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sizing a generator | PowerMark

Simple Guide to choosing generator to motor size

How do you choose a generator motor size?

One question we are always asked when we answer the phone to a customer is, what size generator do I need?

unfortunately 9 times out of 10 it is not as simple as guess work on their annual consumption. The main reason for this is motors have two types of starting methods. DOL (Direct online) and SD (Soft start or Star Delta). Most consumers will calculate the running kW and using PF of 0.8 work out what kVA sized generator they need. This does work on a lot of sites, but is not always the best way to calculate a generator. This is why we here at CPS offer site visits, either through ourselves or with our appointed dealers.


Once on site we will calculate the customers power rating and evaluate the size of the generator. Below is a mocked up idea of a Power rating evaluation, to give you an idea of what we offer. These figures are mocked figures at SD start to give you an idea of how we calculate. Other companies will do things differently, but we find that everyone will be on a par with the end figure.


We also offer the customer a number of solutions as we understand not all motors will start at the same time, plus you will have the added hassle of oversizing your generator, when a lot of the motors may run in sequence. We tailor our reports for our customers so they know the facts and figures before they decide on the right generator for them.

Using our power rating evaluation to help give the client options.

We understand that sizing the generator to all motors will over size the generator, which can have detrimental effects on the generator, but at the same time under sizing will have an impact not only on the engine but also on the application and circuit breaker. It is about finding the middle ground and educating the customer. If the customer only has one option and that is to start every motor on the premises, then their is not a lot of educating needed for the client. If you can educate the customer on the best practice for starting certain motors firstly. Then you can offer the client a smaller sized generator which would be more cost effective and better for the generator. We have had a project where we have had to over-size the generator due to the initial start up of several motors at once. The solution was to add a dummy load on to the generator. This helps with his consumption and efficiency of the engine.


Some Notes for Sizing Generators


if you size the generator to small will cause voltage fluctuations and damage to the generator or equipment winding, a Diesel generator will need to run between 30% and 75%. I always find as a rule of thumb that anywhere between 60% and 75%  rated load for best efficiency and longevity of your Diesel Generator


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

Always warm up your generator before you apply load, we offer water jacket heaters to assist your generator and make sure it is not starting cold.

When running in sequence make sure your motors are ran from smallest to largest if possible. This will also stop your circuit breakers tripping

If you have single phase load then have an electrician balance the load across all phases.

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.