Circuit Breaker Sizing Calculator
Coordinate protective devices with load current and conductor carrying capacity. Select tripping curves, input circuit details, and check coordination compliance instantly.
Circuit Parameters
Standard Cable & MCB Sizing Quick Reference
Typical combinations of copper cables and circuit breakers in residential or commercial settings. Click any row to load the parameters directly into the calculator above.
| Cable Size | Installation Method | Cable Rating (Iz) | Typical Load Limit | Recommended MCB | Application |
|---|---|---|---|---|---|
| 1.5 mm² | Enclosed in Conduit | 14.5 A | 8 A | 10A (Type B) | Lighting Circuits |
| 1.5 mm² | Clipped Direct / Air | 17.5 A | 12 A | 16A (Type B) | Lighting Sub-circuits |
| 2.5 mm² | Enclosed in Conduit | 20.0 A | 15 A | 16A or 20A | Power Point (GPO) Circuits |
| 2.5 mm² | Clipped Direct / Air | 27.0 A | 18 A | 20A or 25A | High load power circuits |
| 4.0 mm² | Enclosed in Conduit | 27.0 A | 20 A | 25A (Type C) | Hot water systems |
| 4.0 mm² | Clipped Direct / Air | 36.0 A | 25 A | 32A (Type C) | Cooktop / Air Conditioner |
| 6.0 mm² | Enclosed in Conduit | 34.0 A | 28 A | 32A (Type C) | Oven / Range feeds |
| 6.0 mm² | Clipped Direct / Air | 46.0 A | 35 A | 40A (Type C) | Sub-mains to outbuilding |
| 10.0 mm² | Clipped Direct / Air | 63.0 A | 40 A | 50A (Type D) | Heavy motor / Solar inverter |
In electrical installations, a circuit breaker does not protect appliances—it protects the cable. Selecting a breaker that is too large means the cable can overheat and catch fire before the breaker trips. Selecting a breaker that is too small leads to continuous nuisance tripping. This calculator evaluates your load and cable capacity to find a standard compliant rating in accordance with AS/NZS 3000.
How Circuit Breaker Sizing Works — The Coordination Rule
To ensure a circuit is safe, the protective device (MCB, RCBO, or fuse) must be properly coordinated with the load and the cabling. The core rule, defined in AS/NZS 3000 Clause 2.5.3, is:
Where:
- Ib: The design load current of the circuit (the actual current the connected equipment draws).
- In: The nominal rating of the protective device (e.g. a 20A breaker).
- Iz: The current-carrying capacity of the cable after applying derating factors (grouping, insulation, ambient temperature) under AS/NZS 3008.
In simple terms, the breaker rating must be large enough to carry the load current without tripping, but small enough that it trips before the current exceeds what the cable can safely carry.
MCB Tripping Curves Explained — Type B, C, and D
Circuit breakers have two tripping mechanisms: a thermal element (trips on slow overloads) and a magnetic element (trips instantly on short circuits). The tripping curve defines how much current is needed to trigger the magnetic trip:
- Type B (3 – 5 × In): Highly sensitive. A 10A Type B breaker trips instantly between 30A and 50A. Standard for residential lighting and GPOs, where inrush currents are negligible.
- Type C (5 – 10 × In): Medium sensitivity. A 10A Type C breaker trips instantly between 50A and 100A. This is the general-purpose standard in Australia, used for fluorescent lighting, small motors, and commercial power.
- Type D (10 – 20 × In): Low sensitivity. A 10A Type D breaker requires 100A to 200A to trip instantly. Designed for heavy starting currents, such as large electric motors, welding equipment, and transformers.
Worked Example 1 — Sizing a Breaker for a Residential GPO Circuit
Let\'s size a circuit breaker for a domestic power circuit wired in 2.5 mm² copper twin-and-earth cable. The cable is run through roof space insulation, derating its capacity (Iz) to 20A. The load (Ib) is estimated at a maximum of 15A.
Step 1 — Identify the Parameters
- Load Current (Ib) = 15 A
- Cable Capacity (Iz) = 20 A
Step 2 — Apply the Coordination Rule
We need to find a standard breaker rating (In) where 15A ≤ In ≤ 20A.
Standard ratings within this range are 16A and 20A.
Step 3 — Select the Breaker
A 16A Type B RCBO is selected. This provides overload protection (tripping at 16A, which is safely below the cable\'s 20A limit) while ensuring the circuit can carry the 15A continuous load. Choosing a 20A breaker is also technically compliant, but leaves zero thermal headroom for the cable if the load is slightly exceeded.
Worked Example 2 — Sizing a Breaker for a Three-Phase Motor
An industrial workshop pump motor draws 22A full load current (FLC). It is wired with a 6 mm² copper sub-circuit enclosed in conduit, giving a derated cable capacity (Iz) of 34A.
Step 1 — Identify the Parameters
- Load Current (Ib) = 22 A
- Cable Capacity (Iz) = 34 A
- Equipment Type = Induction Motor (high starting inrush)
Step 2 — Apply the Coordination Rule
We need to find a breaker rating where 22A ≤ In ≤ 34A. Standard ratings are 25A and 32A.
Step 3 — Account for Inrush and Select Curve
To prevent the motor\'s startup surge from tripping the breaker immediately, we must select a Type D breaker (or a Type C if starting load is light). A 25A Type D triple-pole MCB is the recommended protective device, as 25A provides a safe running cushion above the 22A load, while remaining safely below the cable\'s 34A limit.
MCB vs RCBO — Which One Do You Need?
Modern switchboards use different types of protective devices depending on the safety requirements:
- MCB (Miniature Circuit Breaker): Protects only against overcurrent (overload) and short circuits. It does not protect people from electric shock due to ground leakage.
- RCBO (Residual Current Breaker with Overcurrent): Combines an MCB and an RCD into a single unit. It protects against overload, short circuits, and earth leakage. Under AS/NZS 3000, almost all final sub-circuits under 32A in domestic installations must have RCD protection, making RCBOs the standard switchboard choice.
Common Breaker Sizing Mistakes Sparkies Make
- Ignoring Cable Derating: Installing a 20A breaker on a 2.5mm² cable because "2.5 is always rated for 20A". If that cable runs through thermal insulation or a hot ceiling space, its actual rating might drop to 13A, making a 20A breaker non-compliant and dangerous.
- Matching Breaker Directly to Cable Capacity: Selecting a 32A breaker for a 32A cable when the load is only 10A. Sizing the breaker closer to the load (e.g. 16A) protects the cable and downstream accessories far better.
- Wrong Tripping Curve Selection: Installing a Type B breaker on an air conditioner or pool pump circuit, causing the breaker to trip randomly whenever the compressor or motor tries to kick in.
AS/NZS 3000 Clause 2.5 — Overload Protection Rules
The wiring rules state that every active conductor in a sub-circuit must be protected by one or more protective devices that will automatically disconnect the supply during overload conditions before damage occurs to the cable insulation. Additionally, Clause 2.5.4 specifies that the breaker\'s conventional operating current (the current at which it is guaranteed to trip within an hour, typically 1.45 × In) must be coordinated with the cable capacity.
To check cable sizing and derating parameters in detail, use our AS3008 cable sizing calculator, check loop impedances with the fault loop impedance calculator, or calculate power conversions with the watts to amps calculator.
Frequently Asked Questions
Common questions about miniature circuit breakers (MCBs), RCBOs, and overcurrent protection in Australia