Selecting Circuit Breakers

Quick pick (TL;DR)

Pick the type by current & voltage: MCB (≤125 A final circuits), MCCB (up to ~1600 A LV feeders/motors), ACB (large LV mains), VCB (MV switchgear).
Then rated current In ≥ load, set trip Ir to protect the cable, and the breaking capacity Icu ≥ the prospective fault current.
Use electronic LSIG trip units for selectivity on feeders.

Step 1 — Choose the type

Type	Range	Use for
MCB	≤ 125 A, LV	Final circuits
MCCB	~16–1600 A, LV	Feeders, motors
ACB	~630–6300 A, LV	Main incomers
VCB	MV (3.3–36 kV)	MV switchgear

Step 2 — Worked selection

90 kW motor, 31 kA fault

Given load current = 166 A prospective fault = 31 kA Step 1 — Rated current In ≥ load → In = 250 A frame Step 2 — Trip setting Ir set to protect cable (Iz 340 A) load 166 ≤ Ir 210 ≤ Iz 340 → OK Step 3 — Breaking capacity Icu ≥ fault current Icu 36 kA ≥ 31 kA → OK Answer → MCCB 250 A, Ir 210 A, Icu 36 kA, LSIG

Order: type → In → trip → Icu ≥ fault. Never compromise on Icu.

Step 3 — Trip units & selectivity

Thermal-magnetic: simple, fixed.
Electronic LSIG: adjustable, selective.
Category B for selective feeders.

Breaker selection calculator

Rated current & breaking capacity

In ≥ load; Ir between load & cable Iz; Icu ≥ fault.

Load current (A)

Cable Iz (A)

Fault (kA)

LV MCCB screening — Icu must exceed fault. Use LSIG trip for selectivity.

Common mistakes

Icu below the fault level.
Trip above cable capacity.
No selectivity → nuisance blackouts.

Requisition & datasheet

Load, voltage, current, fault.
Type, In, Ir, Icu, poles, trip unit.

Frequently asked questions

What should I know about Selecting Circuit Breakers?

Pick the type by current & voltage: MCB (≤125 A final circuits), MCCB (up to ~1600 A LV feeders/motors), ACB (large LV mains), VCB (MV switchgear). Then rated current In ≥ load, set trip Ir to protect the cable, and the breaking capacity Icu ≥ the prospective fault current. Use electronic LSIG trip units for selectivity on feeders.

Which standards or codes apply?

IEC 60947-2 (LV breakers), IEC 62271-100 (MV breakers), UL 489, IEC 60909 (fault current).

Standards — IEC 60947-2; IEC 62271-100; UL 489; IEC 60909.