A circuit breaker's casing always shows a letter and a number — for example, C16 or B10. That's not a manufacturer's part code, it's a working spec: the letter describes how much of an overload makes the breaker trip instantly, and the number is the circuit's rated current. Here's what that means in practice and how to avoid getting it wrong.
This won't replace an electrician's calculation for your specific property, but it will help you understand what's being proposed and why.
What a circuit breaker actually does
A breaker protects wiring from two things: sustained overload (when current above the rated level flows through a wire for longer than it should — the wire heats up) and short circuits (a sudden current spike many times the rated value). In both cases the breaker cuts the circuit before the wiring can overheat and ignite.
The breaker's rating (the number — 10A, 16A, 20A, 25A, etc.) has to match the wire's cross-section and the total load on the circuit: too high a rating won't protect the wire, too low a rating trips falsely under normal load.
Trip curves: what the letters B, C, D mean
The letter before the rating is the breaker's time-current curve — it defines how far above rated current has to spike before the breaker trips instantly (short-circuit protection). For sustained overload, the trip threshold is similar across types; the difference is specifically in how each reacts to a brief current surge.
- B — trips at 3-5 times rated current. The most sensitive type, suited to circuits without significant starting currents: lighting, outlets in living spaces.
- C — trips at 5-10 times rated current. The most common type for home and office circuits — tolerates the brief current surge when household appliances, a fridge, or a pump switch on.
- D — trips at 10-20 times rated current. Used for equipment with large starting currents: powerful electric motors, welding equipment, transformers.
How to size a breaker for a specific circuit
A breaker's rating is chosen based on the wire's cross-section and the total load that will be connected to the circuit — not the other way around. The general principle: wire cross-section is sized for the expected load with some margin, and the breaker's rating is set slightly below the wire's allowable continuous current, so the breaker trips before the wire overheats.
A dedicated circuit for a heavy appliance — an electric stove, a water heater, an air conditioner — always gets its own breaker sized for that specific device, not one shared with general outlets. Sizing each circuit's breaker is an electrician's job, done by looking at the wire's actual cross-section and the panel's total load, not by guesswork.
A rough reference table of ratings and curves
| Circuit | Typical rating | Curve |
|---|---|---|
| Lighting | 10A, 16A | B or C |
| Living room/bedroom outlets | 16A | C |
| Kitchen outlets / appliances | 16-20A | C |
| Electric stove | 32-40A | C |
| Air conditioner | 16-25A | C |
| Main incoming breaker (apartment/house) | based on load calculation | C |
This table shows common reference points only, not a ready-made solution for a specific property — the exact rating and curve are determined by the actual wire cross-section and the load being connected.
Breaker vs RCD: what's the difference
A circuit breaker protects wiring from overload and short circuits. An RCD (residual current device) solves a different problem — it cuts the circuit when current leaks, for example if someone accidentally touches a bare wire or an appliance casing is damaged. An RCD doesn't protect wires from overload and is always installed alongside a breaker, not instead of one.
There's also an RCBO — a combined device that packs breaker and RCD functions into one unit. It's more compact in the panel, but if one function fails, the whole unit needs replacing, whereas a separate breaker + RCD pair can be serviced piece by piece.
Common mistakes when choosing breakers
- Oversizing the rating "just in case" — the breaker doesn't trip under overload, and the wire heats up unnoticed.
- One breaker for an entire apartment or house instead of splitting into circuits — any fault cuts power to the whole home.
- Using a B-curve where starting currents are routine (fridge, pump) — the breaker trips falsely every time the device switches on.
- Installing a breaker with no RCD on a circuit where water contact is possible — bathroom, kitchen, outdoors.
If you're not sure what breakers are currently in your panel or whether they match the load, it's worth ordering a diagnostic check — that's cheaper than dealing with the consequences of overheated wiring later.