Diesel Generator Sizing and ATS Selection for Saudi Facilities

Sizing a diesel generator for a Saudi facility starts with the real load list, not the transformer rating on the drawing, and it has to account for how motors start as well as how much they draw once running. Add ambient derating for Saudi summer heat and match the automatic transfer switch to the actual transition the load needs, and you get a set that performs on the day it matters, not just on paper.

Build the Load List Before You Touch a Generator Catalog

The most common sizing mistake is working backward from an existing transformer or panel rating instead of forward from what the facility actually runs during an outage. A hospital, a data hall, and a retail building don't need the same things to stay alive. Life safety loads, medical equipment, chillers, elevators, and IT racks each have a different tolerance for interruption and a different starting behavior.

Start by separating loads into what must run immediately, what can wait a few minutes, and what doesn't need backup at all. This list, not a rule of thumb per square meter, is what a generator sizing exercise should be built on. Skipping it is how facilities end up with a generator that's technically large enough on paper but trips on inrush the first time it's tested under real load.

Step Loading vs Block Loading Changes the Answer

Once the load list exists, how those loads come on matters as much as how big they are. Block loading, where most of the connected load hits the generator at once, demands a much larger set than the same total load applied in stages. A chiller plant and an elevator bank starting together pull a huge inrush current for a few seconds, and a generator sized only for running kW will stall or trip on that spike even though it would handle the steady-state load without strain.

Step loading, where loads are brought on in a planned sequence through load-shedding logic or timed contactors, lets a smaller generator handle the same total connected load because it never sees the full inrush at once. This is a design decision rather than a specification line. A facility with sequenced HVAC and elevator starts can often run on a meaningfully smaller set than one where everything restarts simultaneously after a utility failure, and getting this wrong in either direction costs money, either in an oversized generator that never runs efficiently or an undersized one that fails exactly when it's needed.

Ambient Derating for Saudi Summer Conditions

A generator's nameplate rating is almost always set at a reference ambient temperature that has nothing to do with a Riyadh summer or a Jeddah coastal afternoon. Diesel engines lose output as intake air gets hotter and less dense, and altitude adds a second derating factor on top of temperature for sites further from sea level. A set rated at a cooler reference condition can lose a real chunk of its usable output once it's actually running in 45°C ambient with direct sun on the enclosure.

The fix isn't guesswork. Manufacturers publish derating curves for temperature and altitude, and a proper sizing exercise applies them to the specific site conditions before locking a kVA number, not after the generator is already on order. Skipping this step is one of the most common reasons a generator that looked correctly sized on a spec sheet underperforms once it's commissioned on site in peak summer.

Matching the ATS to the Transition the Load Needs

The automatic transfer switch is not an afterthought bolted onto a sized generator. It has to match how fast and how cleanly the load needs to transition between utility and standby power. An open-transition ATS breaks the connection to one source before making it to the other, which is fine for most commercial loads but introduces a brief interruption that sensitive equipment won't tolerate. A closed-transition or delayed-transition switch, by contrast, is chosen specifically to avoid that gap, and the right choice depends on what's actually connected, not a default spec.

Switch rating, number of poles, and whether the transfer needs to coordinate with load-shedding logic for step loading all follow from the same load list used to size the generator in the first place. Sizing the generator and selecting the ATS are really one exercise done in the wrong order if they're treated as separate purchases.

NFPA 110 and Hospital Standby Power

Hospitals carry a stricter standard than most other facility types, and NFPA 110's requirements for Level 1 emergency power systems shape both the generator and the transfer switch selection along with the wiring downstream. Startup time, fuel supply duration, testing frequency, and the segregation of life safety, critical, and equipment branches all come out of that standard, and a generator sized without those branch requirements in mind usually needs rework once the electrical design is checked against code.

NLS supplies diesel generators and matched automatic transfer switches into hospital and aviation infrastructure projects across the Kingdom, and treats the branch-circuit segregation and testing cadence NFPA 110 requires as a design input rather than a compliance step added at the end.

Putting the Sizing Exercise Together

A correctly sized generator and correctly matched ATS come from the same three inputs every time: an honest load list separated by criticality, a clear-eyed view of whether loads start together or in sequence, and derating applied for the actual site conditions rather than a catalog reference point. Get those three right and the transfer switch selection mostly follows on its own.

NLS sources diesel generators and automatic transfer switches as part of its generators and ATS supply work, including multi-set installations that also need synchronizing panels, for projects across hospitals and healthcare facilities in Saudi Arabia.

FAQ

Q: How do I size a backup generator for a Saudi facility? A: Start with an honest load list split by what must run immediately, what can wait, and what doesn't need backup. Then account for whether those loads start together (block loading) or in sequence (step loading), and apply ambient and altitude derating for the actual site before locking a kVA figure.

Q: Why does my generator underperform in summer even though it's correctly rated? A: Nameplate ratings are set at a reference ambient temperature, usually well below a Saudi summer. Diesel engines lose output as intake air gets hotter, so a set that looks correctly sized on a cooler reference curve can fall short once it's actually running at 45°C with a sun-exposed enclosure, unless derating was applied at the sizing stage.

Q: What's the difference between open-transition and closed-transition ATS? A: An open-transition switch breaks the connection to one power source before connecting to the other, causing a brief interruption most commercial loads tolerate fine. A closed-transition or delayed-transition switch avoids that gap, which matters for loads that can't handle even a momentary drop.

Q: What does NFPA 110 require for hospital standby generators? A: NFPA 110 sets requirements for Level 1 emergency power systems, including startup time, fuel supply duration, testing frequency, and segregation of life safety, critical, and equipment branches. These requirements need to shape the generator and ATS selection from the start, not get added after the electrical design is already fixed.