Generator Ratings Explained

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Generator Set Ratings - Definitions
Intermittent - the highest output obtainable at SAE standard ambient conditions. These levels may be only maintained for operating periods of short duration
Standby - the electrical output of a generator set used for emergency or backup power, for use when normal powerline utilities fail. Usually runs less than 60 hours per year.
Continuous - the output which can be obtained at SAE standard ambient conditions, operating in a continuous duty mode. The electrical load on the generator set will usually be steady.
Prime - the electrical output of a generator set used as the primary source of power, often running 24 hours a day. The electrical load usually varies.

3 phase electricitypf - Power Factor
Three phase circuits will usually assume a "power factor" (p.f.) of 0.8 lagging. The concept of power factor can get a bit confusing but basically it refers to the out-of-phase relationship between the voltage and the current in an electrical system. If actual data are not available, a p.f. of 0.8 is assumed and the power is shown in kiloVolt/Amperes (kVA). There is a direct relationship between the voltage (V), the current (A), and the power in kW or in kVA.

kW - kilowatts - mechanical
kW - kilowatts - electrical
kVA - kilovolt-amperes

kW (kilowatts) or sometimes kWm (kilowatts mechanical) may refer to the power output from an engine driving a generator set or, in other words, the mechanical power driving the generator. To avoid confusion, the electrical output from the generator is often referred to as kWe--the actual generator output after efficiency losses within the generator.

Electrical power is usually measured in Watts (W) or thousands of Watts (kilowatts, kWe).

kWe is sometimes referred to as "real power" while kVA is "apparent power".


If you know the electrical load in Amps and you know the system voltage, you can find the required kilowatts or kVA from the chart. Similarly, the system's current can easily be found if you know the kWe and voltage.

Before attempting to size a generator set, gather as much detail about the actual operating conditions and loads as possible. Sometimes a "custom built" generator set--sized for the specific requirements--can easily pay for its cost in fuel savings, especially where motor starting is a primary consideration.

These charts are handy guides to find the current (Amperes) or kilowatts in an electrical system.

Generator Ratings in Amperes For 3 Phase Outputs at 0.8 Power Factor

KVA

KWe

208V

220V

240V

380V

416V

440V

480V

600V

6.3

5

17.5

16.5

15.2

9.6

8.6

8.3

7.6

6.1

9.4

7.5

26.1

24.7

22.6

14.3

13

12.3

11.3

9.1

12.5

10

34.7

33

30.1

19.2

17.3

16.6

15.1

12

18.7

15

52

49.5

45

28.8

26

24.9

22.5

18

25

20

69.5

66

60.2

38.4

43.7

33.2

30.1

24.1

31.3

25

87

82.5

75.5

48

52

41.5

37.8

30.2

37.5

30

104

99

90.3

57.6

62.5

49.8

45.2

36.1

43.8

35

125

118

108

68

70

59

54

42.2

50

40

139

131

120

76

78

66

60

48

56.3

45

156

147

135

86

86

74

68

54

62.5

50

173

165

152

96

104

83

76

60

75

60

208

198

181

115

130

99.6

91

72

93.8

75

261

247

226

143

139

123

113

90

100

80

278

264

240

154

173

133

120

96

125

100

347

330

301

192

217

166

150

120

156

125

433

413

375

240

260

208

188

150

187

150

520

495

450

288

304

249

225

180

219

175

608

577

527

335

347

289

264

211

250

200

694

660

601

384

434

332

301

241

312

250

866

825

751

480

521

415

376

301

375

300

1040

990

903

576

607

498

451

361

438

350

1220

1155

1053

672

694

581

527

422

500

400

1390

1320

1203

770

868

665

602

482

625

500

1735

1650

1504

960

868

830

752

602

1) Formula used is A = (KVA x 1000) / (1.73 x Volts)
2) Current ratings are "linear". For example, a 750 KVa (600 kWe at 0.8 Power Factor) at 480V produces 902 amps, or double the what is shown in the table for 375 kVa


*NEMA Starting Codes for Three Phase Motors
KVA / HP Required for Locked Rotor Starting

Code

Starting KVA /HP §

Typical Motor Size

A

0 - 3.15

(Special)

B

3.15 - 3.55

(Special)

C

3.55 - 4.0

(Special)

D

4.0 - 4.5

(Special)

E

4.5 - 5.0

(Special)

F

5.0 - 5.6

15 H.P. +

G

5.6 - 6.3

10 H.P.

H

6.3 - 7.1

5 & 7.5 H.P.

J

7.1 - 8.0

3 H.P.

K

8.0 - 9.0

1.5 & 2 H.P.

L

9.0 - 10.0

1 H.P.

M

10.0 - 11.2

< 1 H.P.

* Starting code letter can be found on Motor Name Plate Data. DO NOT confuse "design code" with "starting code".
§ For generator sizing use the higher inrush value. Consult the generator set manufacturer for voltage dip information and for generator cuurent output capability.