How to Choose a Circuit Breaker: Amp Ratings Explained

By ARCK Electrical · Trade counter, North Parramatta · Reviewed May 2026

Quick Q&A — click to expand

What size breaker for 2.5mm cable? +

16A or 20A in a typical domestic install. The exact answer depends on the installation method and derating — a 2.5mm² T&E enclosed in thermal insulation runs cooler-rated than the same cable clipped to a surface. Default to 20A for a kitchen or general-purpose GPO circuit, and step back to 16A if derating or long runs eat into the cable's safe capacity.

Curve B vs Curve C — when does it matter? +

B curve trips at lower magnetic inrush (3–5× rated), C curve at higher (5–10×). For LED drivers, switch-mode supplies, motors, and most modern domestic loads, C curve is the safe default. Pure resistive loads (heaters, hot water elements, incandescent lighting where it still exists) suit B curve. The most common nuisance trip on LED-heavy lighting circuits is a B-curve MCB seeing cold-start inrush — swap to C and it goes away.

Why do my motors trip the breaker? +

Motor inrush is several times the running current for a fraction of a second. A C-curve breaker handles most domestic split-system AC and small motors. Larger motors, compressors, and inrush-heavy equipment trip C curves and need D curve (10–20× rated current) to ride through. If a motor circuit nuisance-trips on starting, the fix is usually curve, not rating.

Can I mix brands on one board? +

Physically, sometimes — modules are mostly DIN-rail-mounted and many will clip in. But busbar arrangements, neutral bars, and incoming connections don't always line up cleanly across brands, and mixing creates a mess for the next sparkie. Stick to one brand per board for new work, and match the original brand when replacing a faulty module.

Choosing a circuit breaker is four decisions in a row: how many poles, what amp rating, which trip curve, and what breaking capacity. Get any one of them wrong and the breaker either nuisance-trips, fails to protect the cable, or doesn't fit the board.

Here's the version we run through at the counter.

Step 1 — Pole count

The pole count is how many conductors the breaker switches.

• 1P (single pole) — switches the active only. Standard for single-phase final sub-circuits (lighting, GPOs).
• 2P (two pole) — switches both active and neutral. Often used as a single-phase main switch, isolators in damp areas, and circuits where switching the neutral is required.
• 3P (three pole) — switches three phases. Used on three-phase final sub-circuits without a switched neutral.
• 4P (four pole) — switches three phases and the neutral. Three-phase circuits where the neutral must be switched (most three-phase final sub-circuits on a TN-C-S installation in Australia).

For a typical Sydney domestic board: 1P MCBs (or 1P RCBOs) on final sub-circuits, a 2P main switch on the consumer mains. Three-phase boards use 4P main switches and 4P RCDs/RCBOs.

Step 2 — Amp rating: size to the cable, not the load

This is the order to think in:

1. Decide the design current of the circuit (what it'll draw).
2. Pick the cable that can carry that current (per AS/NZS 3008), including derating for installation method, bundling, and ambient temperature.
3. Pick the breaker rated at or below the cable's safe current capacity, at or above the design current.

The breaker protects the cable. The cable doesn't protect the breaker.

A 32A MCB on a 2.5mm² cable doesn't protect the cable — it'll let the cable cook to 30A indefinitely. Common combinations on a domestic install:

Circuit	Typical cable	Typical MCB
Lighting	1.5mm² T&E	6A or 10A
Standard GPO	2.5mm² T&E	16A or 20A
Kitchen GPO	2.5mm² T&E	20A
Hot water	2.5–4mm² T&E	20A
Electric oven	6mm² T&E	32A
Ducted AC	6–10mm² T&E	32–40A
Sub-main to granny flat	10mm² T&E	50A
EV charger	6mm² T&E	32A

Adjust for cable derating, route length (voltage drop), and the manufacturer's specifics — these are starting points. Full guidance in our cable sizing chart.

Step 3 — Trip curve

The trip curve describes how the breaker responds to inrush currents. All three common curves trip at the same long-term threshold (e.g. a 20A breaker holds 20A indefinitely on all of them); they differ in how much short-term inrush they tolerate before the magnetic trip fires.

