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Why Motor Starting Drives Sizing
Size to the Starting kVA, Not the Running kW
Most generator sizing mistakes on motor loads come from a single misunderstanding: sizing for running power instead of starting power. A motor draws three to seven times its running current at start-up — a surge called locked-rotor inrush.
Running kW
Real Power
The power the motor consumes at full load. Calculated as HP × 0.746 ÷ efficiency.
Running kVA
Apparent Power
Running kW divided by the motor's power factor. The load the generator sees during steady operation.
Starting kVA
Locked-Rotor Surge
The decisive figure. Found from the motor's NEMA code letter: HP × locked-rotor kVA/HP.
Worked Example: 150 HP Motor
A 150 HP motor with 91% efficiency, 0.91 power factor, and NEMA Code F draws about 123 kW running and 135 kVA running — but a starting kVA of roughly 795 kVA. A generator sized for the running load would be undersized by nearly a factor of six at the moment of start.
The Three Numbers You Need
Every Motor-to-Generator Calculation Reduces to Three Values
1
Running kW
Real power the motor consumes at full load.
2
Running kVA
Running kW ÷ power factor.
3
Starting kVA
HP × locked-rotor kVA/HP from the NEMA code letter. This is the number that sizes the generator.
NEMA Locked-Rotor Code
How to Read the NEMA Code Letter for Generator Sizing
The key to sizing a generator for motor starting is printed on every motor nameplate: the NEMA locked-rotor code letter. This single letter tells you how much apparent power the motor will demand at locked rotor, expressed as kVA per horsepower. Higher letters mean higher locked-rotor current — and a larger generator.
| Code Letter | Locked-Rotor kVA/HP | Approx. Mid-Range |
|---|---|---|
| A | 0 – 3.14 | 1.6 |
| B | 3.15 – 3.55 | 3.3 |
| C | 3.55 – 3.99 | 3.8 |
| D | 4.0 – 4.49 | 4.3 |
| E | 4.5 – 4.99 | 4.7 |
| F | 5.0 – 5.59 | 5.3 |
| G | 5.6 – 6.29 | 5.9 |
| H | 6.3 – 7.09 | 6.7 |
| J | 7.1 – 7.99 | 7.5 |
| K | 8.0 – 8.99 | 8.5 |
| L | 9.0 – 9.99 | 9.5 |
| M | 10.0 – 11.19 | 10.6 |
| N | 11.2 – 12.49 | 11.8 |
| P | 12.5 – 13.99 | 13.2 |
| R | 14.0 – 15.99 | 15.0 |
| S | 16.0 – 17.99 | 17.0 |
| T | 18.0 – 19.99 | 19.0 |
| U | 20.0 – 22.39 | 21.0 |
| V | 22.4 and up | 22.4 |
Source: NEMA MG 1-10.36 / NEC Table 430.7(B).
Always use the conservative (upper) value within the code-letter
range when planning.
Typical Code Letter by Motor HP
| Less than 1 HP | L or M | 9.0 – 11.19 |
| 1.5 – 2 HP | L or M | 9.0 – 11.19 |
| 3 HP | K | 8.0 – 8.99 |
| 5 HP | J | 7.1 – 7.99 |
| 7.5 – 10 HP | H | 6.3 – 7.09 |
| 15 HP and above | G | 5.6 – 6.29 |
| 40 HP and above | F | 5.0 – 5.59 |
The Formula
Starting kVA = Motor HP × locked-rotor kVA per horsepower (from the code letter).
Starting kVA = HP × kVA/HP
Then size the generator to meet or exceed this value.
Then size the generator to meet or exceed this value.
Motor to Generator
Motor HP to Generator kW & kVA Chart
The chart below converts a motor's running horsepower to the minimum generator capacity required for reliable starting, using standard efficiency (90%) and 0.85 power factor, with across-the-line (DOL) starting. For reduced-voltage starting methods, apply the correction factors in the next section.
| Motor kW | Motor HP | Min. Generator kW | Min. Generator kVA |
|---|---|---|---|
| 2.2 | 3 | 6 | 7.5 |
| 5.5 | 7.5 | 12.5 | 15.6 |
| 11 | 15 | 22.5 | 28 |
| 15 | 20 | 30 | 38 |
| 22 | 30 | 45 | 57 |
| 30 | 40 | 60 | 75 |
| 37 | 50 | 75 | 94 |
| 45 | 60 | 90 | 112 |
| 75 | 100 | 150 | 190 |
| 110 | 150 | 210 | 260 |
Cross-checked against industry pump and generator
manufacturers' sizing tables. Final selection must be confirmed
against your motor's nameplate data. For single-phase motors,
size at 2.5 to 3× the motor's running kW.
