Guide to Choosing Generator to Motor Size

Some Notes for Sizing Generators

  • Too small a generator will cause voltage fluctuations and damage to the generator or equipment windings.
  • Too large a generator will cause engine to glaze and carbon up, due to engine loading.

Rule of Thumb

  • Note 1 D.O.L. – Direct On line starting (high current starting).
  • Note 2 S.D. – Star Delta (smooth starting med-high current).
  • Note 3 – A dummy load or extra load will need to be applied to avoid glazing once running.
  1. One horse power (1 hp) equals 0.75 kilowatt i.e. 20hp x 0.75 = 15kW
  2. Warm generator up before applying load
  3. Always engage load from largest to smallest
  4. Have an electrician balance the load across all phases for single phase loads
  5. When motor is operating, after start, requirement will be 1kva for 1hp i.e. 20hp – 20kva used when running.
  6. Size the generator to run between 60-80% full load rating of generator
  7. Amps available on a generator per phase = kva x 1.39. i.e. 20kva = 28amps/phase over 3 phases.
Electric Motor Size Minimum Generator Req’d
(KVA) by Starting Method
Run Power
 HP KW D.O.L to KVA
Size (Note 1)
S.D to KVA
Size (Note 2)
KVA Used When Running
(Note 3)
1 0.75 2.5 2 1
1.5 1.1 3.75 3 1.5
2 1.5 5 4 2
3 2.2 7.5 6 3
4 3 10 8 4
5 3.7 12.5 10 5
6 4.5 15 12 6
7.5 5.5 18.75 15 7.5
10 7.5 25 20 10
12.5 9.3 31.25 25 12.5
15 11 37.5 30 15
20 15 50 40 20
25 19.6 60.5 50 25
30 22 75 60 30
40 30 100 80 40
50 37 125 100 50
60 45 150 120 60
75 55 187.5 150 75
100 75 250 200 100
125 90 312.5 250 125
150 110 375 300 150
175 130 437.5 350 175
200 150 500 400 200
250 185 625 500 250
300 225 750 600 300
400 300 1000 800 400