Single-phase sizing uses the formula kVA = volts × amps ÷ 1,000, while three-phase sizing multiplies the same values by 1.732 (√3) before dividing by 1,000. That single multiplier changes not just your kVA number but the size of the cables, breakers, and transfer switch you need for the same load.
Most sizing guides stop at “add up your loads and add 25 percent.” They skip the part that actually determines whether your calculation holds together: phase. Choose the wrong phase and a mathematically correct kW figure can still leave you with an undersized generator or an overheated cable.
This guide gives you the exact formulas, two full worked examples, and the phase-specific rules our engineers use to size units from 8 kVA to 4,000 kVA. By the end, you will be able to run the numbers yourself and know which phase your calculation demands.
Key Takeaways
- Single-phase uses kVA = V × A ÷ 1,000; three-phase uses kVA = √3 × V × A ÷ 1,000, where √3 ≈ 1.732.
- For the same kW, a single-phase generator draws roughly 1.73× more current than a three-phase unit, so it needs larger cables and breakers.
- Single-phase loads typically use a power factor of 1.0, while three-phase industrial loads use 0.8, changing your kW to kVA conversion.
- Motor starting current, not running current, usually decides the generator size: DOL starting can demand 6–8× full-load current.
- Shandong Huali sizes both single- and three-phase diesel gensets from 8 kVA to 4,000 kVA, with OEM/ODM customization.
kW vs kVA: The Foundation of Generator Sizing
Before any phase-specific math, you need to understand the difference between kW and kVA, because the two numbers are never the same for a generator under load.
kW (kilowatts) is real power, the energy your equipment actually converts into work. kVA (kilovolt-amperes) is apparent power, the total power the generator must produce, including the portion that does no useful work.
The ratio between them is the power factor (PF):
kW = kVA × PF, and kVA = kW ÷ PF
A generator is usually rated in kVA, while most equipment is rated in kW. So the conversion is the first step in any sizing job. If your load is 80 kW and the power factor is 0.8, you need 80 ÷ 0.8 = 100 kVA of generator capacity.
Why Single-Phase and Three-Phase Use Different Power Factors
Here is the part most guides gloss over. The power factor you assume changes with phase.
Residential and small single-phase loads, mostly lights and heaters, have a power factor close to 1.0. Every kVA of capacity delivers nearly a full kW of useful work. Industrial three-phase loads, dominated by motors and transformers, typically run at a power factor around 0.8. This 0.8 convention appears across industry generator sizing guides, including major manufacturers’ own selection tables.
That difference matters. A 10 kVA single-phase unit can feed roughly 10 kW of residential load. A 10 kVA three-phase unit feeding motors only delivers about 8 kW. When you compare a single-phase vs three phase generator sizing, you are not just comparing two formulas. You are comparing two different assumptions about how efficiently the generator turns capacity into useful power.
Single-Phase vs. Three-Phase Generator Sizing Formulas
The formulas are identical except for one term: the square root of three, written √3, which equals 1.732. Three-phase systems include it; single-phase systems do not.
| Calculation | Single-Phase | Three-Phase |
|---|---|---|
| kVA from volts and amps | V × A ÷ 1,000 | √3 × V × A ÷ 1,000 |
| kW from volts, amps, PF | V × A × PF ÷ 1,000 | √3 × V × A × PF ÷ 1,000 |
| Amps from kVA | kVA × 1,000 ÷ V | kVA × 1,000 ÷ (√3 × V) |
| Amps from kW | kW × 1,000 ÷ (V × PF) | kW × 1,000 ÷ (√3 × V × PF) |
For three-phase calculations, V is the line-to-line voltage, such as 208 V, 400 V, or 480 V. For single-phase, V is the line-to-neutral voltage, such as 230 V or 120 V. Do not mix 208 V three-phase math into a single-phase circuit.
A quick way to remember it: three-phase power is what you get from three single-phase circuits working together, offset by 120 degrees. The √3 factor accounts for that offset, and it is why three-phase delivers more power per amp of current.
