Before buying a three-phase UPS, confirm five things: the input supply and phase configuration, the real load expressed in kilowatts, the output power factor and input harmonics, the redundancy mode, and how the unit behaves with a generator. Each of these is a common source of costly surprises, and every one can be checked from the nameplate and a simple site survey rather than learned during the first real outage.
Most three-phase UPS equipment is built for a 400 volt line-to-line, 230 volt line-to-neutral supply, but confirm what your panel actually delivers before ordering. Check the earthing system, typically TN-S or TN-C in industrial buildings, because the UPS neutral and protective-earth arrangement must match it. Verify that the three phases are reasonably balanced; a persistent imbalance beyond about five percent forces derating and shortens component life. If the site still runs a 480 volt or 600 volt legacy distribution, specify a transformer or a unit rated for it rather than assuming compatibility.
The most frequent sizing error is treating kVA as kW. A UPS rated 100 kVA at 0.9 output power factor delivers 90 kW, not 100, and many older units are rated at 0.8, delivering only 80 kW. Size from the actual connected load in watts and apply a load factor so the unit runs in its efficient band. A reliable starting formula is:
kVA required equals total load watts divided by the output power factor divided by a 0.8 load factor for headroom and growth. For a process drawing 120,000 watts with a 0.9 power factor unit, that is 120,000 divided by 0.9 divided by 0.8, or about 167 kVA, so a 200 kVA frame is the practical choice. Always add inrush for motors and variable-speed drives, which can briefly exceed the running load several times over.
Output power factor determines how much real power a given kVA frame supplies, so prefer a unit rated at 0.9 or 1.0 if your load is largely power-factor-corrected IT or variable-frequency drives. Just as important is input harmonic distortion. A six-pulse rectifier can inject around 30 percent total harmonic current back into the supply, which stresses generators and transformers; a modern IGBT rectifier keeps input current distortion below about 5 percent, which protects upstream equipment and simplifies generator sizing. Ask for the THDi figure in writing and match it to what your supply can tolerate.
Decide early whether you need N, N plus one, or N plus X. A single module or frame is cheapest but offers no internal backup; N plus one keeps one unit in reserve so a failure causes no interruption; parallel N plus X scales both capacity and resilience for larger sites. Also confirm the maintenance bypass arrangement, because even a redundant system needs a safe path to service. For most industrial and data applications, parallel redundant three-phase units with a verified bypass give the best balance of cost and continuity.
If a generator backs up the UPS, the two must be specified together. A UPS rectifier presents a large inrush and a nonlinear load to the generator, so the generator should be sized at roughly two to three times the UPS rating, or confirmed against the manufacturer's generator-compatibility curve. Check the UPS input frequency window, because generator frequency wanders during load steps and a unit with a narrow window will reject the source and sit on battery. Confirm that the static transfer to and from bypass is seamless so the load is never exposed during a changeover.
Work through these five checks with your electrical contractor before issuing a purchase order. The current three-phase range and configuration options are listed at https://www.upsboss.com/products/, and detailed sizing and generator notes are collected at https://www.upsboss.com/faq/.
Key takeaway: buy a three-phase UPS on measured kilowatts, matched power factor and harmonics, a deliberate redundancy class, and proven generator behaviour, not on a kVA number alone. The five checks above catch the assumptions that otherwise surface as failures during the first real outage.
Why not just size by kVA from the nameplate?
Because kVA is apparent power and your load is real power in kilowatts. At a 0.8 or 0.9 power factor a 100 kVA frame supplies only 80 or 90 kW, so sizing by kVA alone routinely undersizes the unit and overloads it under real conditions.
Do I always need N plus one redundancy?
Not always, but for any load where an interruption is costly or safety-related it is the sensible default. Single-unit installations save money upfront and accept the risk that the first fault is also the first outage.
Will my existing generator work with a new three-phase UPS?
Often, but not automatically. The UPS rectifier inrush and harmonic current can overload an undersized generator, so confirm the generator rating against the UPS compatibility curve and the input frequency window before assuming a clean handover.
Contact: Frank Zhang
Phone: +86-135 5688 8641
Email: frank@upsboss.com
Add: Jufeng Road, Guangming Street, Guangming District, Shenzhen City, Guangdong Province, China
