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 | ||
|---|---|---|---|---|
| Electric Motor Size HP | KW | Minimum Generator Req’d (KVA) by Starting Method D.O.L to KVA Size (Note 1) | S.D to KVA Size (Note 2) | Run Power KVA Used When Running (Note 3) |
| Electric Motor Size 1 | 0.75 | Minimum Generator Req’d (KVA) by Starting Method 2.5 | 2 | Run Power 1 |
| Electric Motor Size 1.5 | 1.1 | Minimum Generator Req’d (KVA) by Starting Method 3.75 | 3 | Run Power 1.5 |
| Electric Motor Size 2 | 1.5 | Minimum Generator Req’d (KVA) by Starting Method 5 | 4 | Run Power 2 |
| Electric Motor Size 3 | 2.2 | Minimum Generator Req’d (KVA) by Starting Method 7.5 | 6 | Run Power 3 |
| Electric Motor Size 4 | 3 | Minimum Generator Req’d (KVA) by Starting Method 10 | 8 | Run Power 4 |
| Electric Motor Size 5 | 3.7 | Minimum Generator Req’d (KVA) by Starting Method 12.5 | 10 | Run Power 5 |
| Electric Motor Size 6 | 4.5 | Minimum Generator Req’d (KVA) by Starting Method 15 | 12 | Run Power 6 |
| Electric Motor Size 7.5 | 5.5 | Minimum Generator Req’d (KVA) by Starting Method 18.75 | 15 | Run Power 7.5 |
| Electric Motor Size 10 | 7.5 | Minimum Generator Req’d (KVA) by Starting Method 25 | 20 | Run Power 10 |
| Electric Motor Size 12.5 | 9.3 | Minimum Generator Req’d (KVA) by Starting Method 31.25 | 25 | Run Power 12.5 |
| Electric Motor Size 15 | 11 | Minimum Generator Req’d (KVA) by Starting Method 37.5 | 30 | Run Power 15 |
| Electric Motor Size 20 | 15 | Minimum Generator Req’d (KVA) by Starting Method 50 | 40 | Run Power 20 |
| Electric Motor Size 25 | 19.6 | Minimum Generator Req’d (KVA) by Starting Method 60.5 | 50 | Run Power 25 |
| Electric Motor Size 30 | 22 | Minimum Generator Req’d (KVA) by Starting Method 75 | 60 | Run Power 30 |
| Electric Motor Size 40 | 30 | Minimum Generator Req’d (KVA) by Starting Method 100 | 80 | Run Power 40 |
| Electric Motor Size 50 | 37 | Minimum Generator Req’d (KVA) by Starting Method 125 | 100 | Run Power 50 |
| Electric Motor Size 60 | 45 | Minimum Generator Req’d (KVA) by Starting Method 150 | 120 | Run Power 60 |
| Electric Motor Size 75 | 55 | Minimum Generator Req’d (KVA) by Starting Method 187.5 | 150 | Run Power 75 |
| Electric Motor Size 100 | 75 | Minimum Generator Req’d (KVA) by Starting Method 250 | 200 | Run Power 100 |
| Electric Motor Size 125 | 90 | Minimum Generator Req’d (KVA) by Starting Method 312.5 | 250 | Run Power 125 |
| Electric Motor Size 150 | 110 | Minimum Generator Req’d (KVA) by Starting Method 375 | 300 | Run Power 150 |
| Electric Motor Size 175 | 130 | Minimum Generator Req’d (KVA) by Starting Method 437.5 | 350 | Run Power 175 |
| Electric Motor Size 200 | 150 | Minimum Generator Req’d (KVA) by Starting Method 500 | 400 | Run Power 200 |
| Electric Motor Size 250 | 185 | Minimum Generator Req’d (KVA) by Starting Method 625 | 500 | Run Power 250 |
| Electric Motor Size 300 | 225 | Minimum Generator Req’d (KVA) by Starting Method 750 | 600 | Run Power 300 |
| Electric Motor Size 400 | 300 | Minimum Generator Req’d (KVA) by Starting Method 1000 | 800 | Run Power 400 |
NPE provides these calculators and guides to assist with general queries and recommends working with experts to ensure suitability.
NPE assumes no responsibility or liability for any errors, omissions or misuse of the contents of this site. These online tools do not constitute professional advice.
For assistance, contact any of NPE’s Australian branches.