Worked Examples: Sizing Single-Phase and Three-Phase Generators
Formulas make sense when you see them working. Here are two full calculations, one for each phase.
Sizing a Single-Phase Generator
A small workshop runs on a 230 V single-phase supply. Its loads are lighting (2 kW), refrigeration (1.5 kW), two 1.5 kW single-phase motors (3 kW), and a 1 kW miscellaneous of chargers and tools.
- Total running load: 2 + 1.5 + 3 + 1 = 7.5 kW.
- Largest motor starting surge: the 1.5 kW motor draws about 3× its running power at startup, or 4.5 kW. The extra above running is 3 kW.
- Peak load: 7.5 kW running + 3 kW starting = 10.5 kW.
- Convert to kVA: single-phase loads use a power factor near 1.0, so kVA = 10.5 ÷ 1.0 = 10.5 kVA.
- Add a 20% safety margin: 10.5 × 1.2 = 12.6 kVA.
The workshop should select a 12.5 to 15 kVA single-phase generator. This is the math that matches the simple, mostly resistive loads a single-phase unit serves well.
Sizing a Three-Phase Generator
A manufacturing line runs on a 400 V three-phase supply with production motors (40 kW), a compressor (10 kW), HVAC (8 kW), and lighting (2 kW).
- Total running load: 40 + 10 + 8 + 2 = 60 kW.
- Convert to kVA: three-phase industrial loads use a power factor of 0.8, so running kVA = 60 ÷ 0.8 = 75 kVA.
- Largest motor starting surge: the 15 kW production motor on a direct-on-line (DOL) start can demand 6× its full-load current. As a planning figure, add about 3× the motor rating, or 45 kVA.
- Total required: 75 kVA running + 45 kVA starting = 120 kVA.
- Add a 20% safety margin: 120 × 1.2 = 144 kVA.
The line should select a 150 kVA three-phase generator. The starting surge, not the running load, is what pushes the number up, which is exactly why motor-heavy sites almost always need three-phase.
Comparing your own numbers against these examples? See our range of three-phase generators and the how to size a diesel generator guide for the general method.
Why Phase Changes Your Cable and Breaker Sizes
This is the most useful and most overlooked part of single-phase vs three phase generator sizing. Phase does not just change the kVA formula. It changes the current, and current is what sizes your wiring.
For the same power, a single-phase generator draws 1.73× (√3) more current than a three-phase generator. Consider a 15 kW load at 400 V:
- Single-phase: 15,000 ÷ 400 = 37.5 A.
- Three-phase: 15,000 ÷ (1.732 × 400) = 21.7 A.
The three-phase unit carries the same 15 kW on just 58% of the current. That means thinner, cheaper cables, smaller breakers, and a smaller transfer switch.
When Ahmed, a contractor in Cairo, upgraded a small factory from a 30 kVA single-phase set to a 50 kVA three-phase set, his electrician reused the old cables without checking the phase. The old single-phase cabling had been sized for high current, so in this case it was oversized rather than undersized, and it worked. But the transfer switch was a different story. It had been spec’d for single-phase and had to be replaced entirely. Phase changes ripple through the whole installation, not just the generator.
If you size a single-phase generator correctly but run it through cables sized for three-phase current, you will overload and overheat the conductors. Always re-check your cable, breaker, and transfer-switch ratings when the phase changes.
Motor Starting Current: The Hidden Sizing Variable
The running current is easy to calculate. Starting current is what trips undersized generators and dims the lights. Motors draw far more at startup than while running, and the surge is mostly reactive power that the alternator must still supply.
Starting Current by Starter Type
The starting surge depends on how the motor is started:
| Starting Method | Starting Current (× full-load) |
|---|---|
| Direct-on-line (DOL) | 6–8× |
| Star-delta | ~2× |
| Soft starter | 2.5–4× |
| Variable frequency drive (VFD) | ~1.2× |
These multipliers are standard across motor-starting references and generator sizing discussions, including the technical breakdowns on Mike Holt’s electrical forum.
