25A MCB: The Sub-Main Breaker, Its Load and the Wire It Needs

What is a 25 amp MCB?

A 25 amp MCB is a miniature circuit breaker rated to carry 25 amperes continuously. It is the least common of the household ratings, and that is useful to know: a 25A breaker usually means a circuit that feeds something other than a single appliance. In homes it most often protects a sub-main, the cable from the main DB to a second board in a kitchen, a terrace room or a shop at the front of the house, or serves as the main switch of a small flat.

It also has the most awkward wire pairing of any rating, which the drawing below shows. We make 25A breakers in every MCB range in our catalogue at our works in Marol, Andheri East, Mumbai.

Why 4 sq mm falls short of 25A in conduit

Drawn by us, not to scale. Current ratings are from the wire table in our current catalogue.

25 amp MCB load capacity in watts

At 230 V a 25A MCB carries 25 A × 230 V = 5,750 W. On the 80% planning rule that is 20 A or about 4,600 W of steady load.

Very few single household appliances come near that. Where 5,750 W does appear is as the sum of several loads that a sub-board serves:

• Kitchen sub-board: hob, microwave, chimney and counter sockets
• Terrace or garden room: lights, a pump, an AC
• Small shop: lights, a display fridge, a 1 ton AC
• 1BHK main: where the sanctioned load allows it

The arithmetic for every rating is in our MCB size calculation guide.

Which wire goes with a 25A MCB?

By our catalogue’s wire table, 4 sq mm carries 24 A in conduit and 29 A clipped to a surface; 6 sq mm carries 31 A in conduit and 32 A clipped.

So 4 sq mm in conduit sits 1 A below a 25A breaker. It is close, and close is exactly the wrong place to be on a circuit that feeds a sub-board running for hours. Concealed in conduit, a 25A circuit needs 6 sq mm. Run on the surface in open air, 4 sq mm is enough.

That choice decides the price. The wire we sell is made for us by a partner, and its 90 m coils step up steeply:

For short sub-main runs a 45 m coil is usually enough: 6 sq mm is ₹5,085 and 4 sq mm ₹3,272. See our house wire price list.

When to choose 25A, and when not to

Choose 25A when the cable and the load both call for it: a 6 sq mm sub-main in conduit feeding a board whose outgoing breakers are 20A and below, or a small flat whose sanctioned load fits.

Do not choose it as a compromise between 20A and 32A. If an AC circuit needs more than 20A, check the appliance first; if a sub-main needs more than 25A, a 32A needs cable rated above 32 A in conduit, which is larger than 6 sq mm (31 A in our table) and outside our wire range; have the electrician size it. The 20 amp MCB and 32 amp MCB pages cover both neighbours.

A quick test before ordering: write down the cable size, whether it runs in conduit or on the surface, and the largest breaker on the board it feeds. If the cable is 6 sq mm in conduit and the largest outgoing breaker is 20A, a 25A feeder fits neatly between them. If either condition fails, 25A is the wrong rating, and the fix is in the cable or the board rather than the breaker.

Our 25A MCBs: codes and MRPs

Every range we make covers 25A within its span of ratings:

• VIYONA 401: single pole, 10 kA, ₹145, 12 per box
• VIYONA 411: double pole, 10 kA, ₹350, 6 per box
• VIYONA 133 single pole ₹150; 134 double pole ₹300; 177 surface DP ₹350
• VIZA V401 ₹230 and V1301 ₹281, single pole Tiny Trips
• VIZA V402 ₹495 and V1302 ₹543, double pole Tiny Trips
• KEMPS K225 ₹295 single pole; K226 ₹625 double pole, printed for 20A and 25A

The VIYONA 401 to 433 range is made to IS/IEC 60898-1 under BIS licence CM/L-2544151; see the VIYONA 10 kA MCB page.

A 25A double pole breaker for a sub-board

A sub-main is best protected by a double pole breaker, so that switching it off isolates phase and neutral for everything on the sub-board. In the main DB that is the VIYONA 411 at ₹350, set to 25A.

