Technical Tables and Calculators

Generator Sizing Chart by Motor Power: Direct-on-Line and Star-Delta

Recommended generator ratings (kW/kVA) for 2.2–265 kW electric motors with direct-on-line and star-delta starting, with an example and a selector tool.

ORFA Technical TeamUpdated: Oct 30, 20264 min read

Generator sizing for motors is one of the most common calculations when pumps, fans or compressors are to be supplied from a generator set. Induction motors draw several times their rated current when starting; if the generator cannot handle this sudden load, voltage and frequency drop, the motor fails to start or other consumers are affected.

The generator is therefore sized well above the motor rating. The chart below gives recommended generator ratings in kW and kVA for each motor power, for direct-on-line and star-delta starting.

How the Starting Method Affects Generator Size

Direct-on-line: the motor is connected directly to the supply and starting current is highest. The chart recommends a generator of about twice the motor power (more for small motors).
Star-delta: the motor starts in star at reduced voltage and then switches to delta, lowering starting current. The chart recommends about 1.7–2 times the motor power.
Soft starters and variable frequency drives: limit starting current further and can reduce the required generator size; in that case select according to manufacturer data.

The kVA values in the chart are derived from kW using the power factor of 0.8 commonly used for generator ratings (kVA ≈ kW / 0.8).

Example

For a 30 kW (40 HP) pump motor started direct-on-line, the chart recommends a 60 kW / 75 kVA generator. If the same motor is started star-delta, a 52 kW / 65 kVA generator is sufficient.

Generator Size Selector

Motor power (kW)Starting method

The tool selects the motor rating equal to or next above the power entered. Values apply to starting a single motor; with several loads, the total load and the start of the largest motor must be assessed together.

Generator Sizing Chart by Motor Power

Motor power		Generator – direct-on-line		Generator – star-delta
HP	kW	kW	kVA	kW	kVA
3	2.2	6	7.5	–	–
4	3	8	10	6	7.5
5.5	4	10	12.5	8	10
7.5	5.5	12.5	15.6	10.8	13.5
10	7.5	15	18.8	14	17.5
12.5	9.2	18.8	23.5	17.2	21.5
15	11	22.5	28	20.5	25.5
17.5	13	26.5	33	23.6	29.5
20	15	30	38	27	34
25	18.5	40	50	33	42
30	22	45	57	40	50
35	26	52	65	45	57
40	30	60	75	52	65
50	37	75	94	65	81
60	45	90	112	77	97
70	51.5	105	131	90	112
75	55	120	150	102	128
90	66	135	170	115	144
100	75	150	190	128	160
125	92	185	230	158	198
150	110	210	260	190	237
180	132	260	323	228	284
200	147	290	360	254	316
225	165	325	404	285	354
250	185	365	454	320	398
275	200	390	485	345	429
300	220	430	535	380	472
330	240	470	584	415	516
360	265	520	646	458	569

Values are approximate; the generator manufacturer’s motor-starting data take precedence. A dash (“–”) means no value is given for that power.

Tip: If several motors are supplied from one generator, starting them in sequence rather than simultaneously significantly reduces the generator size required. Site altitude and ambient temperature should also be considered.

Frequently Asked Questions

Why is the generator sized at about twice the motor power?

Induction motors draw several times their rated current on starting. The generator must hold its voltage under this sudden load, so for direct-on-line starting a generator of about twice the motor power is recommended.

What is the difference between kW and kVA?

kW is active power and kVA is apparent power. Generators are usually rated in kVA at a power factor of 0.8, so kW ≈ kVA × 0.8.

Can the chart be used for motors with variable frequency drives?

Drives limit starting current but generate harmonics. In that case generator sizing should follow the recommendations of the drive and generator manufacturers.

Conclusion

Sizing the generator by motor power and starting method prevents voltage dips and shutdowns during starting. The chart and selector let you make a quick preliminary selection. For starting and control panels for motor-driven pumps, see our pump accessories and spare parts product group.

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