How to choose an MCB for home circuits

The rule that decides everything else

An MCB is not protecting the appliance. It is protecting the cable. That single fact resolves most of the confusion around choosing one.

A cable of a given size can carry a certain current continuously without overheating, and that figure depends on the size, how the cable is installed, how it is grouped with others and how warm the space is. The MCB sits in front of it to make sure the cable is never asked to carry more than that for long enough to matter. So the rating has to satisfy two conditions at once:

• At or above the design current of the circuit, or it trips during normal use and everyone learns to ignore it.
• At or below the current-carrying capacity of the cable in its actual installed conditions, or the cable can overheat while the MCB sits there perfectly happy.

The second condition is the one that gets broken. Someone puts a 32 A MCB on a circuit wired in 1.5 sq mm to stop nuisance tripping, and has quietly removed the protection the cable depended on. If you take one thing from this page: the wire size and the MCB rating are one decision, not two.

Five steps to a rating

1. Work out what is on the circuit

Add up the loads the circuit will realistically carry at the same time. Appliance nameplates give either watts or a rated current; at a nominal 230 V single phase, current in amps is roughly watts divided by 230. A 3 kW geyser is therefore around 13 A, and a circuit of general sockets is judged on what actually runs together, not on the theoretical maximum of every outlet at once.

2. Find the ceiling the cable sets

This is the step people skip. The cable already run to that point has a capacity, and the MCB cannot exceed it. If the circuit is being designed now, the two are chosen together: the load sets the cable, the cable sets the ceiling, and the MCB lands between them.

3. Pick the nearest standard rating in between

MCBs come in standard steps. The ratings in our catalogue are 6, 10, 16, 20, 25, 32, 40 and 63 A. Choose the smallest standard rating that clears the design current comfortably, rather than the largest one the cable would tolerate.

4. Choose the trip curve

C curve is the general-purpose choice for Indian homes and covers mixed sockets, motors and air conditioners. B curve suits purely resistive loads such as lighting and heating. D curve is for heavy inductive starting loads and is unusual in a home. The MCBs we carry are C curve. Curve and rating are separate decisions. See the fuller explanation in our MCB vs MCCB guide.

5. Choose the number of poles

Single pole switches the live conductor only, and is the usual choice for lighting and general socket circuits on a single-phase supply. Double pole switches live and neutral together, which is what you want on a dedicated high-load appliance point such as a geyser or an air conditioner, because it isolates the appliance completely. Triple and four pole devices belong on three-phase circuits and board incomers.

Typical ratings by circuit

Which MCB for a 1.5 ton AC?

A 16 A or 20 A C curve MCB, double pole, on a dedicated circuit: that is the answer on most Indian domestic schedules. The reasoning behind it is more useful than the number, because it tells you when the number is wrong.

A 1.5 ton split air conditioner draws somewhere around 7 to 10 A while running, and considerably less than that on an inverter unit at part load. What sets the MCB is not the running current but the brief surge as the compressor starts. That is exactly what the C curve exists for: it tolerates a short starting inrush without treating it as a short circuit, so a correctly rated C curve device does not trip every time the compressor kicks in.

• Give the air conditioner a dedicated circuit. Sharing it with sockets means one appliance can trip the other, and it makes the rating harder to choose.
• Use a double pole device on that circuit so the point can be isolated completely for service.
• Check the cable. A 20 A MCB on a circuit wired for less is not a fix for tripping: it removes the protection the cable relies on.
• If the unit trips the breaker only on start-up, that is a curve and coordination question. If it trips at random during running, treat it as a fault and have it looked at.

MCB, RCBO, or both?

An MCB handles overload and short circuit. It does not detect earth leakage, which is the fault that matters for shock protection. There are two normal ways to cover both.

Our circuit-protection listing carries both. If you are deciding between the two, our RCCB guide covers what the residual current device is actually detecting and which sensitivity rating applies where.

Where domestic boards usually go wrong

1. Upsizing an MCB to stop nuisance tripping. The trip is information. Removing the protection does not remove the cause.
2. Choosing the rating before the cable is decided. It is one decision, and doing it in the wrong order means the ceiling is set after the fact.
3. Putting the air conditioner and the kitchen sockets on the same circuit, then wondering why the rating never feels right.
4. Single pole devices on dedicated high-load appliance points, so the point cannot be isolated properly for service.
5. Assuming an MCB provides shock protection. It does not, and an installation with no residual current device anywhere in it has a real gap.