DG Set Sizing Calculator: A Preliminary kVA Rating for a Building

What it calculates

The calculator gives a preliminary kVA rating for a diesel generator (DG) set. It works out the running load in kW and kVA, the capacity needed once an expansion margin and a maximum continuous loading are allowed for, and the peak while the largest motor starts. It takes the larger of the two requirements and rounds it up to the next rating in a list of common DG ratings in India.

It also shows how heavily the recommended set would be loaded: today, with the expansion margin used up, and during the motor start. That tells you whether the set is likely to be oversized as well as whether it is big enough.

The result is for early planning: checking a landlord's backup provision, reserving plant space, or comparing options before an electrical engineer prepares the load schedule.

Inputs

Connected load on the DG is the total rating of everything the generator will feed, in kW. Enter only the essential load if the DG will not back up the whole building. Deciding what is essential comes first; sizing on the full connected load of a building usually produces a set that spends its life lightly loaded.

Demand (diversity) factor is the share of that connected load expected to run at the same time. Lights, air conditioning and pumps rarely all run at full output together.

Largest single motor is the biggest motor that may start while the rest of the load is already running: typically a transfer pump, an air compressor, a lift or a chiller compressor.

Motor starting multiplier is the motor's starting kVA as a multiple of its running kVA. It depends on the starting method. Direct-on-line (DOL) starting gives the highest surge, star-delta and soft starters less, and a variable frequency drive (VFD) the least. The default of 3 is a placeholder; confirm the figure with the DG supplier.

Future expansion margin is the growth you want to allow for.

The assumptions panel holds the load power factor and the maximum continuous loading of the set.

Method

1. Running kW = connected load × diversity.
2. Running kVA = running kW ÷ power factor.
3. Sized on running load = running kVA × (1 + expansion margin) ÷ maximum loading.
4. Motor running kVA = motor kW ÷ power factor.
5. Extra starting kVA = motor running kVA × (starting multiplier − 1). The motor's running kVA is already inside the running load, so only the extra is added.
6. Peak kVA = running kVA + extra starting kVA.
7. Required kVA = the larger of steps 3 and 6.
8. The recommended rating is the next value at or above the requirement in this list of common DG ratings in India: 15, 20, 25, 30, 40, 45, 62.5, 82.5, 100, 125, 160, 180, 200, 250, 320, 380, 400, 500, 625, 750, 1010, 1250, 1500 and 2000 kVA. Ranges differ between manufacturers, so confirm the available ratings with the supplier.

Loading is the running kVA divided by the chosen rating.

Assumptions

Diversity is the assumption most worth questioning. A building with process loads that run around the clock, or a server room with its cooling, can have a demand factor close to 100%. An office with many small sockets can be well below 70%.

Limitations

• It is a single steady-state figure. It does not model the transient voltage and frequency dip when load is applied, which ISO 8528-5 deals with and which the supplier checks for the actual engine and alternator.
• It treats motor starting simply. Starting several motors together, motors on VFDs, and the order in which loads come back after a changeover all change the peak.
• It ignores harmonic loads. UPS systems, VFDs and LED drivers draw distorted current that can call for a larger alternator than the kVA alone suggests.
• It ignores site derating. High ambient temperature and altitude reduce what a set can deliver; the manufacturer's derating tables apply.
• It does not choose between standby and prime ratings. ISO 8528-1 defines different rating categories for different duties, and a set that runs for long hours every day is rated differently from one that only covers occasional outages.

Worked example

A 120 kW essential load at 70% diversity and 0.8 power factor, with a 15 kW pump as the largest motor, a starting multiplier of 3, a 20% expansion margin and an 80% maximum loading:

The next common rating at or above 157.5 kVA is 160 kVA, about 128 kW at 0.8 power factor. At that rating the set runs at 66% today, 79% once the expansion margin is used up, and 89% while the pump starts. Here the running load, not the motor, sets the size.

Change the largest motor to 30 kW with the same multiplier and the picture changes. Its running kVA becomes 37.5, the extra while starting 75 kVA, and the peak 180 kVA. The motor start now governs, and the recommended rating moves up to 180 kVA. A soft starter on that motor could bring the multiplier down and the rating back.

How engineers use it

An electrical engineer starts from a load schedule rather than a single connected-load figure: every circuit on the essential board, its rating, its power factor and how it is used. Diversity is applied load by load. Motors are listed with their starting method, and the start sequence after a mains failure is planned so that large motors do not start together.

The DG supplier then checks the shortlisted set against that schedule: the alternator's voltage dip on the largest motor start, the engine's response to the first block of load, derating for the site, and the rating category for the duty. Noise and emission requirements under CPCB norms apply to the set and commonly drive the acoustic enclosure and the exhaust design, so they are checked for the specific site.

A screening number like this one is still useful at the start. It shows whether a landlord's existing set is in the right range, how much plant space to reserve, and which assumption (usually diversity or the largest motor) the rating is most sensitive to.

When a professional design must replace it

Use the calculator to compare options and to start the conversation with a supplier. Replace it with a load schedule and sizing by a qualified electrical engineer before a set is ordered, before the changeover and distribution design is fixed, and whenever the load includes large motors, lifts, UPS systems, VFDs or process equipment. The engineer and the supplier confirm the rating, the paralleling arrangement if any, and the installation requirements of NBC 2016 Part 8 and the applicable electrical safety regulations.