Circuit Breaker Sizing

Circuit Breaker Selection for AS/NZS 3000:2018

Circuit breaker sizing is one of the most fundamental tasks in electrical design. Every circuit in an installation needs a protective device that will disconnect the supply fast enough to prevent cable damage during an overload or short circuit, while staying closed under normal operating conditions. This calculator helps you select the correct breaker rating, trip curve, and breaking capacity for a given circuit based on the design current, cable current-carrying capacity, and prospective fault current at the point of installation.

Whether you are wiring a residential switchboard, sizing protection for a commercial distribution board, or specifying MCBs for an industrial motor control centre, this tool applies the coordination rules from AS/NZS 3000:2018 Clause 2.5 and the device standards in AS/NZS 60898.1 (MCBs) and AS/NZS 60947.2 (MCCBs).

Key concepts

• Coordination rule (Ib <= In <= Iz). The breaker rated current (In) must sit between the circuit design current (Ib) and the cable current-carrying capacity (Iz). If In is below Ib the breaker nuisance-trips under normal load. If In exceeds Iz the cable overheats before the breaker trips.
• Trip curves (B, C, D). The curve letter defines the instantaneous magnetic trip threshold relative to rated current. Curve B trips magnetically between 3 and 5 times In, suiting resistive and lighting loads. Curve C trips between 5 and 10 times In, appropriate for small motors and fluorescent lighting with moderate inrush. Curve D trips between 10 and 14 times In, required for direct-on-line motor starting, welding equipment, and transformer energisation where inrush can exceed 10 times rated current.
• Breaking capacity (Icu / Icn). The maximum prospective fault current the device can safely interrupt. Common residential MCBs are rated at 6 kA. Light commercial installations typically require 10 kA. Industrial switchboards near large transformers can see fault levels of 25 to 50 kA, requiring MCCBs or high-rupture-capacity MCBs.
• Disconnection times. AS/NZS 3000 Table 2.3 requires socket outlet circuits up to 32 A to disconnect within 0.4 seconds under fault, and fixed equipment circuits within 5 seconds. The breaker must be fast enough at the available fault current to meet these limits.

Common scenarios

1. Residential final subcircuit. A 20 A lighting circuit on 2.5 mm squared TPS cable (Iz approximately 23 A in enclosed conduit at 40 degrees Celsius). Design current is 16 A. A 20 A curve B MCB with 6 kA breaking capacity satisfies the coordination rule and suits the resistive/lighting load profile.
2. Commercial air conditioning circuit. A 7.5 kW three-phase split system drawing approximately 14 A at 400 V. The compressor motor has a locked rotor current of around 70 A (5 times FLC). A 16 A curve C MCB handles the starting inrush without nuisance-tripping, while a curve B at the same rating would trip on every compressor start.
3. Industrial DOL motor starter. A 22 kW motor with a starting current of 280 A (approximately 7 times FLC of 40 A). The prospective fault current at the MCC is 18 kA. A 50 A curve D MCB (or MCCB with adjustable magnetic trip) prevents nuisance-tripping during the 2 to 3 second starting period, and the 25 kA breaking capacity exceeds the available fault level.