculatorI_z = I_table × k_ambient × k_groupingMotor Full Load Current CalculatorI = kW × 1000 / (√3 × V × η × cos φ) [three phase]Earthing Conductor Size CalculatorS = √(I² t) / k