Standard Australian Circuit Breaker Sizes & Cable Coordination Table
Under AS/NZS 3000 Clause 2.5, standard Australian miniature circuit breaker (MCB) and RCBO ratings are 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A, 80A, 100A, and 125A. The fundamental coordination rule (Ib ≤ In ≤ Iz) mandates that the breaker rating (In) must never exceed the conductor's derated current capacity (Iz):
| MCB Rating (In) | Tripping Curve | Minimum Cable (Iz) | Installation Routing | Standard Australian Trade Application |
|---|---|---|---|---|
| 6 Amps | Type B / C | 1.5 mm² TPS (16A) | Enclosed in thermal insulation | Low-energy LED lighting, exhaust fans |
| 10 Amps | Type B / C | 1.5 mm² TPS (16A) | Enclosed in insulation / conduit | Standard domestic lighting, security circuits |
| 16 Amps | Type B / C | 2.5 mm² TPS (21A) | Enclosed in ceiling insulation | Standard residential power points (GPOs) |
| 20 Amps | Type B / C | 2.5 mm² TPS (27A) | Unenclosed / clipped direct | Heavy domestic GPO circuits, trade workshop power |
| 25 Amps | Type C | 4.0 mm² TPS (27A) | Enclosed in conduit / wall frame | Electric storage hot water (3.6 kW), small split system AC |
| 32 Amps | Type C | 6.0 mm² TPS (34A) | Enclosed in conduit | Induction cooktop, built-in wall oven, 7.2 kW EV wallbox |
| 40 Amps | Type C / D | 10.0 mm² TPS (46A) | Enclosed in conduit | Large ducted air conditioning, secondary sub-board feeder |
| 50 Amps | Type C / D | 10.0 mm² TPS (63A) | Clipped direct / open air | Commercial sub-mains, 22 kW 3-phase industrial motor |
| 63 Amps | Type C | 16.0 mm² Copper (68A) | Enclosed in conduit / underground | Single-phase domestic consumer mains, main switchboard |
| 80 Amps | Type C | 25.0 mm² Copper (95A) | Clipped direct / underground duct | Heavy single-phase all-electric home, commercial mains |
| 100 Amps | Type C / MCCB | 35.0 mm² Copper (120A) | Underground wiring enclosure | Three-phase commercial building mains, light factory supply |
| 125 Amps | Type D / MCCB | 50.0 mm² Copper (145A) | Cable ladder / enclosed busway | Commercial switchboard main isolator, heavy plant room |
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:
Ib ≤ In ≤ Iz
- Ib (Design Load Current): The actual continuous current drawn by connected electrical equipment.
- In (Nominal Breaker Rating): The standard rating of the circuit breaker or RCBO (e.g. 16A, 20A, 32A).
- Iz (Derated Cable Capacity): The continuous current-carrying capacity of the cable after applying AS/NZS 3008 derating factors (thermal insulation, grouping, ambient temperature).
In simple terms, the breaker rating must be large enough to carry the load current without nuisance tripping, but small enough that it trips before 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 for slow overloads and a magnetic element for instant short-circuit disconnection. The tripping curve defines how much current triggers instant magnetic trip:
- Type B (3 – 5 × In): Highly sensitive. A 10A Type B breaker trips instantly between 30A and 50A. Standard choice for residential lighting, GPOs, and resistive heating where inrush surges are low.
- 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 for commercial power, fluorescent lighting, and small motor loads.
- Type D (10 – 20 × In): Low sensitivity. A 10A Type D breaker requires 100A to 200A to trip instantly. Required for heavy starting currents, such as large industrial motors, transformers, welders, and solar inverters.
Worked Example 1 — Sizing a Breaker for a Residential GPO Circuit
Consider sizing a circuit breaker for a domestic power circuit wired in 2.5mm² copper flat TPS cable. The cable is run through ceiling insulation, derating its current capacity (Iz) to 20A. The load current (Ib) is estimated at 15A.
- Step 1 — Identify parameters: Ib = 15 A, Iz = 20 A.
- Step 2 — Apply coordination rule: 15A (Ib) ≤ In ≤ 20A (Iz). Standard breaker ratings in range are 16A and 20A.
- Step 3 — Select breaker: A 16A Type B RCBO is selected. This provides complete thermal overload protection safely below the cable's 20A limit while continuously supporting the 15A load.
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 6mm² copper sub-circuit enclosed in conduit, giving a derated cable capacity (Iz) of 34A.
- Step 1 — Identify parameters: Ib = 22 A, Iz = 34 A, Equipment = Induction Motor.
- Step 2 — Apply coordination rule: 22A (Ib) ≤ In ≤ 34A (Iz). Standard ratings in range are 25A and 32A.
- Step 3 — Select curve and rating: To withstand motor starting inrush without nuisance tripping, a 25A Type C or Type D 3-Pole MCB is chosen. 25A provides a comfortable margin above 22A running load while remaining safely below the 34A cable limit.
MCB vs RCBO — Which One Do You Need?
Modern switchboards use specific protective devices based on safety standards:
- MCB (Miniature Circuit Breaker): Protects strictly against overload and short circuits. Does not protect against earth leakage or electric shock.
- RCBO (Residual Current Breaker with Overcurrent): Combines MCB overload protection and RCD earth leakage protection in a single module. Under AS/NZS 3000, almost all domestic final sub-circuits ≤ 32A require RCD protection.
Common Breaker Sizing Mistakes Sparkies Make
- Ignoring Cable Derating: Installing a 20A breaker on a 2.5mm² cable because "2.5 is always 20A". If the cable runs through insulation or hot ceiling spaces, its capacity drops to 13A, making a 20A breaker non-compliant.
- Matching Breaker Directly to Cable Capacity: Fitting a 32A breaker for a 32A cable when the connected load is only 10A. Sizing closer to the load (e.g. 16A) offers superior protection for downstream wiring and accessories.
- Incorrect Curve Selection: Installing a Type B breaker on an air conditioner or pool pump circuit, causing intermittent nuisance trips when the compressor starts up.