This guide walks a building owner or facilities manager through the choices in the order an engineer would make them.
Standby, prime or continuous: pick the duty first
Most commercial buildings need a standby rating. A standby set covers utility outages for a limited number of hours per year and is not designed to run as the main source of power. If the site has a weak grid with daily load shedding, the set may run far more often than standby duty assumes, and a prime rating becomes the safer choice.
The rating on the data sheet changes with the duty. The same engine and alternator can carry a higher standby number than a prime number, so make sure quotes being compared use the same duty.
Sizing for the loads that matter
Start with a load list rather than the main incomer size. Separate life-safety loads such as emergency lighting, fire pumps and lifts from business loads such as servers, cold rooms, point-of-sale and air conditioning. Many owners choose to back up only part of the building, which allows a smaller set.
Motor starting often decides the size more than running load does. Lift motors, chillers and pumps draw several times their running current when they start. A set sized only on running kilowatts may stall or trip on the first large motor start. Staging the loads through the transfer scheme, or using soft starters, keeps the set smaller.
Enclosure, noise and placement
Commercial sites are usually close to neighbours, so the enclosure matters. Options range from a weatherproof canopy to a sound-attenuated enclosure or a dedicated plant room with acoustic louvres. Check local noise limits at the boundary, and allow space around the set for air intake, radiator discharge and maintenance access.
Rooftop and basement installations need extra planning for exhaust routing, structural loads, fuel delivery and fire separation. Those details are cheaper to solve at design stage than after the set arrives.
Fuel and runtime
Diesel remains the usual fuel for commercial standby because it starts fast and accepts large load steps. The base tank or a separate bulk tank sets the runtime; owners commonly plan for a set number of hours at full load and a refuelling contract for longer outages. Fuel should be tested and kept in good condition, since standby fuel can sit for long periods.
Gas sets are an option where a reliable gas supply exists and the building wants to run the set often, for example for peak shaving. Our page on diesel against natural gas generators sets out the trade-offs.
Transfer switch and controls
The automatic transfer switch decides how fast the building gets power back and whether the change-over is open or closed transition. For most commercial buildings an open-transition ATS is enough. Where a short break is unacceptable, a UPS bridges the gap for IT and controls.
Modern Cat control panels allow remote monitoring, so the facilities team can see alarms, run hours and fuel level from a phone. Weekly exercising and an annual load test confirm the set will start when it is needed.
New, used or refurbished?
New sets carry full factory warranty and the latest emissions compliance. A carefully inspected used or refurbished Cat set can make sense for an owner with a tight budget or a short lead time, as long as hours, maintenance records and a load test are verified. Our guide to reading a Cat generator listing lists what to ask.
About the authors
USP&E is an independent EPC and O&M contractor, not a Caterpillar dealer and not a broker. We specify, install and maintain generator sets of many makes, with 150+ power projects in 45+ countries, 25,000 MW contracted since 2002 and 24+ years of field work. Our project engineering team handles larger multi-set plants as well.
Offices: Colorado Springs (1755 Telstar Dr, Suite 300), Knoxville (800 S Gay St, Suite 700), London (162-168 Regent Street), Johannesburg (22 Kildoon Rd, Bryanston), Cape Town (31 Brickfield Road, Woodstock) and China (No. 1 Ningzhou Road, Wuxi). Regions: USA | AFRICA | UK | UAE | CHINA. ISO 9001 and ISO 45001 certified.
Questions about Cat commercial generators
What size Cat generator does a commercial building need?
It depends on which loads must be backed up and on motor starting. A load list, not the main breaker size, is the right starting point.
Is a standby rating enough for a commercial site?
Yes, if the set runs only during occasional outages. Sites with frequent daily outages should consider a prime-rated set.
Can a Cat commercial generator run on natural gas?
Caterpillar offers gas generator sets, but they suit sites with a reliable gas supply and accept load in smaller steps than diesel.
How often should a commercial generator be tested?
Most owners exercise the set weekly or monthly and run a load test at least once a year, following the manufacturer’s maintenance schedule.
Specifying a set for your building?
Share your load list, site drawings and any noise limits, and we will propose a set size, enclosure and transfer scheme.
Phone +27 10 822 2324, email info@uspeglobal.com with your site data, or go through the engineering enquiry page. Inspected Cat sets are listed in the USP&E generator inventory.