Most sizing mistakes come from two things: forgetting motor starting current, and buying exactly the running load with no headroom. This is the method we use when a buyer sends us a load list.
Write down every load that can run at the same time, with its rated power in kW. Lighting, sockets and electronics are simple. Motors (pumps, compressors, crushers, conveyors, air conditioning) need a second look in step 2.
A motor started direct-on-line draws about 6 to 7 times its running current for a few seconds. The generator must carry that without the voltage collapsing. As a rule of thumb:
Start the largest motor first and the others one by one. If several large motors may start together, add them.
Generators are rated in kVA at power factor 0.8. So kVA = kW ÷ 0.8. A 400 kW running load needs at least 500 kVA.
Diesel engines live longest at 50–80% of their prime rating. Running a set continuously near 100% shortens its life; running it below 30% causes wet stacking. Size so that the normal load sits around 70–80%.
If the set is your main source of power, size against the prime rating. If it only runs during grid outages, the standby rating applies. See prime vs standby.
A small mine site has 250 kW of running load, and its largest load is a 75 kW crusher motor with a star-delta starter.
Our sizing tool on the home page does steps 3 and 4 for you.
See our published prices, get an instant delivered estimate, or send your requirement and we reply the same business day.
Request a quote