Plant Room Sizes for a Factory: DG, Transformer, Fire Pump, STP and AHU Rooms

Plant room sizes in a factory come from the equipment, not from the shed's floor area. The National Building Code gives guide areas for substations and DG rooms by capacity, sets conditions for the fire pump house, and expects the STP and AHU rooms to be sized from the vendor's layout. Fix each room's size when the equipment list is fixed, before the architect freezes the plot plan.

What do the codes give for each room?

The table summarises what we could read in the National Building Code of India 2016 (NBC), the CEA safety regulations 2023 and the CPCB noise rules. Where the code gives no area, the table says the vendor's drawing decides.

The Annex figures are stated for general guidance. The NBC says the DG room area and height may finally be fixed according to actual requirements, and that other transformer capacities may change the dimensions slightly. Use them to reserve space on the plot plan, then replace them with the vendor's drawings.

How do you size the DG room?

Start from the load, not the room. The generator size comes from the essential load list; our DG sizing calculator gives a first kVA from that list. Then read the manufacturer's footprint, add service clearances and add the air path: cooling air in, hot air out and an exhaust route to a safe discharge point.

NBC Part 8 Section 2 says an emergency generator should not be installed above the ground floor or below the first basement, that a basement DG room's ceiling should be the ground floor slab, and that a utility building is preferable. In an enclosed room it requires forced ventilation so that the equipment derates as little as possible. It prefers the generator next to the MV switchgear to ease load transfer and keep vibration and noise away from the main building, with a separate direct escape and entry from outside so the electrical supply can be cut in a fire.

Noise is the other design input. CPCB's rules for diesel sets up to 1,000 kVA manufactured after 1 January 2005 set 75 dB(A) at 1 metre from the enclosure, with an integral acoustic enclosure. Where the room is acoustically treated instead, the design is for at least 25 dB(A) insertion loss or the ambient standard, whichever is higher, and the set needs an exhaust muffler of the same loss. The user is asked to bring noise outside the premises within ambient limits by siting and control. State boards regulate this, so check with yours. A bulk diesel tank brings its own safety distances and licence questions, covered in our notes on diesel tank distances and PESO licensing.

What does the transformer room need besides floor area?

Oil and fire. Regulation 46(2) of the CEA regulations requires a fire barrier wall, of the thickness the relevant standards give, between oil-filled apparatus holding more than 2,000 litres of oil and between that apparatus and an adjacent building, unless the building wall is rated for four hours. It requires a suitable oil soakpit, and draining arrangements where more than 9,000 litres of oil sits in one tank or chamber. Spare oil must not be stored near oil-filled equipment. Oil-filled transformers installed indoors in buildings other than residential or commercial ones go on the ground floor or not below the first basement. Cable trenches inside substations are filled with sand or similar non-inflammable material or covered with non-inflammable slabs.

Growth is the usual cause of regret. The NBC suggests allowing for future load, splitting load across more than one transformer and providing for a standby. A room sized for exactly today's transformer has no way to take the second unit. Our note on transformer undersizing covers the cost of finding this out late.

What does the fire pump house need?

NBC Part 4 gives a list. Put the pump house at ground level where possible, directly reachable from outside; if it is below ground, no lower than the second basement, with direct staircase access that does not pass through other occupancies. Separate it by fire walls with 120-minute fire doors, ventilate it, avoid water stagnation, and keep other utility equipment out. Negative suction and submersible pumps are not allowed. It must be large enough for the pumps, valves, pressure reducing valves, the diesel tank and the electrical panel. The static tank must be in two or more interconnected compartments of equal size, and the underground tank not deeper than 7 m from the level of the fire brigade draw-out connection.

One caution for factories: Table 7 of NBC Part 4 states its Group G requirements for small-scale industry units. For other industries, the requirements are worked out from the relevant Indian Standards and in consultation with the local fire authority. So the tank and pump sizes for your plant come from the fire design, and the room follows. See our notes on pump room layout mistakes and tank sizing errors.

What should you give the architect?

Give a room schedule with six entries for each room: floor area, clear height under the beam, floor loading, door and equipment-removal route, ventilation and drain provisions, and cable entry. Give it when the equipment list is frozen, with the vendor's drawings attached where you have them. Architects can then place rooms on the plot plan with correct clearances, road access for the fire tender and a route for crane and truck. Our page for architects sets out how we coordinate MEP rooms with the building design.

How Secured Engineers helps

Secured Engineers designs electrical, fire, HVAC and plumbing systems in-house, so the room schedule and the equipment layouts come from one design. Fire NOC, CEIG and DISCOM filings are handled in-house, and MEPF and solar can be placed under one contract. We work from offices in Noida (Delhi NCR) and Ludhiana. The company has delivered 535+ projects across 18+ states, with 89.77% repeat clients.

If you have a plot plan and an equipment list, we can review the plant room schedule against the code points above and mark the rooms that look short before the drawings freeze. Start with the DG sizing calculator, then send the plan by WhatsApp, call +91 70099 85817, or ask for a free proposal that begins with a budget and layout review.