Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones
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•
Obtain
all
current rate schedules that might
be
applicable
to the
station.
If an
abbreviated
rate
schedule
is
given,
ask for a
complete
rate
book.
•
Discuss
the
advantages
and
disadvantages
of
each
schedule with
the
utility representative.
•
Analyze demand charges, seasonal demand
or
energy charges, basic energy charges, power
factor
clauses,
and the
basis
of the
demand charge
(15-min
demand,
30-min
demand, connected horsepower,
etc.)
and
discuss them with
the
utility representative.
• Use an
aerial photographic survey
to
mark
the
loca-
tion
of the
pumping station
and the
preferred point
of
service. Review
the
proposed location
in the field
with
the
utility representative
and
send
a
marked
copy
to the
utility.
•
Obtain
the
utility's agreement
on
acceptable start-
ing
methods
and the
frequency
of
starts
for the
pro-
posed pumping station drives.
If
they
do not
allow
across-the-line
starting
for
pump drives
of the
pro-
posed size, discuss
the
alternatives,
select
the
best
one for
reliability
and
economic factors,
and
inform
the
utility
of the
selection.
•
Discuss transformer ownership
and the
utility's
maintenance policy
if the
transformer
is
provided
by
the
owner.
•
Obtain
the
utility's transformer specifications
and
find the
size
they
would likely provide,
its
imped-
ance,
and its
primary
fuse
size
and
insulation (dry
or
oil).
•
Check
the
utility's
policy
on
overhead
and
under-
ground
service installation
and
determine whether
they
share costs
or
furnish
the
entire service.
If
they
furnish
it,
determine whether underground services
are
encased
in
concrete. Cable
for
pumping stations
should
be in a
concrete-encased conduit
and not
directly
buried.
Telephone
Company
Contact
Obtain agreement
with
the
telephone company
on the
following:
•
Approximate required date
of
service
•
Service point (e.g.,
pole)
•
Routing
of
telephone company cables
and the
loca-
tion
of
their equipment
in the
owner's facilities
•
Clear
definition
of
items (and their interface) fur-
nished
by
telephone
company
and by
owner (con-
tractor)
•
Telephone terminal facilities: type (plywood panel,
enclosure, etc.)
and
size,
120-
V
power require-
ments,
and
grounding provisions
•
Telephone extension conduit size
and
type
•
Telephone extension outlet
box
size, type, location,
and
mounting height
•
Pumping station equipment interface requirements,
for
example,
for
telemetry (space, proximity
to
120-
V
sources, need
for
terminal block
or
surge
suppressors).
9-4. Construction
Information
to
Utilities
The
following information should
be
sent
to the
elec-
tric power utility following
the
completion
of the
design.
•
Specifications:
special
switchboard, substation,
and
special grounding specifications;
•
Drawings: site plan, building power plan, single-
line
diagram, service gear elevation (switchboard,
MCC, etc.),
and
substation plans
and
elevations;
and
•
Other data:
the bid
opening date
and the
approxi-
mate date service
is
needed.
The
telephone company should
be
sent
•
Specifications:
terminal cabinet specification,
including size;
•
Drawings:
site plan
and
building plan containing
telephone provisions;
and
•
Other data:
the bid
opening date
and the
approxi-
mate date
service
is
needed.
9-5.
Load
Estimation
To
facilitate
the
design
of the
electrical power service
entrance equipment, substation, motor control center,
and
so on, a
load
or
motor list such
as
that shown
in
Example
9-1
should
be
utilized.
The
motor
and
equip-
ment
lists should
be
started
at the first
stage
of
electri-
cal
design.
If no
data sheets
are
available
from
the
designers
at the first
meeting between
the
various
design team members,
a
tentative list should
be
pre-
pared
by the
electrical
engineer
and
copies should
be
distributed
for
general discussion.
The
motor
and
equipment list must
be
kept
up to
date during
the
course
of
design
as
motors
and
loads change
in
rating
or
are
added
or
deleted.
One of the
most subtly impor-
tant
points
of the
motor
and
equipment list
is an
early
decision
on a
name
for
each equipment item exactly
as
the
designer wants
it
inscribed
on the
nameplates.
Even though
the
exact name
of a
particular item
may
change during
the
course
of
design,
all
parties
involved
will
use the
same name
to
describe
a
piece
of
equipment.
