UFC 3 -520-01
June 10, 2002
Total harmonic distortion (voltage and current).
13-4.3.6 UPS systems are available in single -phase and three-phase configurations.
In some cases, the load requirements (three -phase versus single phase or magnitude of
the loads) will establish a need for a three-phase UPS. With a three-phase system,
care must be taken to balance loading and load power factors between phases to
minimize the imbalance in the UPS output voltage.
13-4.3.7 Once the UPS has been sized for a given kVA, the next size -related design
issue is one of how long the battery must be capable of operating. The battery has to
be capable of providing the specified power for the specified duration. Refer to
Appendix E for sizing of UPS batteries.
Selection of a Large UPS System or Multiple Smaller UPS Systems.
13-4.4.1 Paragraph 13-4 .3 provides the recommended approach for UPS sizing. This
approach applies regardless of the UPS size in terms of rated kVA. If the total
connected load is large, another design selection decision must be made regarding
whether to select a single distrib ution-type UPS system or multiple smaller point-o f-use
UPS systems. Several smaller UPS systems are preferred over a single large UPS
system for the reasons discussed in the following paragraphs.
13-4.4.2 Large UPS systems have one advantage in that they are usually well-
designed, include self-monitoring capability, and are well supported by the
manufacturer. They are also expensive, require a large dedicated facility space for
installation, the battery tends to be very large, and maintenance can require special
training. Furthermore, loss of a single large UPS can possibly cause the loss of multiple
mission critical functions.
13-4.4.3 Smaller point -of-use UPS systems are often self-contained, require less floor
space, and provide additional flexibility for relocation when equipment is moved.
Smaller UPS systems are more easily added to the facility without requiring extensive
rework of the facility electrical system. Rather than select a single large UPS, the use of
smaller 10 kVA to 20 kVA UPS syste ms is preferred. Ideally, the UPS systems should
be designed as multiple identical units that are easily interchangeable and relocated.
Such a design arrangement allows UPS systems to be relocated and interchanged as
necessary under contingency situations to allow continuation of critical missions.
13-4.4.4 Smaller point -of-use UPS systems require periodic monitoring of the backup
power source the battery. Smaller UPS systems might use lower quality batteries and
are often forgotten by the maintenance program until they fail. Manufacturer's literature
for UPS systems rated up to 15 kVA often specify 10-year life valve-regulated lead-acid
batteries (and even some larger UPS systems rated at 150 kVA have lower-life
batteries). These batteries often fail within 3 years. Smaller UPS systems should have
their batteries replaced more frequently than usually suggested by the manufacturer.