The calculator answers one question: how much apparent power (kVA) must the generator deliver, for the first few seconds, to bring the motor up to speed? It builds the answer in three steps.
A motor's starting kVA (SkVA) comes from the nameplate starting current: SkVA = √3 × V × starting amps ÷ 1000, or from the NEMA code letter: SkVA = HP × locked‑rotor kVA per HP.
Direct‑on‑line (DOL) draws the full inrush; star‑delta reduces it by a third; a soft starter or VFD reduces it further. The calculator multiplies the SkVA according to the selected method.
The alternator must deliver that SkVA while holding the voltage dip within your limit. The tool compares your SkVA against published transient ratings and returns the required generator kVA.
For multiple motors, the calculator sums the largest motor's SkVA plus the running kVA of everything else — because only the largest motor starts at a time. This is the standard plant‑room rule, and it prevents oversizing the generator to an unrealistic all‑motors‑starting condition.
The locked‑rotor apparent power the motor draws during the first few seconds of startup — the value that determines the generator's transient burden.
The generator capacity needed to start the motor while keeping the voltage dip within your acceptable limit — typically 10–15% for most industrial applications.
The temporary voltage drop at the generator terminals during start. Helps identify whether contactors will drop out or sensitive equipment will be affected.
Enter the code letter from the motor nameplate — the calculator uses it to estimate locked‑rotor kVA per horsepower when starting current data is not available.
The calculator follows this sequence — Step 1: Find the SkVA from NEMA code or nameplate current. Step 2: Adjust for starting method (DOL, Star-Delta, Soft Starter, VFD). Step 3: Size for the alternator's transient capability.
Enter the motor horsepower and NEMA code letter from the nameplate. The calculator estimates locked-rotor kVA using the code letter range.
Enter the motor's nameplate starting current directly. Use this method when the NEMA code is not available.
Locked-rotor kVA — usually written SkVA — is the apparent power a motor demands at the instant it is energized, while the rotor is still stationary. It is the number that determines whether a generator can start the motor, because it is far larger than the motor's running kVA.
| NEMA Code Letter | SkVA per HP | NEMA Code Letter | SkVA per HP |
|---|---|---|---|
| A | 0–3.15 | L | 9.0–10.0 |
| B | 3.15–3.55 | M | 10.0–11.2 |
| C | 3.55–4.0 | N | 11.2–12.5 |
| D | 4.0–4.5 | P | 12.5–14.0 |
| E | 4.5–5.0 | R | 14.0–16.0 |
| F | 5.0–5.6 | S | 16.0–18.0 |
| G | 5.6–6.3 | T | 18.0–20.0 |
| H | 6.3–7.1 | U | 20.0–22.4 |
| J | 7.1–8.0 | V | 22.4+ |
| K | 8.0–9.0 | — | — |
How the motor is started changes everything, because it changes the inrush current — and therefore the SkVA — the generator must absorb. The gentler the start, the smaller the generator.
Starting current: 6–8× full-load current
Generator sizing: 2.5–4× motor kW
DOL applies full voltage immediately and creates the highest surge. Best for smaller motors where full breakaway torque is required and a larger generator is acceptable.
Starting current: 2.5–3× full-load current
Generator sizing: 1.5–2.5× motor kW
Star-delta roughly halves the starting current and torque. Often suitable for fans, pumps, and centrifugal loads that do not need full torque at zero speed.
Starting current: 3–4× full-load current
Generator sizing: 1.25–1.75× motor kW
Soft starter provides 30–70% current reduction with a gentle ramp. Particularly useful for retrofit projects where an existing generator is already installed and motor starting remains marginal.
Starting current: 1.1–1.5× full-load current
Generator sizing: 1.1–1.3× motor kW
VFD provides 80–95% inrush reduction with near-unity power factor. The smallest generator, but the harmonic load can still force a larger alternator configuration.
The pattern is simple: the gentler the start, the smaller the generator. But there is a trade-off — reduced-voltage starting also reduces starting torque, which matters if the motor starts under load. Always match the starting method to the load torque profile.
DOL: 2.5–4× |
Star-Delta: 1.5–2.5× |
Soft Starter: 1.25–1.75× |
VFD: 1.1–1.3×
A 100 HP pump motor on DOL may need a 300 kW generator. The same motor on a soft starter can often run from a 175 kW set. Starting method is the strongest lever available.
When a motor starts, the generator voltage sags before the governor and excitation recover. How much sag is acceptable depends on what else is connected. The alternator's transient capability determines whether the motor will start cleanly.
