MCB Size Calculator

Turn a load in watts or amps into a design current and the next standard MCB rating from the IEC 60898-1 preferred list — with the trip curve that suits the load and the check that matters most: the breaker must protect the cable.

Last updated 22 Sep 2026

MCB rating for a load

Load → design current → rating

Load given as

Load power (W)

Load current (A)

Voltage (V)230 V single-phase in India and most of the Gulf, 120 V in North America; for three-phase the line-to-line voltage.

System

Power factor1.0 for heaters and geysers; about 0.8–0.9 for motors, pumps and air conditioners.

Continuous-load factor (%)125% is what the NEC requires for a load that runs three hours or more (210.20(A)); IEC and IS installations size the device directly on the design current, so enter 100 unless your code’s continuous-load rule applies.

Cable ampacity Iᴢ from your code’s table (A), 0 = skip the checkThe derated current-carrying capacity of the cable you intend to use, for its insulation, installation method, ambient temperature and grouping. No code table is reproduced here.

Trip curve

Ratings to choose from

Supply, breaker, cable, load. The breaker's job is to protect the cable between it and the load: it turns red if its rating is above the cable ampacity you entered, because then the cable can overheat without the breaker ever tripping. The dots show the size of each RMS current; a real 50 Hz or 60 Hz current reverses a hundred or a hundred and twenty times a second, so the direction drawn is only a convention. The supply is drawn with the battery symbol, the only source symbol in this set; on AC the + marking is just a reference direction.

20AExample

a 3,000 W geyser at 230 V single-phase, power factor 1, with the NEC’s 125% continuous-load factor

Design current and rating

I_B = P ÷ (V × PF) (or ÷ √3 × V_LL × PF); I_design = I_B × factor; I_n = next preferred rating ≥ I_design, subject to I_B ≤ I_n ≤ I_Z

I_B: design current: the current the circuit is meant to carry
I_n: rated current of the breaker, from the preferred list
I_Z: the cable’s derated current-carrying capacity, from your code
I₂: the current at which the breaker trips conventionally: 1.45 × I_n for an MCB, which is why I₂ ≤ 1.45 I_Z is satisfied automatically when I_n ≤ I_Z

Worked example

a 3,000 W geyser at 230 V single-phase, power factor 1, with the NEC's 125% continuous-load factor
I_B = 3,000 ÷ (230 × 1) = 13.04 A
Design current = 13.04 × 1.25 = 16.30 A
Next preferred rating at or above it: 20 A (16 A would be next below, and is the answer if you size on the design current alone)
Curve C: it will not trip instantaneously below 100 A and must by 200 A; thermally it holds 22.6 A for an hour and must trip by 29.0 A
The cable must then be good for at least 20 A after derating — check Iᴢ in your code’s table before accepting this rating

Trip curves (IEC 60898-1 instantaneous tripping ranges)

The ranges are from IEC 60898-1; the applications are ordinary installation practice, not part of the standard. All three curves have the same thermal behaviour for slow overloads — they differ only in how fast they clear a short circuit.
Curve	Instantaneous trip	Example at 16 A	Typical use
B	3 to 5 × I_n	48–80 A	Resistive and lighting circuits, sockets, long cable runs in domestic installations
C	5 to 10 × I_n	80–160 A	Mixed loads with moderate inrush: small motors, fluorescent and LED lighting banks, air conditioners
D	10 to 20 × I_n	160–320 A	High inrush: transformers, welding sets, capacitor banks, larger motors started direct on line

Design current for common loads (computed by this page)

Worked through this page’s own formula. They are examples, not recommendations: the cable, the installation method and your local code decide the final rating, and the breaker may never exceed the cable’s ampacity.
Load	Assumed	Design current	Rating at 100%	Rating at 125%
LED lighting circuit	400 W at 230 V, PF 0.9	1.93 A	6 A	6 A
Sockets, general purpose	2,300 W at 230 V, PF 1.0	10.00 A	10 A	13 A
Water heater (geyser)	3,000 W at 230 V, PF 1.0	13.04 A	16 A	20 A
1.5 ton air conditioner	1,600 W at 230 V, PF 0.9	7.73 A	8 A	10 A
Induction hob	3,600 W at 230 V, PF 1.0	15.65 A	16 A	20 A
1 HP water pump	900 W at 230 V, PF 0.8	4.89 A	6 A	8 A
11 kW three-phase machine	11,000 W at 415 V, PF 0.85	18.00 A	20 A	25 A

Choosing an MCB rating

A miniature circuit breaker does two jobs with two mechanisms. A bimetallic strip handles slow overloads: it bends as it warms, and the more the current exceeds the rating the sooner it trips. An electromagnet handles short circuits: above a multiple of the rating it throws the contacts open in milliseconds. The rating I_n is the current it will carry indefinitely; to IEC 60898-1 it must not trip within an hour at 1.13 × I_n (22.6 A for a 20 A device) and must trip within that hour at 1.45 × I_n (29.0 A).

