A generator is not the network. Its voltage falls when a large load starts, and it falls further the smaller the set. So a set has two jobs: carry the running load, and start the motors without too deep a dip. On a site with a big motor, the start is often the larger job. This page explains how the generator sizing calculator works both.
The example
A farm board with 10 kW of lights and sockets, and a 7.5 kW water pump at a power factor of 0.85. The supply is three‑phase 400 V. The lights come on first, then the pump starts.
Running kVA: why kVA do not add
Loads at different power factors do not add as kVA. Each load draws real power, in kW, and reactive power, in kvar. Those add on their own. The kVA comes from both, as the square root of kW² + kvar².
| Load | Running kVA | Power factor |
|---|---|---|
| Lights and sockets | 10 | 1 |
| Water pump | 9.8 | 0.85 |
| The two kVA added | 19.8 |
The board draws 18.33 kW and 5.16 kvar, so the running load is 19.05 kVA, less than the two added. The gap grows as the power factors spread apart.
A set is rated in kVA at a power factor, often 0.8. Its engine gives the kW, so the set must also give 18.33 kW at 0.8. That needs 22.92 kVA, more than the 19.05 kVA. The tool takes the larger.
Starting kVA
A motor draws far more than its running current as it starts. The tool takes the motor's running kVA from the motor tool's full load current, then multiplies it by the start:
| Start | Multiple of running kVA | Where it comes from |
|---|---|---|
| Direct on line | 6 | An AmpSize assumption |
| Star-delta | 2 | A third of direct on line |
| Soft starter | 3 | Your input, 3 by default |
| Variable speed drive | 1 | The tool takes the running kVA |
These are the tool's assumptions, and it states them on every result. Take a soft starter's current limit from its settings. Loads given the same step number start together, so their starting kVA add.
Voltage dip
As a step starts, the set's voltage dips. The tool works the dip from the alternator's subtransient reactance, x, and the step's starting kVA:
- dip = x × step kVA ÷ (set kVA + x × step kVA)
Turn it round and a set keeps the dip to d or less when:
- set kVA ≥ x × step kVA × (1 − d) ÷ d
Both x and d are yours. Take the reactance from the alternator's data sheet. Set the dip to what the loads on the set can ride through: contactors, drives, lights and electronics all have a limit. AmpSize holds no generator standard, so it gives no dip limit of its own. The tool's defaults are 15 % for each.
For the pump direct on line, the start needs a 50 kVA set, against 22.92 kVA running. The pump's start sets the size. Open the pump direct on line.
The dip limit moves the size a long way. Hold the same start to a 10 % dip and the set needs 79.41 kVA. Open the pump with a 10 % dip limit.
Starting methods
A gentler start lowers the starting kVA, and the set it needs falls in step.
| Pump start | Pump step needs | Set size | Set by |
|---|---|---|---|
| Direct on line | 50 kVA | 50 kVA | Starting step 2 |
| Star-delta | 16.67 kVA | 22.92 kVA | The running load |
| Soft starter, 3 × FLC | 25 kVA | 25 kVA | Starting step 2 |
| Variable speed drive | 8.33 kVA | 22.92 kVA | The running load |
Open the pump on star-delta, on a soft starter or on a drive.
Once the start needs less than the running load, a gentler start saves nothing more. A star-delta starter and a drive give the same set here.
Starting in steps works the same way. Motors in one step add their starting kVA. Put them in separate steps and each step is worked on its own, so the largest single start sets the size.
Derating
A set gives less than its nameplate in heat and at altitude. The maker's data gives the reduction. The tool divides the larger need by what is left. On star-delta, with 10 % off, the pump needs a set rated 25.46 kVA. Open the pump on star-delta, derated 10 %.
What the tool leaves out
The dip here comes from the reactance alone. A real set's voltage regulator pulls the voltage back up, the engine slows under the load step, and motors already running draw more current as the voltage falls. A maker's sizing program models those. Use the tool's size as a first pick, then check it on the maker's data.
The tool does not cover a set in parallel with the network, or the installation of a set. That is covered by AS/NZS 3000 Cl 7.3. For the current and fault level of a chosen set, use the generator calculator. A worked dairy shed is in What size generator for a dairy shed.