30% is a common planning figure for general loads. Motor contactors may drop out below 65% voltage — a real failure if the dip is too deep. Starting kVA falls roughly with the square of the dip — a 30% dip cuts starting kVA to about half (0.7 × 0.7).
The alternator's subtransient reactance (X″d) sets how much the voltage dips for a given SkVA. Lower reactance means a stiffer source — better tolerance of motor starting surges and harmonic currents.
A low-reactance alternator with PMG (permanent-magnet generator) or AREP excitation — both fitted across the Huali range — recovers faster and holds a shallower dip. PMG feeds the AVR independently, providing stable excitation even under severe voltage dips.
Reference points at 20% voltage dip:
400 kW → ~765 SkVA |
800 kW → ~2,000 SkVA |
1,000 kW → ~2,525 SkVA.
The calculator compares your SkVA against these figures, so the "will it start" answer is based on a real alternator.
A 400 kW generator can deliver nearly twice its continuous rating for a few seconds — about 765 SkVA at 20% dip. This transient overload capability is what allows a generator to start motors that are much larger than its steady-state rating would suggest.
A reduced-voltage start that saves inrush kVA also cuts starting torque. If a loaded pump or compressor never reaches speed, the cause is usually torque collapse — not generator size. Always confirm the motor's breakaway torque against the reduced-voltage starting torque.
When a motor fails to start on a generator, the cause is almost always one of four things. Every one is a design problem you can catch before ordering — by running the SkVA and voltage-dip calculation.
The generator's transient kVA is below the motor's SkVA, so the voltage collapses before the motor reaches speed. The fix: increase generator size, choose a lower-reactance alternator, or use a reduced-current starting method.
The starting dip pushes voltage below the contactor's hold-in threshold (around 65%), and the starter drops out mid-start. Solutions: use a softer start, increase the generator, or specify contactors with a wider voltage tolerance.
The voltage regulator's response clashes with the inrush, causing hunting or a stall before the governor catches up. PMG excitation and properly tuned AVR settings are the proven fix for this persistent issue.
A reduced-voltage start that saves inrush kVA also cuts starting torque, so a loaded pump or compressor never reaches speed. The fix: confirm breakaway torque, use a higher-torque starting method, or unload the motor before start.
Every one of these is a design problem you can catch before ordering — by running the SkVA and voltage-dip calculation, then matching the motor to the right alternator and excitation. Use the calculator to confirm the match before you buy.
Final motor-starting verification should be checked against the motor's nameplate data, the generator's transient capability curve, the starting method, and the acceptable voltage dip for connected equipment. Our motor starting generator sizing calculator runs these checks automatically.
We build diesel generator sets from 8 kVA to 4000 kVA around Stamford alternators — the brand whose own application notes define motor-starting practice — with PMG and AREP excitationand electronic governors that hold voltage steady under inrush.
A: Find the motor’s starting kVA (SkVA) from horsepower and NEMA code letter, or √3 × V × starting current ÷ 1000. Adjust for the starting method, then choose a generator whose alternator can deliver that SkVA at your allowed voltage dip.
A: Locked-rotor kVA (SkVA) is the apparent power a motor draws at the instant it is energized, before the rotor turns. It is typically 5–7× the running kVA and is the figure that determines generator size.
A: A direct-on-line motor draws 6–8× its full-load current at start. Star-delta reduces this to 2.5–3×, a soft starter to 3–4×, and a VFD to 1.1–1.5×.
A: The NEMA code letter is a nameplate marking that encodes the locked-rotor kVA per horsepower. For example, Code G means 5.6–6.3 kVA per HP, so a 100 HP Code G motor draws about 600 SkVA at start.
A: A 30% dip is a common planning figure for general loads. Contactors may drop out below about 65% voltage, and starting torque falls with the square of the dip, so critical loads often need a shallower dip.
A: Yes. A soft starter cuts starting current to about 3–4× full-load current, letting you size the generator at roughly 1.25–1.75× the motor kW instead of 2.5–4× for direct-on-line starting.
A: A VFD allows the smallest generator (about 1.1–1.3× motor kW) because it reduces inrush 80–95%, but it adds harmonics and cost. A soft starter is a cheaper middle ground at 1.25–1.75× motor kW.
A: Add the largest motor’s starting kVA to the running kVA of all other loads. If two motors start simultaneously, add both starting kVA figures together.