Starting Method Comparison
How Starting Method Changes the Generator Size
The single biggest lever in motor-driven generator sizing is how you start the motor. Across-the-line starting slams the full locked-rotor current into the generator; reduced-voltage starters limit that inrush, allowing a smaller — and less expensive — generator set.
01
Direct-On-Line (DOL)
Full voltage applied instantly. Highest inrush current — typically 6 to 8 times full-load amperage.
Starting current: 6–8× FLA
Starting kVA: 600–800% of running
Largest generator required
02
Star-Delta (Wye-Delta)
Reduces starting current by connecting windings in star configuration during start, then switching to delta.
Starting current: 2–3× FLA
Starting kVA: 200–300% of running
~33% of DOL size
03
Soft Starter
Gradually increases voltage to the motor, limiting inrush current and mechanical stress.
Starting current: 2–3× FLA
Starting kVA: 200–300% of running
Smaller generator
04
Variable Frequency Drive (VFD)
Controls both voltage and frequency, allowing the motor to accelerate smoothly with minimal inrush.
Starting current: 1–1.5× FLA
Starting kVA: 100–150% of running
Smallest generator required
Why This Matters in Practice
Consider three 50 kW motors that must start together. Started DOL, they produce a combined starting surge of roughly 1,050 kVA. The same three motors fitted with soft starters draw about 375 kVA; with VFDs, the surge drops to roughly 180 kVA. The difference can move you from a 1,000 kVA generator down to a 400 kVA unit — a substantial saving in purchase price, fuel, and footprint.
Voltage Dip
Voltage Dip When Starting a Motor on a Generator
Starting kVA is only half the story. What limits your generator's ability to start a motor is voltage dip — how far the terminal voltage falls while the alternator supplies the locked-rotor surge.
How Far Does the Voltage Dip?
A 30% voltage dip reduces the motor's starting kVA by about half (0.7 × 0.7 = 0.49), because both voltage and current fall together. But it also cuts starting torque to roughly 49% of rated locked-rotor torque — so a heavily loaded motor may fail to accelerate.
Target: 20–30% max instantaneous dipWhat Else Is on the Line?
Most control relays and contactors tolerate a 40% dip, but sensitive electronics can chatter or drop out above a 20% dip. For mission-critical starts — medical equipment, safety systems, process-critical pumps — keep the dip to 15% or less and request the manufacturer's starting-kVA curve.
Critical loads: ≤15% dipLet's talk
Get a Factory-Direct Quote
Shandong Huali Electromechanical Co., Ltd. manufactures diesel generator sets from 8 kVA to 4,000 kVA — single-phase and three-phase, at 50 Hz or 60 Hz, built to start and run the most demanding motor loads. Every unit is 100% pre-delivery tested in our national-standard testing center, certified to ISO9001, CE, and CCC, and supported by more than 80 engineers and 25+ years of manufacturing experience.
Frequently Ask Questions
Q: What size generator do I need to run a 40 HP motor?
A: For a 40 HP, 3-phase motor started DOL, plan on roughly 90 to 112 kVA (about 75 to 90 kW). The exact figure depends on the motor’s NEMA code letter, efficiency, and power factor. Use the chart above as a starting point, then confirm against the nameplate.
Q: How do I calculate generator size for a motor?
A: Multiply the motor’s HP by its locked-rotor kVA/HP from the NEMA code letter to get starting kVA. Calculate running kW as HP × 0.746 ÷ efficiency, and running kVA as running kW ÷ power factor. Size to the larger value.
Q: What does the NEMA code letter mean for generator sizing?
A: The NEMA code letter on a motor nameplate indicates locked-rotor kVA per horsepower. A Code G motor draws 5.6 to 6.29 kVA/HP at start-up; a Code L motor draws 9.0 to 9.99 kVA/HP. Higher letters require larger generators.
Q: Why is the generator bigger than the motor's running kW?
A: Because a motor draws 3 to 7× its running current at start-up. The generator must supply this locked-rotor surge without excessive voltage dip, so it is sized to the motor’s starting kVA — typically 2.5 to 3× the running kW for DOL starting.
Q: Does a soft starter or VFD reduce the generator size I need?
A: Yes. A soft starter limits starting current to roughly 2 to 3× FLA, and a VFD to about 1 to 1.5×. This can reduce the required generator by 50% or more compared to DOL starting.
Q: What is locked-rotor current?
A: Locked-rotor current is the current a motor draws when its rotor is stationary at the moment of start-up. It is several times the motor’s full-load running current and is the dominant factor in generator sizing for motor loads.