TPN MCB Box: Three Phase DB Sizes, Ways and What Goes Inside

What is a TPN MCB box?

A TPN MCB box is a distribution board for a three phase supply. TPN stands for triple pole and neutral: the board carries three phase busbars, R, Y and B, plus a neutral link and an earth bar. It takes a four pole main switch, and each of its “ways” is a set of three positions, one on each phase.

We make the VIYONA TPN double door board in 4, 6 and 8 way, from ₹2,620 to ₹3,350 MRP, plus two versions with T.V. and telephone provision, at our works in Marol, Andheri East, Mumbai. We also make the four pole and three pole MCBs and isolators that go in it.

How a TPN board is laid out

Drawn by us, not to scale. The usual layout of a TPN board; confirm the exact arrangement of each size with your dealer.

TPN MCB box sizes and prices

All our TPN boards are double door: an inner cover that shows only the MCB levers and an outer door over the front. Printed MRPs from our current catalogue:

Per TPN way the larger boards are cheaper: about ₹655 at 4 way, ₹508 at 6 way and ₹419 at 8 way. Photos are on the MCB distribution double door product page.

Do you need a TPN board?

Only if your supply is three phase. Look at the meter or the electricity bill: a three phase connection brings four wires, three phases and a neutral, and is common in bungalows, row houses, larger shops, clinics, offices and small factories.

Most flats are single phase and take an SPN MCB box instead. A TPN board on a single phase supply does nothing useful; an SPN board on a three phase supply cannot take the other two phases.

Homes move to three phase when the sanctioned load grows: several ACs, an EV charger, a lift or a large pump. Our single phase vs three phase guide explains when that happens, and SPN vs TPN distribution board compares the boards.

What does “way” mean on a TPN board?

On an SPN board one way is one single pole MCB. On a TPN board one way is usually a set of three positions, one on each of R, Y and B. So a 4 way TPN board can normally hold 12 single pole MCBs (four on each phase), a 6 way 18 and an 8 way 24.

A three phase load takes a whole way: a three pole MCB for a motor with no neutral, or a four pole MCB where the neutral is switched too. Single phase circuits each take one position on one phase.

Our MCB box size list gives every SPN and TPN size in one table.

The main switch: four pole MCB or isolator

A TPN board takes a four pole main, so that one lever cuts all three phases and the neutral. There are two ways to do it from our range:

• Four pole MCB: 432 (40A, ₹855) or 433 (63A, ₹1,035). Switches and trips.
• Four pole isolator: 461 (40A, ₹480) or 462 (63A, ₹530). Switches only.

An isolator main is acceptable where the supply cable is already protected upstream and every outgoing circuit has its own MCB. Where the main must protect the cable, use the four pole MCB.

Rate the main for the supply, not for the sum of the outgoing breakers. A three phase home sanctioned for a modest load is usually well served at 40A per phase; a bungalow with a lift, several ACs and a borewell often needs 63A. The electricity company’s sanction and the size of the meter cable set the ceiling.

The isolator switch page covers the isolators and 3 pole vs 4 pole MCB explains when the neutral must be switched. The VIYONA 401 to 433 MCBs are made to IS/IEC 60898-1 under BIS licence CM/L-2544151; see the VIYONA 10 kA MCB page.

Three phase loads: three pole MCBs

A three phase motor, a lift, a borewell pump or a workshop machine with no neutral connection takes a three pole MCB. Ours come in 40A (422, ₹655) and 63A (423, ₹815).

Those ratings suit larger machines and sub-boards. For a small three phase motor needing a lower rating, the breaker must be sized to the motor, and our catalogue lists three and four pole breakers in 40A and 63A only.

For pumps, our MCB for water pump guide and motor starter guide cover sizing and starting.

Balancing the three phases

The point of three phases is to share the load. A good TPN layout spreads the heavy circuits across R, Y and B so that each phase carries roughly a third of the house.

• ACs on different phases: three ACs, one per phase
• Geysers spread: never all on R
• Kitchen on its own phase: hob and microwave together
• Lighting split: so one phase failing does not darken the whole house

An unbalanced board loads one phase to its limit while the other two idle, and that phase’s fuse at the meter goes first. Our electrical load calculation guide shows how to add up each circuit before assigning it.

What a TPN board costs, filled

For a bungalow with twelve single pole circuits, four per phase, a full 4 way board leaves no spares and no room for an RCCB, so size it as a 6 way TPN board: two spare positions per phase. With a 63A four pole MCB as the main, our parts at MRP come to:

• 388 6 way TPN double door: ₹3,050
• 433 63A four pole MCB: ₹1,035
• 12 × VIYONA 401 single pole MCBs: ₹1,740

That is ₹5,825 before RCCBs, wire and labour. Where the supply cable is already protected upstream, a 462 63A four pole isolator (₹530) can be the main instead, at ₹5,320.

T.V. + Tel. TPN boards

Two of our TPN boards are listed with T.V. + Tel. provision: 390 (6 way) and 391 (8 way), both ₹3,015 MRP. They suit buildings where the TV and telephone wiring is brought to the same location as the power board.

Ask your dealer for the layout of these two before ordering, since the space given to TV and telephone affects how many power circuits fit. Our RJ45 and TV socket guide covers the outlets at the room end.

Mistakes people make with TPN boards

Three phase boards fail in their own ways:

• Three pole main on a TPN board: the neutral is not isolated.
• Two phases on one switch board: 400 V within reach.
• All ACs on one phase: that phase trips, the others idle.
• TPN for a single phase flat: money spent on two empty busbars.
• Ways counted as poles: a 4 way TPN holds 12, not 4, single pole MCBs.

If one phase keeps tripping, the board is usually unbalanced. Our MCB tripping guide lists the safe checks.

TPN MCB box price and packing

Our TPN double door boards run from ₹2,620 (387, 4 way) to ₹3,350 (389, 8 way), with the T.V. + Tel. versions at ₹3,015. All are packed 10 per box and sold in full boxes only through dealers. Single phase double door boards, for comparison, run ₹750 to ₹1,410; see the double door MCB box page.

Every board, breaker and isolator we make is in our MCB price list.

4, 6 or 8 way: which TPN size?

List every circuit, assign each to a phase, and count the busiest phase. That count, plus two spare per phase, is the number of TPN ways you need; check whether the printed way count includes the four pole main and RCCB, and go one size up if it does.

• Row house: 3 lighting, 3 room, 3 AC, 2 geyser, 1 kitchen = 12 circuits, four per phase. A 4 way board (387, ₹2,620) is full on day one; the 6 way (388, ₹3,050) leaves two spare per phase.
• Clinic or office floor: 15 to 18 single phase circuits plus one three phase AC unit. Choose 8 way (389, ₹3,350).
• Bungalow, lift and borewell: two three phase loads take two whole ways; the rest is spread over the phases. Usually 8 way.

The step from 4 to 6 way costs ₹430 at MRP and adds six single pole positions, which is cheap insurance against adding a board later.

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.