Getting the size right is the difference between a UPS that protects the load for a decade and one that overheats, trips, or wastes money idling. Here is how we size three-phase systems, step by step.
Most sizing questions come from someone who already has a number in hand, a figure off a spreadsheet, a panel schedule, or the back of every piece of equipment, and wants to know which UPS to buy. The honest answer is that the number you start with is usually wrong, and starting from the right one saves you from a unit that is two or three sizes too big.
The common mistake is adding up the nameplate ratings on every device and matching a UPS to the total. Nameplate is the worst-case draw the manufacturer will admit to, it carries margin for the highest-spec configuration, full options, and a hot day, all stacked together. Real operating draw is almost always far below it. Size to the sum of nameplates and you buy a frame you will run lightly loaded for its entire life.
Measure instead. Put a clamp meter or, better, a power-quality analyzer on the existing feed and log real demand over a representative period, a busy day, and any startup surges, not a quiet afternoon. If the load does not exist yet, work from realistic per-device operating figures rather than nameplate and sum those. What you want is the actual design load, in both kW and kVA, measured or estimated honestly, before you add a single thing for the future.
A three-phase UPS carries two ratings: kVA (apparent power) and kW (real power). The ratio between them is the power factor. kW is what the load actually consumes and turns into work and heat; kVA is what the UPS and the upstream wiring, breakers and transfer gear have to carry. You have to satisfy both ratings, a load can reach the kW limit or the kVA limit first, and whichever it hits sets the ceiling.
This matters most when you compare quotes or reuse an old spec. Legacy three-phase UPS were rated at 0.8 power factor, so a 100 kVA unit delivered only 80 kW. Modern IT loads draw close to unity, and most current platforms, the Eaton 93PM, the APC Galaxy VS and their peers, are rated at unity power factor, where kVA ≈ kW. Match an old unit’s kVA without checking its power factor and you can quietly come up 20% short on the kW that actually powers the load. Always size against the real numbers, in the units the datasheet uses.
Never size for exactly the measured load. Leave margin for measurement error, for the inrush when equipment powers up, and for the load you will add over the system’s life. A sensible starting point is the design load plus a comfortable cushion, then a sanity check against where the site will realistically be in a few years.
There is a limit in the other direction, though, and it is the one people forget. A double-conversion UPS is most efficient in the upper part of its load range and least efficient when lightly loaded; run one chronically below roughly a quarter to a third of capacity and you waste energy every hour and have paid for a frame you are not using. Grossly oversizing “to be safe” is its own failure mode, higher capital cost, worse efficiency, and a unit that never reaches the band it was designed for. The target is loaded enough to be efficient, with enough room to absorb growth and surges, not a unit sitting at 10% waiting on a build-out that may never happen. Where real growth is planned, a modular system you populate over time almost always beats one oversized frame bought up front.
Redundancy is a sizing decision, not an afterthought, and it changes the answer before you ever pick a model. A single unit sized to the load is N, no spare, and any module fault or service window drops the load. The moment you need to tolerate a failure, or take a unit down for maintenance while the load keeps running, you move to N+1: enough capacity that any one module can drop out and the rest still carry the full load.
That reshapes the numbers. If the load needs three modules’ worth of capacity, N+1 is four, and each runs at about three-quarters load in normal operation, which conveniently keeps every unit in an efficient band. A 2N design doubles the count again for full path redundancy. Decide the redundancy level first, because it sets how many units you buy and how hard each one works. We lay out the trade-offs in the N, N+1 and 2N guide.
Battery runtime is sized in minutes, not hours, and the real question is what those minutes are for. If a standby generator backs the site, the UPS only has to ride through the outage and cover the generator’s start and transfer, a few minutes of autonomy is usually plenty, and padding the battery past that just adds cost, weight and footprint. If there is no generator, the battery is the entire backup, so runtime has to cover either a graceful shutdown of the load or the longest outage you are prepared to survive on batteries alone.
Runtime and UPS size interact: more autonomy means more battery, and the battery string and charger have to suit the unit. When a generator is in the picture, frequency and waveform tolerance matter as much as minutes, the UPS has to accept a generator’s slightly less stable output without kicking back to battery, or you defeat the purpose of having the generator at all. We work through that pairing in the UPS and generator sizing guide.
Two more things shape the final spec. First, the nature of the load. Switch-mode power supplies and variable-frequency drives are non-linear, they draw current in pulses rather than a clean sine, which stresses the UPS and can push harmonic current back onto the input. Modern double-conversion units with active front ends handle this well and keep input harmonics low, but on some load mixes, or with older designs, you may need to derate the unit or add filtering. It is one more reason measured data beats a tidy assumption: harmonics and crest factor do not show up on a nameplate.
Second, the voltage and phase configuration. Confirm what the load actually needs before anything else. Many three-phase UPS are three-phase in and three-phase out (3-in/3-out), but some sites run three-phase in and single-phase out (3-in/1-out) to balance a single-phase load across the supply, and input and output voltages have to match the building and the equipment. Getting the configuration and voltage right is part of sizing, not a detail to settle on the dock.
Tell us the load, measured if you have it, estimated if you do not, the upstream source, the runtime you need and how much you expect to grow, and we will size it across any brand and lay out the options. Browse systems by application on the data-centre page, or compare platforms like the APC Galaxy VS family.
No. Nameplate is the manufacturer’s worst-case figure and is almost always well above the real draw. Measure the actual load with a clamp meter or power analyzer, or estimate from realistic operating figures, then size to that plus headroom. Summing nameplates leads to a UPS two or three times larger than needed, which then runs lightly loaded and inefficiently for its whole life.
kW is the real power the load consumes; kVA is the apparent power the UPS and wiring must carry. The ratio is the power factor, and you must satisfy both ratings. Modern unity-power-factor units have kVA roughly equal to kW; older 0.8-PF units deliver only 80 kW per 100 kVA, so check the power factor before matching an old unit’s kVA.
Enough to cover measurement error, startup surges and planned growth, but not so much that the unit runs chronically below about a quarter to a third of load, where double-conversion efficiency drops. Size for the design load plus a sensible cushion. For large planned growth, a modular system you populate over time beats grossly oversizing one frame up front.
Yes. A single unit sized to the load is N, with no spare. N+1 adds a module so any one can fail or be serviced while the rest carry the full load, which means buying more capacity than the load alone requires and running each unit below its rating. 2N doubles the count again. Decide the redundancy level before final sizing, because it sets the unit count.
Tell us the application and we will come back within one business day, sizing, the right system, install and a price. Three-phase installs usually need a licensed electrician, so let us know if you have one.