Following
the
motor
and
equipment list,
the
elec-
trical calculations
for the
pumping station include
the
following:
•
Sizing
the
branch circuit breaker
or
fused
switch,
starter,
and
conductors
for
each
of the
main pumps
•
Sizing
the
circuit breaker, starter,
and
conductors
for
the
sump pumps
and
other incidental motor
loads
•
Sizing
the
lighting
and
small power transformers,
circuit
breakers,
and
conductors
•
Sizing
the
heating
and
ventilating equipment panel
circuit breakers
and
conductors
•
Sizing
the
main (service)
overcurrent
protection
device, transformer, primary
and
secondary protec-
tion,
and
conductors
•
Lighting calculations, layouts,
and
circuiting
•
Sizing
the
power
factor
correction capacitor bank
•
Sizing
the
standby engine-generator.
These calculations
are
illustrated
in
Examples
9-2
through
9-
10.
Example
9-1
Electrical
Load Estimation
Problem: Estimate
the
service electrical load
for a
pumping station with
four
constant-speed,
25
hp, 720
rev/min
motors driving three
duty
pumps
and a
standby pump.
The
maximum planned
capacity
is
4500
gal/min
at a
maximum
TDH of
40.7
ft for a
Hazen-Williams
C of 120 and
38.3
ft
for a C of
145.
The
present required capacity
of
3600
gal/min
can be
achieved with
two
pumps.
The floor
elevations are: ground,
112.5
ft;
basement, 102.5
ft;
pump room, 90.0
ft; and the
three
floors are
21
by 42 ft in
plan.
The
ground
floor
contains
a
trolley hoist,
the
motor control
center,
and an
engine-generator.
Air
handling units
are in the
basement.
The wet
well
floor is at
elevation 89.25
ft, and the wet
well plan size
is 8 by 34 ft.
Solution:
Pump
motors. According
to NEC
Table
430-150,
the
full-load current
(7
FL
)
for a
25-hp,
Code
F.
720
rev/min,
460-V,
three-phase,
60-Hz
motor
is 34 A.
(Note: check
the
manufacturer's data
for
720
rev/min motor
to
determine
the
actual
full
load current. Slow-speed motors
[<1200
rev/
min]
typically have lower power
factors
and
therefore draw more current than
is
shown
in NEC
tables,
so
compute with
the
manufacturer's data
if
they
are
available.
If
power factor correction
at
the
motor terminals
is
included
in the
design,
the
full-load
current
may be
reduced
to the NEC
value
[or
even
less].)
Heating
and
ventilating.
The
power requirements
for
heating
and
ventilating
(H&V)
must
come
from
the H&V
engineer,
who
determines them
on the
basis
of the
size
of
rooms, insula-
tion value
of the
building envelope, heat sources,
and
climate
(as
explained
and
calculated
in
Chapter 23).
The H&V
loads used
in
this example
are
higher than normal
and
would
be
appro-
priate
for a
pumping station
in a
severe climate.
The H&V
load
is
typicaly less than
10% of the
total load.
Other
electrical loads. Estimates
for
other loads
can be
made either
by the
project leader
or
by
an
experienced electrical engineer.
As the
design progresses
and the
loads become known
more exactly,
the
estimates below
may be
modified.
Power
Item Full load
Service
Essential
load
Name
No. hp kW A V
Phase
Continuous Intermittent Design
A on
generator,
A
Pump
1 25 34 460 3 X 34 34
2
25 34 460 3 X 34 34
3 25 34 460 3 X X 34 34
4 25 34 460 3 X
Stdby Stdby
Sump
pump
5 0.5 1 460 3
Xl
1
6 0.5 1 460 3 X
Stdby
Instrument
air
compressor
7 0.5 1 460 3
Xl
1
Item
Name
Pump
room
exhaust
fan
Control
room
exhaust
fan
Control
room
exhaust
fan
Engine
room
exhaust
fan
Air
handling
unit
Duct
heater
Pump
room
H&V
control
Wet
well
intake
fan
exhaust
fan
duct
heater
Wet
well
H&V
control
Lighting
transformer
Totals
Power
Full
load
Service
No.
hp
kW A V
Phase
Continuous
Intermittent
Design
A
8
0.5 1 460 3 1
9
0.5 1 460 3 1
10
0.5 1 460 3 1
11
0.5 1 460 3 1
12
0.5 1 460 3 1
13
16.5
21 460 3 21
14
0.5 1 460 1 1
15
0.5 1 460 3 1
16
0.75
1.5 460 3 1.5
17
34.8 43.5
460 3
43.5
18
0.5 1 460 1 1
(9
kVA)
19
8.5
10.5
460 3
10.5
62.75
222.5 187.5
Essential
load
on
generator,
A
1
1
1
1
1
1
1
1.5
1
9
121.5
Maximum
power required (for
a
corrected power factor
of
95%)
is
P
3
^
=
73
x 460 x
187.5
x
0.95/1000
=
14.9
kW(
149.4 kVA)
Demand factor: actual
=
0.85. Demand factor used
=
1.0. (The demand factor
is an
arbitrary per-
centage
of
connected, rated design loads that
is
used
to
determine actual operating loads.
