MCB rating guide, how to size a circuit breaker correctly
Undersized and your MCB trips constantly, oversized and it stops protecting the wire. Here's the actual process electricians use to pick the right amperage, explained without the jargon.
The basics, what an MCB does
A miniature circuit breaker sits in your distribution board and automatically cuts power to a circuit if too much current flows through it, either from overload or a short circuit. Unlike an old-style fuse, it can be reset with a switch once the fault is cleared, rather than replaced.
Every MCB carries a current rating in amperes, stamped on the front, the standard sequence used in India and most of Asia runs 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, and 63A. Picking the wrong one from this list is the single most common mistake in a home electrical setup.
The golden rule, protect the cable, not just the appliance
Most people size an MCB by looking at what they are plugging in. That is the wrong starting point. An MCB's real job is to protect the wire in the wall, not the appliance at the end of it. If the breaker is rated higher than the wire can safely carry, the wire can overheat well before the breaker ever trips.
In practice this means MCB size and wire gauge are chosen together, not separately. The reference table further down pairs the two as they are typically matched in Indian residential wiring.
The formula, working out your circuit's load
Start with the total wattage of everything that circuit will realistically power at once, then convert to current.
The 1.25 multiplier exists because a load running for more than three hours continuously is treated differently from a brief load, the extra headroom stops the breaker from nuisance-tripping under normal daily use while still protecting the circuit.
Reference, typical ratings by circuit type
This is how MCB rating, wire gauge, and circuit type typically pair up in Indian residential wiring. Treat it as a starting reference, not a substitute for an electrician's load calculation on your specific site.
| Circuit type | Typical MCB | Wire gauge | Trip curve |
|---|---|---|---|
| Lighting only | 6A | 1.0 to 1.5 sqmm | B |
| Lights, fans, and light sockets | 10A | 1.5 sqmm | B |
| General power sockets, TV, fridge | 16A | 2.5 sqmm | C |
| Heavier sockets, small AC | 20A | 2.5 sqmm | C |
| 1.5 ton AC, dedicated line | 25A | 4.0 sqmm | C |
| Geyser, 2 ton AC | 32A | 6.0 sqmm | C |
Trip curve, B versus C
The letter printed alongside the amperage on an MCB is its trip curve, and it matters as much as the rating itself.
- B curve trips at roughly 3 to 5 times the rated current. It suits steady, resistive loads, lighting, heaters, and plain sockets with no motor.
- C curve trips at roughly 5 to 10 times the rated current. It is the common default for home use because most modern appliances, ACs, fridges, mixers, washing machines, draw a brief surge of current on startup that a B curve breaker would misread as a fault.
If you are unsure which curve a circuit needs, C curve is the safer general-purpose default for anything with a motor in it.
Common mistakes
- Sizing by appliance alone. The wire, not the plug, is what needs protecting.
- Oversizing to stop nuisance tripping. If a breaker keeps tripping, the fix is investigating the load or the wiring, not fitting a bigger breaker.
- One rating for everything. Lighting, sockets, and AC circuits should each be on their own appropriately sized breaker, not sharing one.
- No dedicated line for high-load appliances. Geysers and ACs should ideally run on their own dedicated circuit, not shared with sockets.
- Buying uncertified breakers. A breaker that fails to trip reliably under fault conditions is not a safety device, it is a false sense of one.
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