Getting the size right is a short, logical exercise. This guide walks through the steps we use before recommending a Cat set or a group of them, so that the model choice follows from the load rather than the other way round. It is general guidance; final ratings always come from the manufacturer’s data for the specific package.
Start with a proper load list
List every load the generator will carry, with its rated power, how it starts and whether it runs continuously, cyclically or only occasionally. Group the loads into those that must run together and those that can be staged. A load list that separates motors, lighting, electronic loads and heating is far more useful than a single total.
If the site already has a supply, metered data from a power quality logger is better still. A few weeks of recorded demand shows real peaks, typical power factor and how quickly the load changes.
Running kW, power factor and kVA
Engines produce kilowatts; alternators are rated in kilovolt-amperes. The link between them is power factor. Cat generator sets are commonly rated at 0.8 power factor, so a set rated 1,000 kVA delivers 800 kW. If the site load has a lower power factor, the alternator may reach its limit before the engine does.
Calculate total running kW and kVA from the load list, then check both against the candidate set. Sizing on kW alone is one of the most common causes of overheated alternators.
Motor starting and step loads
Large motors draw several times their running current when started direct on line. The generator must supply that inrush while holding voltage and frequency within limits that keep contactors closed and drives running. Soft starters and variable frequency drives reduce the inrush but bring their own effects on the alternator.
Work out the largest single step the set must accept, often the biggest motor starting with others already running, and compare it with the set’s transient response data. Cat publishes load acceptance information for its packages; ask for it for the exact engine, alternator and control combination being offered.
Non-linear and sensitive loads
Variable speed drives, rectifiers, UPS systems and LED lighting draw distorted current. That distortion heats the alternator and can upset voltage regulation. Where these loads are a large share of the total, the alternator is often upsized relative to the engine, and a permanent magnet excitation system helps the set hold voltage under distortion.
Tell the supplier the share of non-linear load at the outset. It changes the alternator selection more than the engine choice.
Choose the right duty rating
The same Cat engine can carry different ratings depending on duty. Standby ratings apply to sets that run only when the utility fails, with limited annual hours. Prime ratings allow unlimited hours at variable load, and continuous ratings cover constant base load for long periods. A set that will run every day must be sized on its prime or continuous rating, not its standby figure.
Our guide to reading Cat 3516 specifications explains where these ratings appear on a data sheet and how to compare them.
Derate for altitude and temperature
Engine output falls as air becomes thinner and hotter. A set rated at sea level and moderate temperature can lose a meaningful share of its capacity on a high plateau or in desert heat. Cooling systems also have limits; a radiator chosen for a mild site may not hold coolant temperature in high ambient conditions.
Apply the manufacturer’s derating curves for the actual site, using the highest expected air temperature at the radiator, not the average for the month.
One large set or several smaller ones
A single set is simpler and usually cheaper to install. Several smaller units running in parallel offer redundancy, allow maintenance without an outage and keep each engine better loaded when demand varies. For loads above roughly 1 MW, or where downtime is costly, a multi-unit plant is often the better answer.
The comparison in our article on choosing between the 3512 and the 3516 shows how the decision changes as the load grows.
Leave room for growth, but not too much
Add a margin for future load and for ageing, typically by planning for the next unit rather than oversizing the first one. Diesel engines should not spend long periods at very light load. If the early load is small, consider a smaller first set or a load bank programme rather than a much larger engine.
Record the sizing assumptions in writing. When loads change, the record makes it easy to check whether the installed sets are still suitable.
A worked outline
Suppose a plant has 600 kW of running load at 0.85 power factor, with a 150 kW pump motor that starts direct on line while the rest is running. Running demand is about 706 kVA. The set must also absorb the motor’s starting step without the voltage falling too far. The answer might be a single set of around 1,000 kVA with good transient response, or two smaller sets in parallel for redundancy. The figures are illustrative only; confirm them with manufacturer data for the package offered.
For heavy industrial sites, our page on Cat industrial generators covers enclosure, cooling and control choices once the size is settled.
About the authors
These sizing notes are from our generation engineers, who specify and install Cat and other diesel plant for industrial, mining and utility clients. USP&E is the EPC and O&M contractor on that work, not a broker, and brings 150+ power projects in 45+ countries, 25,000 MW contracted since 2002 and 24+ years of delivery. How we deliver projects is described on our project delivery page.
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Cat generator sizing questions
How do I convert kW to kVA for a Cat generator?
Divide kW by the power factor. At the usual 0.8 rating, 800 kW corresponds to 1,000 kVA. Check the site’s actual power factor, because a lower value needs more alternator capacity.
Should I size on the standby or prime rating?
Use the prime or continuous rating for any set that will run regularly or carry base load. Standby ratings are only for emergency duty with limited annual running hours.
Why does motor starting matter so much?
A motor started direct on line can draw several times its running current. The set must supply that step while keeping voltage and frequency within limits, so the largest motor often sets the minimum size.
Is it bad to oversize a diesel generator?
Long running at very light load can cause carbon build-up and fuel in the exhaust. It is better to size close to the load and plan for an extra unit later.
Get a sizing check
Share the load list or a logger file and the site altitude and temperature, and our engineers will propose a set size and configuration for review.
If a document set is easier than a conversation, send it to info@uspeglobal.com or upload a summary through the generator enquiry page; to talk it over, dial +27 10 822 2324. Cat and other sets that may match the result are listed in our current generator inventory.