At the sub-board itself, an incomer lets someone working there cut the supply without walking back to the main DB. A double pole isolator does that job; our VIYONA 442 is a 40A double pole isolator at ₹230. An isolator only switches. It does not trip on overload, which is why the 25A breaker in the main DB is still needed.

The isolator switch page lists all six of our isolators, and isolator vs MCB explains the difference.

The sub-board itself

A kitchen or terrace sub-board is small, but count it the same way as a main board: outgoing circuits + 2 ways for a DP incomer + 2 for an RCCB + 2 spare, and check whether the box’s printed way count includes the incomer. Three outgoing circuits with a DP isolator and two spares come to 7 ways, a 6–8 way box; add an RCCB and it is 9, so a 10–12 way box or a separate RCCB enclosure. A 2–4 way box suits only one or two circuits. Our SPN white line MCB boxes start at ₹380 (code 371, 2–4 way) and ₹480 (372, 6–8 way). For a single breaker on the wall, the PVC surface MCB box for 1 and 2 ways is ₹40 (375).

The SPN MCB box page covers sizes and the MCB box size list gives every box we make.

Mistakes people make at 25A

Because 25A is uncommon, the mistakes tend to come from treating it as a slightly bigger 20A:

• 4 sq mm in conduit: rated 24 A behind a 25 A breaker.
• 25A outgoing on a 25A feeder: with equal ratings, either may trip first.
• Main above sanctioned load: the main must match the supply, not the wish list.
• No neutral isolation: a single pole feeder leaves the sub-board neutral connected to the system.

On the main switch question, the sanctioned load and the size of the meter cable are set by the electricity company; our single phase vs three phase guide explains when a home outgrows single phase altogether.

25 amp MCB price and boxes

A 25A single pole breaker from us is ₹145 (VIYONA 401) to ₹332 (KEMPS K625 grey). A 25A double pole is ₹300 (VIYONA 134) to ₹698 (KEMPS K626 grey).

A small kitchen sub-board at MRP, with a VIYONA 411 25A feeder (₹350) in the main DB, a 372 SPN box (6–8 way, ₹480) and three VIYONA 401 outgoing breakers (₹435), comes to ₹1,265 before the isolator, RCCB and wire. We sell in full boxes only, through dealers. Every breaker and box is in our MCB price list.

Single appliances that ask for 25A

A few appliances do need a 25A circuit on their own. The test is the nameplate, not the category: divide the rated watts by 230 V, or read the maximum current the maker prints.

• 4,500 W instant heater: about 19.6 A steady. With headroom, a 25A circuit on 6 sq mm in conduit, with a 30 mA RCCB in a bathroom.
• Large inverter AC: some 2.5 and 3 ton units print a maximum current near 20 A; the installer’s manual then names the breaker.
• Commercial kitchen equipment in a small restaurant or tiffin service: a tandoor oven or a large fryer.

Most homes have none of these. A 2,000 W storage geyser, a 1.5 ton AC or an ordinary induction cooktop all sit comfortably on 16A or 20A circuits, which are cheaper to wire.

25A as the main switch of a small flat

In a 1RK or a small 1BHK without ACs, the electricity company may sanction a load low enough for a 25A double pole main breaker to make sense. The main then protects the cable from the meter and cuts the whole flat, phase and neutral, with one lever.

The moment the flat gains an AC, the arithmetic changes: lights and sockets plus a 20A AC circuit can together run above 25A for hours on a hot evening, past what the main and its cable are sized for, and the main eventually trips. That is the point to talk to the electricity company about the sanctioned load, and to move the main up to 32A or a 40A breaker such as VIYONA 412 at ₹435.

Our MCB for home guide shows a full set of breakers for a typical flat, and the load calculation guide adds up the total.

Where we supply

Everything on this page is made at our works in Marol, Andheri East, Mumbai, and sold through dealers across Maharashtra and in many other states. See where we supply in Maharashtra and how we supply the rest of India. Orders placed through our office are delivered all over India, in full boxes only.