A 15 kW motor running at about 30 A full load will pull roughly 180 A on a DOL start, but only around 36 A with a VFD. The starting method can dramatically cut the required generator size.
Ravi, the maintenance lead at a packaging plant in Kuala Lumpur, switched the plant’s largest conveyor from DOL to a soft starter. The starting surge fell enough that the facility kept its existing 200 kVA generator instead of upgrading to 300 kVA. The soft starter cost a fraction of the larger generator. Before you buy a bigger generator, ask whether a VFD or soft starter can shrink the starting surge instead.
How Motor Starting Decides Generator Size
Because locked-rotor current is mostly reactive, with a power factor around 0.2 to 0.3, the engine must supply enough real power to accelerate the motor while the alternator holds voltage for the reactive surge. That is why a rule of thumb says to size the generator at about 300% of the largest motor’s rating for direct starting.
Sequence your loads too. Start the largest motor first, then bring on smaller loads. If you start everything at once, you need a generator sized for the combined surge, not the running load.
A Phase-Aware Generator Sizing Checklist
Use this checklist to run the calculation in the right order, with phase considered at every step:
- List every load and note its running watts, voltage, and whether it is resistive or inductive.
- Identify the phase of your equipment. Motors above roughly 4–5 kW are almost always three-phase.
- Total the running kW, then add the largest motor’s starting surge on top.
- Convert kW to kVA using the correct power factor: 1.0 for single-phase residential, 0.8 for three-phase industrial.
- Apply the phase-specific formula to confirm amps: single-phase without √3, three-phase with √3.
- Check cable and breaker ratings against the calculated current, remembering single-phase runs hotter for the same kW.
- Add a 20–25% safety margin and confirm the unit fits your site’s supply voltage and frequency.
This order matters because each step depends on the one before it. Skip the phase check and you will likely calculate the right kW and still select the wrong generator.
Still not certain which phase your loads require? Shandong Huali’s engineers will review your load list, run the calculation for both phases, and specify the correct unit at no obligation. Contact us to request a custom sizing consultation.
Frequently Asked Questions
How do you size a single-phase vs three phase generator?
Total your running loads in kW, add the largest motor’s starting surge, convert to kVA using the right power factor (1.0 for single-phase, 0.8 for three-phase), and add a 20–25% margin. Use √3 in the formula for three-phase, but not for single-phase.
What is the formula for three-phase generator sizing?
Three-phase kVA = √3 × volts × amps ÷ 1,000, and three-phase kW = √3 × volts × amps × power factor ÷ 1,000, where √3 ≈ 1.732 and volts is the line-to-line voltage.
Does single-phase or three-phase use more power?
For the same kW, a single-phase generator draws about 1.73× more current than a three-phase. Three-phase is more efficient for high loads, but single-phase is simpler and cheaper for small loads.
What power factor should I use for generator sizing?
Use 0.8 for three-phase industrial loads dominated by motors, and 1.0 for single-phase residential or small-tool loads. Check the motor nameplate power factor if available.
How much current does a three-phase generator draw?
Amps = kVA × 1,000 ÷ (1.732 × volts). For example, a 100 kVA three-phase generator at 400 V draws about 144 A.
Conclusion
Single-phase vs three phase generator sizing comes down to one multiplier and a few assumptions. Single-phase uses V × A ÷ 1,000 and a power factor near 1.0. Three-phase multiplies by √3 and typically assumes 0.8. The phase you choose also sets the current, which decides your cable and breaker sizes.
Here is what to remember:
- Three-phase includes √3 (1.732); single-phase does not.
- Single-phase draws 1.73× more current for the same kW, so it needs larger conductors.
- Motor starting current usually sets the size, not the running load.
- Convert kW to kVA before comparing generators, since the ratings use different units.
Get the math right and you avoid both the overload that trips an undersized unit and the wasted fuel and wet stacking of an oversized one. Shandong Huali Electromechanical Co., Ltd. manufactures single- and three-phase diesel generator sets from 8 kVA to 4,000 kVA, with Cummins, Perkins, Weichai, and Yuchai engines and full OEM/ODM customization.
