Circuit Breaker Sizing

Parameters

Every field on the form maps to one input on the calculation engine. This is what each one means, what it is measured in, the range it accepts, and where it actually changes the answer.

Design current, Ib (amperes)

The current the circuit is expected to carry in normal service, after diversity has been applied. Take it from the load schedule or from the cable sizing calculation.

Typical range. About 5 A for a small lighting circuit up to 125 A for a submain. The field accepts 1 to 500 A.

Gotcha. Ib is the load current, not the cable rating. If you enter a design current above the cable capacity the calculator stops, reports the cable as undersized, and returns no breaker rating at all.

Cable current-carrying capacity, Iz (amperes)

The continuous current the cable can carry as installed. That is the AS/NZS 3008.1.1 table value after every derating factor has been applied: ambient temperature, grouping, thermal insulation, and installation method.

Typical range. Around 20 to 25 A for 2.5 mm squared thermoplastic cable enclosed in a wall, and 34 to 40 A for 6 mm squared. The field accepts 1 to 500 A.

Gotcha. Use the derated figure, not the raw table value. Entering the underated value inflates Iz and lets the calculator pick a breaker the cable cannot actually support.

Load type (resistive, lighting, motor, mixed, capacitive)

Selects the trip curve. Resistive and lighting return curve B. Motor returns curve C, or curve D when the starting multiple is above 8. Mixed and capacitive both return curve C.

Gotcha. Load type changes the curve letter and the justification wording only. It does not change the selected rating or the breaking capacity.

Motor starting multiple (multiples of rated current)

The ratio of motor starting current to breaker rated current. Direct-on-line induction motors typically sit at 6 to 8 times. This field only appears when load type is set to motor.

Typical range. The field accepts 1 to 15. Above 8 the curve moves from C to D. Above 10 the calculator raises a warning to confirm that direct-on-line starting is viable at all, or whether a soft starter is needed.

Gotcha. For every other load type the value is not passed to the engine, so changing it has no effect.

Continuous load (checkbox)

Marks loads that run for more than three hours at a time, such as EV chargers, resistive heating, and process equipment.

Gotcha. Ticking this box adds a note to the result and nothing else. The calculator does not apply a 125 percent or any other continuous-duty factor to the rating. If your design rule requires one, apply it to Ib before you enter it.

Supply phase (single phase or three phase)

Records whether the circuit is single phase or three phase and, when RCD protection is ticked, feeds the RCD type rule of thumb.

Gotcha. The selected rating, curve, and breaking capacity are identical for both settings. Phase balance, neutral loading, and three-phase derating are not modelled.

Prospective fault current (kiloamperes, optional, under advanced options)

The maximum prospective short-circuit current available at the point where the device is installed.

Typical range. About 3 to 6 kA at a domestic switchboard, 10 kA at a light commercial board, and 15 to 50 kA close to a distribution transformer. Leave the field blank and the calculator assumes 6 kA.

Gotcha. The calculator returns the smallest standard breaking capacity from 6, 10, 15, and 25 kA that is at or above your figure. Enter more than 25 kA and it still returns 25 kA, but the breaking capacity check then fails. That failure is the signal to move to a moulded-case device.

RCD protection required (checkbox, under advanced options)

When ticked, the calculator adds an RCD type suggestion. Motor and capacitive loads return Type B, other three-phase circuits return Type AC, and everything else returns Type A.

Gotcha. This is a coarse rule of thumb. It does not consider EV chargers, solar inverters, or the New Zealand Type A requirement, and it does not return a sensitivity in milliamperes. Use the RCD selection calculator for that decision.

Assumptions and limits

What the tool assumes

• Only the standard rating ladder of 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, and 125 A is considered. Fuses, adjustable moulded-case trip units, and non-standard ratings are not offered.
• It returns the smallest rating that satisfies Ib <= In <= Iz and stops there. If no standard rating fits inside that window it returns nothing and issues a warning rather than rounding either way.
• Breaking capacity is chosen from a four-value list of 6, 10, 15, and 25 kA.
• The trip curve is a lookup from load type. The calculator does not compare a device let-through characteristic against a motor starting curve or a transformer inrush curve.
• The trip bands quoted with each curve, roughly 3 to 5 times rated current for B, 5 to 10 for C, and 10 to 20 for D, are the generic bands. Individual manufacturer curves vary within and across those bands.

What this calculator does not do

• No overload coordination check against 1.45 times Iz, which is the second half of the AS/NZS 3000 Clause 2.5.3 rule.
• No adiabatic short-circuit check. It does not confirm that the cable withstands the let-through energy of the selected device.
• No earth fault loop impedance or disconnection time check against AS/NZS 3000 Table 8.1, so it cannot tell you whether the device clears a fault within 0.4 or 5 seconds.
• No discrimination, cascading, or backup protection check against the upstream device.
• No temperature correction of the device itself. Miniature circuit breaker thermal elements are calibrated at a reference ambient of 30 degrees Celsius and lose capacity in a hot switchboard.
• The RCD suggestion is a rule of thumb, not an assessment against AS/NZS 3000 Clause 2.6.

Do not treat any output here as a verified selection. Check the result against the current edition of AS/NZS 3000 and AS/NZS 60898.1, against the manufacturer published time-current curves and breaking capacity, and have the design signed off by the person responsible for the installation. Nothing on this page has been validated or certified by a chartered professional engineer.

Worked examples

Four complete runs through the same method, using the calculator formulas exactly as implemented.

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 Clause 1.5.5.3(d) and Clause 5.7.2 require final subcircuits supplying socket outlets rated up to 63 A, hand-held Class I equipment, or portable equipment to disconnect within 0.4 seconds under fault, and other circuits including submains and circuits supplying fixed or stationary equipment within 5 seconds. Table 8.1 lists the maximum earth fault loop impedance that achieves these times for common MCB and fuse ratings. 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.