In short. A commercial building does not need one generator; it needs a power-continuity plan that says which services run during an outage, in what order, and for how long. Life-safety systems come first by regulation, then lifts, pumps, security and tenant IT, then comfort cooling if the owner chooses to pay for it. Size on the retained loads and the starting demand of lifts, pumps and chillers, design the room for an occupied building, and test safely.
Four classes of building load
Office towers in Hyderabad’s IT corridors, mixed-use blocks in Secunderabad and smaller commercial buildings across Telangana share one planning error: the owner asks for a generator “for the building” and the supplier quotes the transformer rating. The useful conversation starts with classes of load.
| Class | Typical building services | Who decides | Transfer expectation |
|---|---|---|---|
| Life safety | Fire pumps, emergency and exit lighting, smoke extraction and pressurisation fans, fire lift, public address, fire alarm | Statutory, fire authority, building codes | Automatic, within seconds |
| Essential | Passenger lifts, domestic water and sewage pumps, security and access control, BMS, common area lighting, basement ventilation | Owner and facility team | Automatic, short break |
| Tenant critical | Server rooms, UPS input, trading or call-centre floors, tenant process loads | Lease and tenant agreements | UPS no-break, generator behind |
| Comfort | Chillers, air handling units, tenant air conditioning, façade and decorative lighting | Commercial choice | Delayed or shed |
The classes also decide the architecture. Life safety and essential loads are almost always on an automatic transfer arrangement. Comfort cooling can be a second step added by a load management controller once the first loads are stable, or excluded entirely in a building where the outage history is short. In multi-tenant buildings the tenant-critical class is usually metered separately on generator so that DG units can be billed. The classification method is explained on critical, essential and mission-critical power.
Sizing: retained load plus the motors that start together
The nameplate total of a building says little about the generator. What matters is the running load of the retained classes, measured where possible from the building’s energy meters, and the starting events that follow a transfer. In a commercial building those events are lifts, which have high inrush and regenerate on descent; domestic and fire pumps, often started direct-on-line; and chillers, which are by far the largest step if comfort cooling is retained.
S_gen = (P_life-safety + P_essential) / PF + k × S_UPS + S_largest-step
Here S_gen is the indicative generator kVA, P_life-safety and P_essential are the running kW of those classes, S_UPS is the tenant UPS rating with k the generator-to-UPS ratio for its rectifier type, PF is the power factor and S_largest-step is the transient kVA of the largest single start after the base load is connected. For a ten-storey office with 60 kW of life-safety load, 180 kW of essential load, a 200 kVA modern UPS with k of 1.2, PF of 0.85 and a 45 kW domestic pump starting direct-on-line, S_gen is roughly 520 kVA plus about 350 kVA of transient, pointing to a 750 kVA class set with comfort cooling excluded (illustrative). Add two 400 kW chillers and the answer becomes a paralleled plant of 1,500 kVA or more. That difference is the real commercial decision for the owner. The method is set out on generator sizing.
One set or a paralleled plant
Buildings whose retained load exceeds about 750 to 1,000 kVA, or which cannot accept losing all backup when one set is under service, generally do better with two or three sets on a common bus. Load management then adds chillers only when capacity allows and sheds them first when a set trips. The arrangements are compared on generator paralleling.
Transfer and distribution interfaces
Provide the existing single-line diagram and identify the circuits proposed for generator supply. In a new building, the essential and life-safety loads are usually fed from a dedicated section of the main switchboard through an automatic transfer switch or a four-pole changeover; in an existing building the first task is often to separate these loads from the general distribution so that transfer does not energise the whole building. Fire pump supplies have their own code requirements for routing and protection. Neutral switching, earth-fault protection on generator supply and the coordination of breakers on the lower generator fault level all need a review rather than an assumption. Transfer options are described on automatic transfer switches.
The generator room in an occupied building
Where the generator sits matters more in a commercial building than anywhere else because people live with it. The main points are these.
- Ventilation must remove radiator heat and supply combustion air; a basement room without a proper inlet and outlet path will overheat the set within an hour at Hyderabad summer ambient.
- The exhaust stack must meet the CPCB requirement, which for diesel sets is a stack height of the building height plus 0.2 times the square root of the kVA rating, discharging away from air intakes and neighbours.
- Noise at the room boundary or enclosure must meet the CPCB limit of 75 dB(A) at one metre for enclosed sets, and the room should be treated acoustically if offices are adjacent.
- Fuel storage needs containment, ventilation and a location acceptable to the fire authority, with a day tank near the set and bulk storage outside the building where possible.
- Maintenance access, including a route to remove a radiator or an alternator, should be designed in before the room is enclosed.
The ventilation design is explained on generator room ventilation and the noise question on acoustic engineering.
Testing in an occupied building and handover
A commercial building generator must be tested on load without putting tenants and safety systems at risk. The sensible pattern is a monthly transfer test at a published time, usually early morning or a weekend, with tenants informed, lifts parked and UPS units confirmed healthy beforehand, and an annual load-bank test to prove rating. At commissioning the full sequence, mains failure, generator start, transfer, lift and pump restart and retransfer, should be witnessed and recorded. Handover should assign who operates the set, who refuels it, who receives alarms and who holds the service contract; buildings where this is undefined are the ones whose generators fail quietly. The programme is described on commissioning.
Common mistakes
- Sizing from the transformer or sanctioned demand. The set is two or three times too large, runs lightly loaded, and costs more to own and maintain than the building needed.
- Putting comfort cooling on the generator without load management. Chiller starts trip the set during the first summer outage and the lifts stop with it.
- Ignoring stack height and ventilation in a basement room. The set overheats and the exhaust re-enters the building’s fresh air intakes.
- Testing only on no load because tenants complain. The engine has never carried a lift start and nobody knows whether it can.
- Leaving tenant UPS input out of the load study. The rectifier’s distorted current and recharge demand were never counted.
What to send us
Send the building single-line diagram, a load list by class with motor ratings for lifts, pumps and chillers, the tenant UPS ratings, the proposed generator location with its dimensions, ventilation openings and distance to neighbours, and any fire authority or lease requirements you already hold. For an existing building, photographs of the main switchboard, the generator nameplate and the room help. Send your building power requirement.
Questions engineers ask
What size generator does an office building need?
For life-safety, essential and tenant-critical loads without comfort cooling, many mid-rise offices land between 500 and 1,000 kVA; with chillers included the figure can double or triple (illustrative). The answer comes from the retained loads and the largest motor start, not the floor area or the transformer rating.
Should the chillers be on the generator?
Only if the owner is willing to pay for a much larger plant and to add load management so that chillers start after the essential loads are stable. In buildings with short outages, comfort cooling is usually excluded or limited to one chiller.
Can the generator be in the basement?
Yes, if ventilation, exhaust routing, fuel storage and maintenance access are designed for it. Many basement installations fail on cooling air rather than on the generator itself, so the inlet and outlet path must be sized, not assumed.
How often should a building generator be tested on load?
A monthly transfer test on real building load at a published time, and an annual load-bank test to confirm rating. No-load running alone does not prove the set can carry a lift or pump start.
How is generator power billed to tenants?
Tenant feeders on the essential or tenant-critical bus are usually metered with meters that record units during generator operation, and the owner recovers fuel and maintenance cost at a declared rate. The metering arrangement should be part of the electrical design, not an afterthought.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.