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.
- One horse power (1 hp) equals 0.75 kilowatt i.e. 20hp x 0.75 = 15kW
- Warm generator up before applying load
- Always engage load from largest to smallest
- Have an electrician balance the load across all phases for single phase loads
- When motor is operating, after start, requirement will be 1kva for 1hp i.e. 20hp – 20kva used when running.
- Size the generator to run between 60-80% full load rating of generator
- 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 |