MCB Size Calculation Explained: Watts to Amps, Cable and Rating

MCB size calculation: the short answer

MCB size calculation comes down to three steps. Work out the current the load draws (watts divided by 230 for a single phase supply). Add headroom for loads that run for hours. Then pick the next standard MCB rating above that figure, and check that the cable on the circuit can carry at least that rating.

So a 3,000 watt load draws about 13 A. With headroom that is about 16 A, which points to a 16A or 20A MCB, and the wire behind it must be rated for whichever one is fitted. The last check matters most, because an MCB exists to protect the wire, not the appliance.

This guide gives the formula, a watts-to-MCB chart, worked examples for common loads, the three phase version of the sum, and the mistakes that lead to a breaker that trips all the time or, worse, never trips. Which MCB each circuit in a home usually gets is covered in our which MCB for home guide; here we show how the number is worked out.

We make MCBs in our VIYONA 10 kA range from 6A to 63A at our works in Marol, Andheri East, Mumbai. The sums below are general practice. A licensed electrician confirms the final rating against the cable actually laid.

How to calculate MCB rating step by step?

To calculate MCB rating, find the load in watts from the appliance nameplates, divide by the supply voltage to get amps, multiply by about 1.25 for loads that run for long periods, round up to the next standard MCB rating, and confirm that the installed cable is rated at or above that MCB.

1. Add the watts 2. Watts ÷ 230 = amps 3. Add 25% headroom 4. Next standard rating 5. Cable ≥ MCB

Step 1 uses the input watts on the nameplate, not the output or the tonnage. Our appliance power consumption chart lists typical figures if the plate is unreadable.

Step 3 is common practice for loads such as geysers, heaters and ACs that run for hours: an MCB is built to carry its rating, and electricians commonly keep the steady load below 80% of it so the breaker and the terminals do not run hot.

Step 5 is the one people skip. If the cable cannot carry the rating you arrived at, the answer is a heavier cable, not a bigger breaker on the old one. Our wire size for AC and geyser guide works through the cable side.

What is the formula for MCB size?

The basic formula for MCB size on a single phase supply is current in amps equals power in watts divided by 230 volts, and for motors and other loads with a power factor below one the current is watts divided by 230 times the power factor, which gives a higher current for the same watts.

I = P ÷ 230 for heaters, geysers, irons and lights, where power factor is close to one.

I = P ÷ (230 × pf) for motors, pumps and compressors. A power factor of 0.8 raises the current by a quarter compared with the plain sum.

I = P ÷ (1.732 × 415 × pf) for a balanced three phase load, giving the current in each phase.

Then apply headroom and round up:

MCB ≥ I × 1.25 for loads that run for hours, as common practice.

The formula does not cover a motor’s starting surge, which can be several times its running current for a moment. That surge is handled by the MCB’s trip curve, not by a bigger rating; our B curve vs C curve MCB guide explains how. Voltage in your area may sit above or below 230 V; the sum is still close enough for choosing a breaker.

MCB rating chart: watts to MCB

An MCB rating chart converts a load in watts to its current at 230 volts and the next standard MCB above that current with headroom, so 1,000 watts needs about 4.3 A and a 6A MCB, 2,000 watts about 8.7 A and a 16A MCB, and 5,000 watts about 21.7 A and a 32A MCB.

The table below this section adds the headroom step and the MCB each figure points to. Two things to notice. The step from 16A to 20A falls right around a 3 kW load, which is why geysers of that size sit on either, depending on the cable. And anything above about 10 kW on one single phase circuit is beyond a 63A MCB and is a question of whether the home needs three phase, covered in our single phase vs three phase guide.

Which MCB for a 3000 watt load?

A 3,000 watt load draws about 13 A at 230 volts, and with the usual 25% headroom for a load that runs for long periods that becomes about 16.3 A, so it is commonly put on a 20A MCB with cable rated for 20 A, or on a 16A MCB where the load is short-duty and the electrician accepts it.

The sum: 3,000 ÷ 230 = 13 A then 13 × 1.25 = 16.3 A then next rating: 20A.

Why both answers appear in practice: a 16A MCB carries 13 A all day without tripping, and many 3 kW geysers sit on one. Electricians who apply the headroom rule step up to a 20A MCB, which then needs at least 4 sq mm cable in conduit by our catalogue’s table (24 A), where 2.5 sq mm is rated 18 A.

The same arithmetic covers other common loads:

1,500 W: 6.5 A → 10A 2,000 W: 8.7 A → 16A 2,500 W: 10.9 A → 16A

For an AC, use the nameplate input watts and the breaker in its manual; our MCB for 1, 1.5 and 2 ton AC guide gives the usual ratings by tonnage.

