B Curve vs C Curve MCB, Plus D: Trip Bands and Where Each Fits

B curve vs C curve MCB: the short answer

The difference between a B curve and a C curve MCB is how big a sudden surge has to be before the breaker trips instantly. A B curve trips instantly at 3 to 5 times its rating, a C curve at 5 to 10 times. Both trip the same way on a slow overload.

That one difference decides where each fits. A B curve reacts to smaller fault currents, which suits lighting and heating circuits and long cable runs. A C curve rides through the brief starting surge of motors, compressors and pumps without tripping, which is why it is the common general-purpose choice for mixed home circuits.

A third type, the D curve, trips instantly only at 10 to 20 times its rating and belongs to heavy-inrush equipment such as large motors and transformers. It is rare in homes.

The curve is printed as a letter in front of the rating, so a B16 and a C16 are both 16A breakers that differ only in their instant trip band. How an MCB’s thermal and magnetic parts work is drawn in our MCB working principle guide. Here we compare the curves, show which suits each kind of circuit, and explain why the curve should never be used as an excuse to fit a bigger rating.

What is the difference between B curve and C curve MCB?

The difference between a B curve and a C curve MCB is the instant, magnetic trip band: under IS/IEC 60898-1 a B curve trips instantly between 3 and 5 times its rated current and a C curve between 5 and 10 times, so a B16 trips instantly from 48 A and a C16 only from 80 A.

Read the chart as a band, not a point. A B16 will not trip instantly below 48 A and must trip instantly by 80 A. A C16 has the same promise between 80 A and 160 A.

Below those bands, both breakers rely on the thermal trip, which works the same way on every curve. So for a slow overload a B16 and a C16 behave alike. The curve only matters when current jumps suddenly, on a fault or a motor start.

What are the types of MCB curves?

The MCB curve types used in homes and small buildings are B, C and D, set out in IS/IEC 60898-1, and they differ only in the multiple of rated current at which the breaker trips instantly, with B the most sensitive, C the general-purpose middle and D the most tolerant of surges.

B: 3–5 × rating lighting, heaters, geysers, long runs to outlying points.

C: 5–10 × rating mixed home and office circuits, sockets, ACs, fridges, small pumps.

D: 10–20 × rating transformers, large motors, welding sets and other heavy inrush.

You may also see K and Z curves. These come from a different breaker standard and are for specialist jobs: Z is very sensitive and protects electronics and instrument circuits, K is built for motors and transformers with a strong start. Neither is a normal choice in a home board.

All three letters share the same thermal overload behaviour and the same rating. What they do not share is the cable they can protect at the far end of a long run, which the next sections explain. Breaking capacity, the kA figure, is a different property again; our 6kA vs 10kA MCB guide covers it.

Which curve MCB is best for home?

For most homes a C curve MCB is the common general-purpose choice, because household circuits mix lights with motors, compressors and electronic loads that draw a short surge at switch-on, while a B curve is often preferred for pure lighting or heating circuits and for long cable runs where the fault current is low.

A sensible split, as general practice:

Lights and fans: B or C Geyser, heater: B or C Sockets: C AC, fridge: C Pump, motor: C Main incomer: per electrician

The safest way to decide is to let the electrician choose the curve along with the rating, and then check the face of the breaker you buy. Mixing curves in one board is normal. What matters is that each circuit gets the curve that suits its load and its cable length.

Ratings for each home circuit are in our which MCB for home guide, and the arithmetic behind them in our MCB size calculation guide.

When is a D curve MCB used?

A D curve MCB is used on equipment with a very large switch-on surge, such as transformers, big motors, welding sets, X-ray machines and some industrial power supplies, because it only trips instantly at 10 to 20 times its rating, and it is rarely the right choice for an ordinary home circuit.

The trade-off is real. A D16 needs between 160 A and 320 A of fault current to trip instantly. On a long, thin home circuit the fault current may never reach that, so a short circuit at the far end could be cleared only slowly by the thermal part, leaving the cable to overheat in the meantime.

That is why a D curve should be chosen by an electrician who has measured or calculated the fault current at the far end of the circuit, not picked off a shelf because a breaker keeps tripping. In homes, the loads that tempt people towards D are usually a pump or an old fixed-speed AC, and both are normally fine on a C curve with the right rating and a healthy motor.

Why does the curve matter on long cable runs?

The curve matters on long cable runs because a fault at the far end of a long, thin cable produces less current than one close to the board, and a breaker only trips instantly if that fault current reaches its magnetic band, which a B curve reaches at a much lower current than a C or D curve.

