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

> DG set sizing works out the kVA rating a diesel generator needs to carry a building's backup load. A screening calculation converts the running load to kVA, adds an expansion margin, keeps the set below its maximum continuous loading, checks the peak when the largest motor starts, and rounds up to the next common rating.

## 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

| Assumption | Default | Basis |
| --- | --- | --- |
| Demand (diversity) factor | 70% | A planning figure for mixed office and light commercial loads |
| Load power factor | 0.8 | The power factor DG sets in India are normally rated at |
| Motor starting multiplier | 3 × running kVA | A placeholder; it depends on the starting method and the motor |
| Expansion margin | 20% | Room for growth in the essential load |
| Maximum continuous loading | 80% | Headroom below the set's rating for load steps and hot days |

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:

| Step | Calculation | Result |
| --- | --- | --- |
| Running load | 120 kW × 70% | 84 kW |
| Running load in kVA | 84 ÷ 0.8 | 105 kVA |
| With expansion margin | 105 × 1.2 | 126 kVA |
| Sized on running load | 126 ÷ 80% | 157.5 kVA |
| Motor running kVA | 15 ÷ 0.8 | 18.75 kVA |
| Extra while it starts | 18.75 × (3 − 1) | 37.5 kVA |
| Peak during the start | 105 + 37.5 | 142.5 kVA |
| Required | larger of 157.5 and 142.5 | 157.5 kVA |

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.

## Frequently asked questions

### Why are DG sets rated in kVA and not kW?

The alternator has to carry the full current of the load, and current depends on apparent power (kVA), not just real power (kW). A set's kW capability is its kVA multiplied by the power factor it is rated at, which in India is normally 0.8. A load with a poor power factor draws more current for the same kW, so it needs more kVA.

### Why not size the DG to run at 100% load?

A set that runs flat out has no headroom for a load step, a motor start or a hot day, and the continuous rating a manufacturer publishes depends on the duty. Planning to run at around 80% of the rating is a common way to leave that margin. The supplier's rating for your duty, standby or prime, is the figure that decides it.

### Why does the largest motor matter so much?

A motor draws several times its running current while it accelerates. On a generator, that surge causes a voltage dip that can stall the motor or trip other equipment. If the largest motor is big compared with the set, the start rather than the running load can decide the rating. A gentler starting method reduces the surge.

### Is one large DG better than two smaller ones in parallel?

It depends on the load. Two paralleled sets give redundancy and let one set run alone at night or at weekends, so neither runs lightly loaded for long. They need synchronising and load-sharing controls and more space. One set is simpler and costs less to install. The choice is part of the electrical design, not this calculator.

### How much diesel will the set use?

Consumption depends on the engine and on how heavily the set is loaded, and it is published by the manufacturer as litres per hour at 25%, 50%, 75% and 100% load. Use that table with the loading this calculator shows. A rough fuel figure without the datasheet is not reliable enough for sizing a fuel tank.

## Sources

- [ISO 8528-1](https://www.iso.org/standards.html) — Generating sets: application, ratings and performance
- [ISO 8528-5](https://www.iso.org/standards.html) — Generating sets: performance, including response to load changes
- [NBC 2016, Part 8, Section 2](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Electrical and allied installations
- [CPCB norms](https://cpcb.nic.in/) — Emission and noise limits for diesel generator sets

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Source: https://hagerstone.com/calculators/dg-set-sizing-calculator