Generator sizing from a load list
Size a generator from a load list with motor starting.
Try a job 4
Each job fills in example values. Replace them with your values.
- Size a generator for a bore pumpThe set for a 5.5 kW bore pump started direct on line after the lights.
- Size a generator with a star-delta startHow much smaller the set is when the pump starts star-delta.
- Size a standby set for a dairy shedThe set for milking plant, vacuum pump and hot water started in turn.
- Size a generator for a building chillerThe set for lighting, a chiller on a drive and lifts, derated for heat.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Phases
- Three phase
- Voltage
- 400 V
- Loads, with the step each starts in
- 2 rows
- Largest voltage dip allowed
- 15 %
- Subtransient reactance
- 15 %
- Rated power factor
- 0.8
- Derating for altitude and heat
- 0 %
Generator rating, at least
50kVA
- Running load
- 19.05kVA
- Size set by
- Starting step 2
- Largest dip at this size
- 15%
- Full load current at this size
- 72.17A
- Fault current at this size
- 481.13A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Load 1: 10 kW, step 110kVA
- Load 2: 7.5 kW motor, direct on line, step 29.8 kVA running, 58.82 kVA starting
- Running real power18.33kW
- Running reactive power5.16kvar
- Running load19.05kVA
- Set kVA for the running load, at least22.92kVA
- Step 1: set kVA for a 15 % dip8.5kVA
- Step 2: set kVA for a 15 % dipResult50kVA
- Size set byStarting step 2
- Largest dip at this size15%
- Full load current at this size72.17A
- Fault current at this size481.13A
About this calculator
How this is calculated
A generator has to do two things. It has to carry the running load, and it has to hold its voltage when a large load starts. A small set can run a pump but still dip so far as the pump starts that lights flicker and contactors drop out. This tool works both and gives the larger as the size.
You list the loads with the power each draws, its power factor and the step it starts in. Loads in the same step start together. For the running load the tool adds the real power and the reactive power of every load, then takes the kVA from both. Loads at different power factors do not add as kVA. The set must also give the real power at its own rated power factor, which is often 0.8.
A motor draws 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: six for direct on line, a third of that for star-delta, the multiple you enter for a soft starter, and one for a variable speed drive. For each step the tool finds the smallest set whose reactance keeps the dip within the limit you set. The largest need, running or starting, divided by what is left after any reduction for altitude and heat, is the size. The generator tool then gives the full load current and fault current of a set at that rating.
AmpSize holds no generator standard. The dip limit, the reactance and the soft starter multiple are inputs, and the result says so.
| Step | Table or clause | What it decides |
|---|---|---|
| Running load | P and Q added, kVA from both | The set kVA for the running load |
| Rated power factor | Set kW = kVA × rated pf | The set kVA for the running kW |
| Motor starting kVA | The motor tool's full load current × the start multiple | The kVA each step draws as it starts |
| Voltage dip | Dip = x × step kVA ÷ (set kVA + x × step kVA) | The set kVA for each step |
| Altitude and heat | Size ÷ (1 − the reduction) | The rating to buy |
| Current and fault level | The generator tool | The full load current and fault current at that rating |
Worked examples
- Farm lights, then a 7.5 kW water pump direct on line needs 50 kVA. The pump start sets the size, as Starting step 2, against a running load of 19.05 kVA.
- The same pump on a star-delta starter needs 22.92 kVA. The running load sets the size now, at a running load of 19.05 kVA.
- A dairy shed: milking plant, vacuum pump and hot water in turn needs 50 kVA. The vacuum pump's direct-on-line start, Starting step 2, sets it, against a running load of 34.96 kVA.
- A building's lighting, chiller on a drive and lifts, derated 10 % for heat needs 97.22 kVA. The running load sets the size at a running load of 79.44 kVA.
Limits
The dip is worked from the set's subtransient reactance alone. It leaves out the voltage regulator's recovery, the engine's speed dip, and loads already running, which a maker's sizing program models. Use the 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, a non-linear load's effect on the alternator, or the installation rules for a generating set.
See also: Generator, Motor and Maximum demand.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How do I size a generator for a motor?
A motor draws several times its running current as it starts, and the set's voltage dips. The tool works the starting kVA of each step and the set size that keeps the dip within the limit you set.
Why is the running load not just the kVA added up?
Loads at different power factors add as real and reactive power. The tool adds the kW and the kvar, then takes the kVA from both. The set must also give the kW at its rated power factor, often 0.8.
What voltage dip should I allow?
It depends on the loads. Lighting flickers at a smaller dip than a motor can ride through, and contactors can drop out at a large one. AmpSize holds no generator standard to set the limit, so you enter it.
How much smaller can the set be with a soft starter or VSD?
The starting kVA falls with the starting current. Star-delta starts at a third of direct on line. A soft starter starts at the multiple you enter, and a variable speed drive near its running current.
Does it allow for altitude and heat?
Enter the maker's derating as a percent. The tool divides the size by what is left. It also gives the full load current and fault current of a set at that rating, from the generator tool.
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