Electricians check this as the earth fault loop: the total resistance of the path a fault current takes. The longer and thinner the cable, the higher that resistance and the lower the fault current. A garden light, outhouse or terrace point at the end of a long run is the classic case where a B curve protects better than a C.

A healthy earth matters as much as the curve. Our how to check earthing guide covers the safe checks.

Does the curve change the overload trip?

No, the curve letter does not change how an MCB trips on a slow overload: B, C and D breakers of the same rating share the same thermal behaviour under IS/IEC 60898-1, which is to carry 1.13 times their rating for an hour without tripping and to trip within an hour at 1.45 times their rating.

1.13 × rating: no trip 1.45 × rating: trips

For a 16A breaker of any curve, that means about 18 A is carried for an hour and about 23 A trips it within an hour, faster as the current rises. The thermal part bends slowly as it heats, which is why an overload takes minutes and a short circuit takes a fraction of a second.

So a breaker that trips after an AC or geyser has run for a long time is not a curve problem. It is too much current for the rating, a loose connection heating the breaker, or a wrong rating for the load. Changing the curve will not fix that, and neither should a bigger rating on the same cable. Our guide to an MCB tripping again and again sorts trips by pattern.

B curve or C curve for AC, fridge and pump?

Where the curve is marked, ACs, fridges and pumps commonly go on C curve MCBs, because their compressors and motors draw a starting surge that can reach the instant band of a B curve breaker and trip it at every start, while a C curve rides through that surge and still trips quickly on a real short circuit.

AC: C common Fridge: C common Washing machine: C Pump: C Inverter supply: per manual

Inverter ACs start gently, so the surge is smaller than with a fixed-speed unit, but a C curve is still the usual choice. The AC-specific ratings by tonnage are in our MCB for 1, 1.5 and 2 ton AC guide, and pumps in our MCB for water pump guide.

A geyser is a resistive load with almost no surge, so either curve suits it; the rating and cable decide. If a fridge or AC trips a breaker only at start-up, the curve is worth checking. If it trips after running for a while, look at the rating, the wiring and the appliance.

Can LED lights trip a B curve MCB?

Yes, a circuit with many LED lights or drivers on one breaker can occasionally trip a B curve MCB at switch-on, because each driver draws a brief charging surge at power-up, and many of them together can briefly reach the instant band of a B curve, while the same circuit on a C curve usually does not trip.

The surge lasts only a very short time, but a breaker’s magnetic part responds to the peak. One or two lamps are no problem. A long row of panel lights or a shop ceiling full of downlights on one switch is where it shows up.

The fixes, in the order an electrician usually tries them: split the lights across two circuits, switch them in groups, or use a C curve of the same rating. Not a bigger rating.

For lighting layouts in a shop or office, our LED lights for shop guide covers how panels and downlights are grouped. And if lights flicker rather than trip, our LED light flickering guide covers the other causes.

How to read the curve on an MCB

The curve is printed on the face of the MCB as a letter in front of the rating, such as B16, C20 or D32, usually next to the breaking capacity in kA and the standard it is made to, so you can read the curve, rating and kA together without opening anything.

Letter: curve Number: rating in amps kA: breaking capacity Standard: IS/IEC 60898-1

If a breaker shows only a number, check its packaging or ask the dealer before relying on any curve. Do not guess from the price or the size of the body; B, C and D breakers of one range usually look the same.

On our devices, the face carries the rating, the 10 kA breaking capacity of our VIYONA 10 kA range, IS/IEC 60898-1 and licence CM/L-2544151. Our catalogue does not state a trip curve, so we do not claim one. VIYONA 411 is a double pole 10 kA MCB rated 6A to 32A with an MRP of ₹350.

The full range, single to four pole, is on our VIYONA 10 kA MCB page.

Before you change an MCB curve

Before you change an MCB from one curve to another, find out why the old one tripped, check that the new curve will still trip instantly on a fault at the far end of that circuit, keep the same rating unless the cable is also changed, and have a licensed electrician fit it with the supply off.

1. Find the cause 2. Same rating 3. Check the far-end fault 4. Electrician fits it

Changing a curve is a small swap on the rail, but it changes how the circuit is protected. Moving from B to C doubles the fault current needed for an instant trip; moving to D doubles it again. On a short run with a healthy earth that is usually fine. On a long run it may not be.

The way an MCB is replaced in the board is covered in our how to replace MCB guide, and how to tell a good breaker from a faulty one in how to check MCB is good or bad.