Why the cable decides the MCB size

The cable decides the MCB size because the MCB is there to protect the cable from overheating, so its rating must be no higher than the current the cable can carry as installed, which means a load that needs a bigger MCB also needs a heavier cable, never a bigger breaker on the existing wire.

Read across: 1.5 sq mm suits 10A, 2.5 sq mm suits 16A, 4 sq mm suits 20A and 6 sq mm suits 25A. A 16A MCB on 1.5 sq mm lets 14 or 15 A flow for hours through a wire rated 13 A, and nothing trips.

Our VIYONA wire is made for us by a partner to IS 694, and the table above is printed in our catalogue. Your electrician adjusts it for the actual run under IS 732.

How to calculate MCB size for the main switch

The main switch MCB is sized from the maximum demand of the whole home, not from the sum of every MCB in the board, so you add the loads likely to run together, convert that to amps at 230 volts, and choose a double pole rating at or above it that matches the sanctioned load and the incoming cable.

Adding every breaker gives a silly number: ten circuits of 16A and 20A add up to well over 150 A, which no home draws. The real peak is what runs at once on a hot evening.

A worked example: two ACs at 1,500 W each, a 2,000 W geyser, lights, fans, fridge and kitchen loads of about 1,500 W gives 6,500 W. That is 28 A, so a 40A DP main gives sensible headroom where a 32A main would run close to its limit.

The full method, with connected load, maximum demand and sanctioned load, is in our electrical load calculation guide. For the part: VIYONA 412 is a 40A double pole 10 kA MCB at ₹435 MRP, and 413 is the 63A version at ₹530.

MCB size for motors and pumps

MCB size for a motor or pump starts from its running current, found from the nameplate amps or from watts divided by 230 times the power factor, and is set above that current, allowing for the starting surge, which is why motor circuits commonly use a C curve breaker and often a motor starter rather than a bigger MCB.

A 1 HP motor is about 746 W of output. Its input is higher, and at a power factor around 0.8 the running current on single phase comes to roughly 5 to 6 A. The nameplate figure is always better than the sum.

The starting current can be several times the running current for a second or two. A breaker sized only to the running current, with a sensitive trip curve, may trip at every start. Raising the rating to cure that leaves the motor’s cable unprotected; the trip curve and a proper starter are the fix.

Our MCB for water pump guide works through 0.5 HP to 1.5 HP pumps, and our motor starter for water pump guide covers the KEMPS K223 starter, which protects the motor itself.

MCB size calculation for three phase

For a balanced three phase load, the current in each phase equals watts divided by 1.732 times 415 volts times the power factor, so a 10 kW heater load draws about 14 A per phase and a 10 kW motor load at 0.85 power factor about 16 A, and a three or four pole MCB is chosen above that figure.

10,000 ÷ (1.732 × 415) ≈ 13.9 A at power factor one.

10,000 ÷ (1.732 × 415 × 0.85) ≈ 16.4 A for a motor, about 16A per phase.

That is why three phase spreads a big load so well: the same 10 kW on single phase would draw 43.5 A on one circuit. In a home, though, the three phases rarely carry equal load. Lights and sockets are spread across phases, and each phase must be checked on its own; the heaviest phase sets the incomer.

Our three pole MCBs come in 40A (422, ₹655 MRP) and 63A (423, ₹815), and our four pole MCBs in 40A (432, ₹855) and 63A (433, ₹1,035). When to use three pole or four pole is covered in our 3 pole vs 4 pole MCB guide.

Standard MCB ratings: why there is no 13A or 30A MCB

MCBs come in a fixed set of standard ratings, commonly 6A, 10A, 16A, 20A, 25A, 32A, 40A and 63A in homes, so a calculated current of 13 A or 30 A is always rounded up to the next rating on that list, provided the cable can carry it, and never rounded down below the load.

6A 10A 16A 20A 25A 32A 40A 63A

Some ranges also offer 50A, and larger frames go above 63A, where an MCCB usually takes over; our MCB vs MCCB guide covers that line.

VIYONA 401 is a single pole 10 kA MCB rated 6A to 32A with an MRP of ₹145.

Common MCB size calculation mistakes

The most common MCB size calculation mistakes are fitting a bigger MCB to stop nuisance tripping, sizing the breaker to the appliance while ignoring the cable, adding every breaker’s rating to size the main switch, and using output power or AC tonnage in place of the input watts on the nameplate.

Upsizing to stop trips A breaker that trips is reporting overload or a fault. Our guide to an MCB tripping again and again sorts the causes.

Ignoring the cable The cable as installed sets the ceiling, every time.

Summing every breaker The main is sized to maximum demand, not to the board total.

Tonnage or output watts A 1.5 ton AC is a cooling figure, and a motor’s HP is output. Use input watts.

Forgetting future load An extra AC later can push the main over its rating. Planning spare ways and a 63A main costs little at the wiring stage.