New member

Generator Capacity

I have a relatively simple problem but I need a quick answer, please


We are running two 320 ekW 400 kVA 60 Hz 1800 rpm 480 Volt sets for 12 hours a day,  each set runs on alternate days. The generators are running at 50% load (200kVA). We have been told by a generator expert that serious engine damage can occur running the generators at this load. I'm not an expert but I find this claim strange. One because If I use the same analogy for a diesel vehicle that means if I drive at 50 kms/hr rather than 100 kms/hr the engine will wear out faster and two why would CAT give no warnings of this in their specification sheets and declare the fuel consumption at 50% capacity on the spec sheets?.





Super Contributor

Re: Generator Capacity

Extended run times at lower loads can cause a number of issues, such as poor combustion, "slobber" and deposits forming on cylinders and turbo hot sections.  Newer engines depend on cylinder pressures generated by higher loads to properly seal the combustion chamber for best combustion. Also newer engines are more highly turbocharged and operate at settings designed not for optimum combustion but to have best balance between performance and emissions. This narrows the operating range and long term operation at lower loads, typically below 45%, can reduce engine service life.


While I don't find your stated loads levels particularly alarming, you will likely find that there could be increased combustion chamber deposits, coking on the turbo hot wheel, and possbily deposits forming in your exhaust system.  Ideally if you could operate the engine at a higher load for a few hours each week, you could probably mitigate most of those problems.


In practise, you just meed to be aware of the symptoms and take appropriate action before small problems turn into bigger ones.


Has the visible exhaust smoke for your "normal" load levels increased?


How is you lube oil life? Do you use sampling?


Do you note any signs of slobber, black streaks coming out of exhaust joints at the head and turbos?


Do the engines maintain stable frequency? Do they start OK?


You also did not state you real power load, in kW. This equates to the flywheel horsepower and is what the engine sees. You may have a low power factor load that would give you a lower kW (real power) load while your kVA (apparent power) is higher. If that is the case then you may be operating the engine at a point where it could have problems.


This is a concern among all engine manufacturers, especially of engines in the size ranges to meet strict US and European emissions standards. Newer design engines are less fuel and load factor tolerant than older designs, the trade off for better fuel consumption and lower emissions, and usually better power density.


Hope that answers you question.


Mike L.

New member

Re: Generator Capacity

[ Edited ]



Pretty comprehensive and informative reply. We will check the signs that you have elaborated upon and act if they manifest.


Thanks and regards


New member

Re: Generator Capacity

Reference to 50% Load


Minimum recommended load is about 33% for short periods anything under even then a load bank is recommended. Prolonged underloading without a loadbank will cause Generator  to start glazing.

Diesel engines can suffer damage as a result of misapplication or misuse - namely internal glazing and carbon buildup. This is a common problem in generator sets caused by failure to follow application and operating guidelines. Ideally, diesel engines should be run at least 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Internal glazing and carbon buildup is due to prolonged periods of running at low speeds and/or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, (misuse); if the engine powering the set is over-powered (misapplication) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (misuse).


Your sescription indicates a prime power rating and if so it appears you have oversized the Generator for the load required


Lawrence De Silveira

Perth UON PTY Ltd


Re: Generator Capacity

I would advise you to look up the Caterpillar OEM recommendations, The best possible load factors to run a generator is between 75% to 90%.

Mike has given you some sound advice here.

I have opened up engines that have been operating on low load conditions only to find a lot of carbon build up on valves,stems, causing valves to stick and making contact with the piston, I always recommend the load factors as recommended by Caterpillar.


Cat Dealer

Re: Generator Capacity

Hello All, Regarding the subject point which related to the Generator Load Capacity, the reply from Mikel is found detailed and good, As stated, you have to check the Mechanical Loads on the Engine, and not the Electrical Load of the Generator Set, because it might be the Real Power (kW) might be lower than the measured Load Percentage (50%), the Percentage is based on the Active Load of the Generator Set (kVA), which includes the Real (kW) and Reactive Powers (kVAR). knowing that the Reactive Power (kVAR) is produced from the Magnetic Rotative System of the Alternator (Rotor), and it is pure Electrical, althougth, the Real Power (kW) is initially produced mechanically by the Engine in hp, then converted to the electrical power by the alternator. As a result of above, the Engine Load in (horse Power) or Real Power (kW) has to be measured, instead of the Electrical Active Power (kVA), in order to make sure how much the effective load percentage on the engine, In addition, The best recommended is to Load the Generator Set at a 75% of its total Rated Power, so as to maintain the Mechanical Equipments and Parts in a good conditions for a longer life time cycle possible. and it is highly recommented not to operate the geneset with a load less than 30% for more than one to two hour, in order to avoid the Mechanicals issues, like (Deposits forming on Cylinders, Turbo, and Exhaust Systems.. Reducing Lube Oil Life, and accelerate its change period, because of low Cylinder Pressure ..etc), Hope above clarification and information would be helpful, Best regards, Wissam HAYEK
New member

Re: Generator Capacity

Running the generators at 50% load is not dangerous or harmful, but you must not go below 50% load to avoid certain engine fault that could develope.

However, I think it is better to run the Cat generators continuously until it is due for routine maintenance as long as you have all the elements(water, engine oil & fuel ) okay. Cat generators that are managed that way serves better and lasts longer than the alternate option you are currently adopting.






New member

Re: Generator Capacity


what model do you have?

New member

Re: Generator Capacity

Howdy folks! I have a sort of odd problem. WE have a small gold mill that uses a handful of 600v motors. The load is approx 30kw. The only genset we can get is Cat 3306 w/175kw generator. I have read all the comments regarding under load issues.


My question is, Can you mitigate the issues by simply running engine at full throttle for an hour or two each day?We need to run the plant for about 1400 hours this summer.


This unit was a back up power gen for a hospital. It has 532 total hours on it. Showroom condition, averaging 15 hours runtime per year to comply with government spec.

New member

Re: Generator Capacity

Hi, am running a 650kVA generator on 3x80kVA UPSs connected in parallel,my UPSs are slightly loaded less than 10%,immikdiately I load them into the generator, the generator power factor shifts from 1 to 0.3% then is switches off


Kindly advise on what could be happenning and how to solve this problem am stack