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S
S
YSTEM
IZING
Sizing a power center to meet present and future
needs is a fundamental requirement.
Present Requirements
Estimating the present system size in kilovolt
amperes (kVA) is done in a number of ways. Typical
sources include computer site planning manuals,
equipment nameplate data, and electrical service
data. kVA requirements are estimated using any of
the following formulas:
V = volts
A = amperes
kVA = kilovolt-amperes BTU/hr = British Ther-
kW = kilowatts
1. Power profile of equipment. (This is the most
reliable base from which to estimate present
kVA loading.)
For three-phase systems:
kVA
=
A
=
For single-phase equipment:
kVA
A
=
2. Kilowatts (kW) and Power Factor (pf).
kVA
(If pf is not given, assume 0.8.)
3. Ampere specifications for the electrical
service feeding the site:
kVA
=
(for three-phase systems)
4. BTU/hr or Kcal/hr specifications:
BTU
kW
=
- - - - - - - - - - - - - - - -
3413
(If pf is not known, assume 0.8.)
After the present kVA requirement has been deter-
mined, the anticipated growth and the special char-
acteristics of the load must be considered.
pf = power factor
• = "multiplied by"
mal Units per hour (heat
output)
Kcal/hr = kilocalories per
hour (heat output)
⋅
⋅
(
)
V
A
1.73
- - - - - - - - - - - - - - - - - - - - -
1000
⋅
kVA
1000
- - - - - - - - - - - - - - - - -
⋅
(
)
V
1.73
⋅
V
A
=
- - - - - - - -
1000
⋅
kVA
1000
- - - - - - - - - - - - - - - - -
V
kW
=
- - - - -
pf
⋅
⋅
(
)
V
A
1.73
- - - - - - - - - - - - - - - - - - - - -
1000
⁄
( )
⁄
( )
hr
kcal
hr
=
- - - - - - - - - - - - - - -
860
kW
kVA
=
- - - - -
pf
Growth Requirements
The power center should be sized to anticipate
growth. Growth rates associated with data process-
ing centers double power requirements in a short
time. Therefore it is reasonable to size the system for
twice the present kVA load. Even in a minimum
growth environment, the power center should be
sized for 125% of the estimated kVA load.
Special Load Characteristics
For special load characteristics, factory application
engineers should be consulted for recommended sys-
tem sizing.
System Grounding Considerations
The grounding of any power conditioning system is
critical to its performance. The National Electrical
Code provides for a safe electrical system. The
ground path required by the NEC for safety should
be enhanced or improved for system performance,
never defeated or eliminated. For detailed system
grounding considerations, refer to the Precision
Power Center Installation Operation and
Mainenance Manual.
13
System Sizing
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