August 2017
Power Distribution Systems
Typical Components of a Power System
1.7-1
Sheet 01 081
Typical Power System
Components
The System One-line on Page 1.2-4,
illustrates schematically the various
types of power distribution equipment
that an engineer will encounter during
the design of a power system. It is
important to consider the various
physical attributes of the various pieces
of electrical equipment that will be
utilized as well as the constraints that
will be encountered in their application.
Electrical equipment that distributes
power has a heat loss due to the
impedance and/or resistance of its
conductors. This heat is radiated into
the electrical room where the equipment is placed and must be removed
to ensure excess heat does not cause
failures. Table 1.7-1 provides heat loss
in watts for typical power distribution
equipment that may be used in the
sizing of HVAC equipment.
As indicated on the one-line, a
number of distribution components,
are provided with a description of the
physical structure in which they are to
be enclosed. The National Electrical
Manufacturers Association (NEMA)
has developed a set of standards to
ensure the consistent application
performance of enclosures.
As an example, the panelboard shown
in Figure 1.2-2 is called out as being
NEMA 4X. Table 1.7-2 and Table 1.7-3,
show the various performance data
for these enclosures in indoor and
outdoor applications respectively.
Table 1.7-4 covers enclosures to be
installed in explosive or hazardous
environments.
Because the majority of medium- and
low-voltage switchgear is mounted
indoors, they are typically provided
in NEMA 1A enclosures. In these
applications, ventilation openings are
normally provided to allow heat to
escape from the enclosures. Where
required, optional dust screens and
gasketing can be provided.
Power Equipment Losses
Many indoor applications are in basements or areas where condensation
on the ceiling may leak on top of the
switchgear. Additional concerns may
arise where sprinklers are provided
above the switchgear or alternately,
on the floor above. Eaton can provide
“sprinkler resistant” low-voltage
switchgear or low- and mediumvoltage switchgear with a drip hood.
Medium-Voltage Switchgear (Indoor, 5 and 15 kV)
For outdoor environments, this equipment may be mounted in a NEMA 3R
drip-proof enclosure. Where equipment
is located outdoors, the humidity in the
air may condense during evening
hours, resulting in water droplets falling on the bus bars in the equipment.
Under these circumstances, an optional
space heater may be provided and
wired to a thermostat or humidistat
for control.
Because many countries around
the world refer to International
Electrotechnical Commission
standards (IEC), designers should
reference Table 1.7-5 to determine the
appropriate alternate enclosure rating.
Table 1.7-1. Power Equipment Losses
Equipment
Watts
Loss
i
ii
Medium-Voltage Switchgear (Indoor, 5 and 15 kV)
1200 A breaker
2000 A breaker
3000 A breaker
4000 A breaker
600
1400
2100
3700
600 A unfused switch
1200 A unfused switch
100 A CL fuses
500
750
840
1
2
3
Medium-Voltage Starters (Indoor, 5 kV)
400 A starter FVNR
800 A starter FVNR
600 A fused switch
1200 A fused switch
600
1000
500
800
Low-Voltage Switchgear (Indoor, 480 V)
800 A breaker
1600 A breaker
2000 A breaker
400
1000
1500
3200 A breaker
4000 A breaker
5000 A breaker
2400
3000
4700
Fuse limiters—800 A CB
Fuse limiters—1600 A CB
Fuse limiters—2000 A CB
200
500
750
4
5
6
7
8
Fuse truck—3200 A CB
Fuse truck—4000 A CB
3600
4500
Structures—3200 A
Structures—4000 A
Structures—5000 A
4000
5000
7000
9
High resistance grounding
1200
10
Panelboards (Indoor, 480 V)
225 A, 42 circuit
300
Low-Voltage Busway (Indoor, Copper, 480 V)
11
800 A
1200 A
1350 A
44 per foot
60 per foot
66 per foot
12
1600 A
2000 A
2500 A
72 per foot
91 per foot
103 per foot
13
3200 A
4000 A
5000 A
144 per foot
182 per foot
203 per foot
14
Motor Control Centers (Indoor, 480 V)
NEMA Size 1 starter
NEMA Size 2 starter
NEMA Size 3 starter
39
56
92
15
NEMA Size 4 starter
NEMA Size 5 starter
Structures
124
244
200
16
Adjustable Frequency Drives (Indoor, 480 V)
Adjustable frequency drives
> 96%
efficiency
Note: The information provided on power
equipment losses is generic data intended
to be used for sizing of HVAC equipment.
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CA08104001E
For more information, visit: www.eaton.com/consultants