PROPERTY OF SOLIDSTATE CONTROLS, INC. 7/1998
The first step in sizing the UPS is to perform a thorough Load Study. When performing the Load
Study, it is important that all possible considerations be made for present needs and any planned
future expansion. All loads need to be analyzed with respect to various parameters, as described
below. The following is a discussion and examples of which loads need to be considered for a
computer room and how to use this information in sizing the UPS. The same procedure can be
used for load studies for any critical load:
1. Computer Main Frame or CPU
2. Printers
3. Disk Drivers
4. Plotters
5. C.R.T. Terminals
6. Any Other peripherals
7. Emergency room lights, alarms, security, and all other loads which must be supported
when the computer is in operation.
8. Any other DC loads
9. Any projected expansion in the near future. Consider the planned addition of any
peripherals, CRTs, printers, etc., or new or additional computers. (It is more
economical to purchase one large UPS than two smaller ones to equal the same KVA
rating.)
Identify the voltage of each load, whether it is single phase, two phase, or three phase, and the
frequency.
In an effort to minimize the size of the UPS, and correspondingly the cost, one should consider which
devices, if any, need not be powered from the UPS. For example, if a computer room application
requires only time to properly power down the system, then you need not necessarily power your tape
drives. Also, suppose there are ten (10) terminals connected. Only one is necessary to power down. In
this way, one can minimize the initial cost by purchasing a smaller size UPS, which is likely to require
less floor space, less air conditioning, and less annual operating costs.
If some loads have different input voltages, it should be determined if they are convertible. For
example, the main frame may be able to accept either 208Y/120 VAC, three phase, or 240 VAC,
three phase inputs. If all other voltages are 120 V or 208Y/120 V, then 208Y/120 V should be
selected to power the main frame. This will eliminate the need for, and additional cost of, special
voltage transformers.
If any of the loads are three phase, then a three phase U PS is required.
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