• B curve — trips at roughly 3–5× rated current under magnetic conditions. For purely resistive loads (lighting, heating elements, hot water) and circuits with no significant inrush. Common in domestic lighting circuits.
• C curve — trips at roughly 5–10× rated current. The default for mixed domestic and commercial circuits — handles the inrush of LED drivers, switch-mode power supplies, small motors, and general use. This is what most domestic boards use across the board.
• D curve — trips at roughly 10–20× rated current. For motor loads, large inrush devices, transformers, welding outlets. You'll see D curve on workshops, larger compressors, and inrush-heavy industrial loads.

For a typical domestic board, C curve is the safe default. The exception is LED-heavy lighting circuits with cheap drivers — occasionally a B-curve MCB nuisance-trips on cold-start inrush, and the fix is C curve.

Step 4 — Breaking capacity (kA rating)

Breaking capacity is the maximum prospective short-circuit current the breaker can interrupt without exploding. It's stamped on the breaker as Icn / Icu — usually 6kA, 10kA, or higher.

The rule of thumb:

• 6kA — domestic installations and small commercial. Most residential MCBs and RCBOs you'll see are 6kA.
• 10kA — larger commercial, industrial, or any installation where the prospective fault current at the board exceeds 6kA. Mandatory if the network calculation says so.

How do you know? In most domestic jobs, 6kA is fine — the fault current at a typical suburban meter board doesn't exceed it.

For larger sites, three-phase commercial boards, or boards close to a transformer, run the network fault-current calc or use the supply authority's published values. If in doubt on a borderline job, go 10kA.

Step 5 — Brand and board compatibility

MCBs and RCBOs aren't all interchangeable — they have to fit and clip into the board they're going in. The main brands and ranges you'll meet in Sydney:

• Clipsal Resi MAX (MX9) — current domestic range. Clipsal MAX MCBs and RCBOs fit Clipsal Resi MAX enclosures. Older Clipsal MAX9 modules are not interchangeable with all newer boards — check the catalogue.
• Hager Resi9 — Hager's domestic and small commercial range. Hager modules fit Hager boards. Some sizes will clip into other DIN-rail boards if you're careful, but stick to brand-matched for a clean board.
• NHP / Terasaki / Schneider — commercial-grade. NHP Slimline, Schneider Acti9 and similar. Stocked for commercial and three-phase boards.
• Connected Switchgear (CS) — value-priced domestic range, fits standard DIN rail. We stock it for budget jobs and for sparkies who like a leaner price point on RCBOs.

The bigger question to ask when you walk in: what's already on the board? If you're replacing a single MCB on an existing Clipsal Resi MAX board, you don't want to drop a Hager module in.

They'll usually fit physically but the busbar arrangements and neutral bars don't always line up. When in doubt, bring the old breaker in or send a photo.

We'll match it.

Replacing vs upgrading

Two scenarios that come up a lot:

Replacing a faulty breaker. Match brand, range, amp rating, pole count, curve, and breaking capacity. Same-for-same is the fastest call.

Upgrading from an old board to RCBO-per-circuit. Often the cleanest answer is a new enclosure with modern busbar and RCBOs across the board. If you're trying to fit RCBOs into an old enclosure where the MCBs are 30 years old, the spacing and the busbar arrangement often don't suit modern modules. A full board change usually ends up faster and tidier than a piecemeal upgrade — and the customer gets compliant surge protection and Type A RCDs at the same time.

Order from ARCK

Clipsal Resi MAX (MX9), Hager Resi9, NHP, Schneider Acti9 and Connected Switchgear — MCBs, RCDs and RCBOs in 1P, 2P, 3P and 4P, B/C/D curves, 6kA and 10kA. Same-day pick-up from the North Parramatta trade counter.

Bring the old breaker in or send a photo and we'll match it. Browse the circuit protection range, or call the counter on (02) 9890 9693.

Mon–Fri 6:30am–5pm, Sat 7:30am–1pm.