Start from the design current. Work out what the circuit will actually draw — watts ÷ (volts × power factor), or √3 × volts × power factor for three-phase — then apply whatever continuous-load factor your code demands. The NEC requires the device and the conductor to be sized at 125% of a load that runs three hours or more; IEC 60364 and IS 732 instead require only I_B ≤ I_n ≤ I_Z, with the inrush of motors handled by the trip curve rather than by oversizing. Take the next preferred rating at or above the result.

The breaker protects the cable, not the appliance. This is the point people get wrong. If a 16 A cable is protected by a 32 A breaker, a 25 A fault will sit there heating the insulation forever without ever tripping. Never fit a larger breaker to stop nuisance tripping: find out why it trips. If the circuit genuinely needs more current, the cable has to grow first. Enter the cable’s derated ampacity above and the page checks I_n against it.

Curves. B, C and D differ only in the instantaneous threshold — 3–5, 5–10 and 10–20 times the rating. A motor drawing six times its running current for a second will trip a B curve and hold on a C; a transformer’s magnetising inrush may need a D. But a higher curve needs more fault current to operate quickly, so on a long circuit with a high earth-fault loop impedance a D curve may not clear a fault in the required time at all. That calculation — loop impedance, disconnection time, breaking capacity (6 kA or 10 kA), discrimination with the upstream device and the RCD or RCBO for shock protection — is the electrician’s, not this page’s.

Size the cable for the drop with the voltage drop calculator or the wire size calculator, convert the load with the watts to amps calculator, and check what it all costs with the electricity consumption calculator.

Frequently asked questions

What size MCB do I need for a 3 kW geyser?

At 230 V and unity power factor the load is 13.04 A. Sized on the design current alone that is a 16 A device; with the NEC’s 125% continuous-load factor it is 16.30 A, so 20 A. Either way the cable must be good for the rating you choose.

What is the difference between B, C and D curves?

Only the instantaneous trip threshold: B trips between 3 and 5 times the rating, C between 5 and 10, D between 10 and 20. Use B for lighting and resistive loads, C for mixed and small-motor loads, D for heavy inrush. The slow-overload behaviour is identical.

Can I fit a bigger MCB to stop it tripping?

No. The breaker is sized to protect the cable; a larger one leaves the cable unprotected. Find the cause — an overloaded circuit, a faulty appliance or a genuine fault — or have the circuit rewired in larger cable and then uprated.

Is an MCB the same as an RCD or RCBO?

No. An MCB protects against overload and short circuit; an RCD detects current leaking to earth and protects people. An RCBO is both in one module. Most codes now require residual-current protection on socket and wet-area circuits as well as the MCB.

What are the standard MCB ratings?

IEC 60898-1’s preferred values are 6, 8, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100 and 125 A; 8 A and 13 A are rarely stocked outside a few markets, and ratings below 6 A are also made. Above 125 A the device is a moulded-case circuit breaker to IEC 60947-2.

Related calculators

References

IEC 60898-1:2015, Circuit-breakers for overcurrent protection for household and similar installations: preferred rated currents and the type B, C and D instantaneous tripping ranges (reproduced in BEAMA, Guide to Low Voltage Circuit-breaker Standards, March 2022).
IEC 60364-4-43:2008, Low-voltage electrical installations — Protection against overcurrent, clause 433.1: I_B ≤ I_n ≤ I_Z and I₂ ≤ 1.45 × I_Z.
NFPA 70, National Electrical Code: 210.19(A)(1) and 210.20(A) (noncontinuous load plus 125% of the continuous load) and the informational notes to 210.19(A) and 215.2(A) (3% branch, 5% total voltage drop, advisory).
IS 732:2019, Code of practice for electrical wiring installations (Bureau of Indian Standards).
Schneider Electric, Complementary technical information — tripping curves (CA908024E, 2021): the conventional thermal points for EN 60898-1, 1.13 I_n (no trip) and 1.45 I_n (trip).