It
allows
for
nonconcurrent
loads
and
motors that
are not
running
at
full
load.)
Minimum
feeder
to MCC =
187.5
A + 25% of
largest motor
=
187.5
+
(0.25
x 34) =
196.0
A.
Minimum main circuit breaker
=
187.5
A + 50% of
largest motor
=
187.5
+
(0.5
x 34) =
204.5
A.
Minimum
standard circuit breaker
= 225 A.
Design load plus
20%
"cushion"
=
187.5
x 1.2 = 225 A.
Anticipated power needed (for
a
corrected power factor
of
95%)
is
P^
=
73x460x225x0.95/1000
=
170.2
kW( 179
kVA)
Minimum
feeder
to MCC = 225 +
(0.25
x 34) =
233.5
A.
Minimum feeder conductor (NEC
Table
310-16,
75
0
C
column)
= 250
kcmil
(thousands
of
circular mils) rated
for 255 A.
Minimum
main circuit breaker
= 225 +
(0.5
x 34) =
242.0
A.
Next smallest standard circuit
breaker rated
for 300 A.
Minimum feeder conductor
= 350
kcmil rated
for
310
A.
Minimum
standard transformer
= 225
kVA.
Use:
225
kVA, 12.47
to
480/277-V
transformer;
300-A
three-phase main circuit breaker
in
MCC;
350-kcmil
feeder conductors (rated
for the
main circuit breaker);
and
#2
AWG
(American Wire Gauge) ground conductor.
9-6. Overcurrent Protection
and
Conductor
Sizing
The NEC has
definite
standards
for
selecting overcur-
rent protective device ratings
and for
sizing conduc-
tors. Conductor sizing
is
based
on the
load current
of
the
equipment connected
to the
circuit
and is
influ-
enced
by
circuit length (voltage drop)
and
ambient
temperatures.
It is
usually wise
to
allow spare capac-
ity.
The NEC and
local
and
state codes
and
rules must
be
followed
as a
minimum
safety
standard
for
design.
Furthermore, consider designing
for
long-time service
and
load growth. Overcurrent protection
is
accom-
plished
by
applying current-sensing techniques
at
many
points
in the
utility's distribution system
as
well
as
within
the
user's system. Sensing equipment
may
be
self-destructive
(e.g.,
fuses),
resettable
(e.g.,
circuit
breakers),
or
remotely located (e.g., protective relay
applications).
The
electrical design engineer's role
is to
develop
a
system
so
coordinated that
an
overloaded motor will
be
removed
from
service before
the
motor insulation
or
bearings
are
damaged,
and
only
the
affected
equip-
ment
(in
this example,
the
overloaded motor) will
be
removed
from
the
service.
A
properly coordinated
overcurrent
system will enable
fault
conditions
to be
isolated
from
the
system without
affecting
the
opera-
tion
of
other equipment. Coordination studies range
from
simple
to
very complex.
A
thorough discussion
of
the
method applicable
to
system studies
and
coordi-
nation
is
contained
in
IEEE Standard 242.
A
single-line diagram
of the
pumping station
of
Example
9-1 is
shown
in
Figure
9-1.
The
utility
fuses
are
pole
mounted,
and
they protect
(1)
the
utility
from
overloads (overcurrent,
faults)
within
the
pumping
station
electrical
systems,
(2) the
transformer,
and (3)
the
service cable connecting
the
transformer
to the
utility.
Fuses
Fuses
are
commonly used
by the
utility
to
protect their
system
from
faults
in the
customer's
electrical
system.
Generator
Standby
Pump
Sump
lnstr
Pump
Wet
Lighting
control
generator
no. 1
pump.
air Rm
well
panelboard
panel
(typical
Items*
comp.
(see (see
(11OV)
of
4
)
5-6
Item*
Figure
Figure
9-3)
Item*
Items*
7
9-2).
Items*
19
1_4
Items*
15-18
8-14
Figure 9-1.
Pumping
station
electrical
single-line
diagram.
Htr,
heater;
lnst,
instruments;
Comp,
compressor;
H&V,
heating
and
ventilating;
AHU,
air
handling
unit;
Cont
Rm EF,
control
room
exhaust fan;
lnt,
intake.
*See
load
esti-
mates
in
Example
9-1.
Automatic
transfer
switch
Several classes
of
fuses
are
encountered
in
industrial
design such
as
that
for
pumping stations.
An
example
is
pole-mounted
fuses
used
by
utilities. These
may be
of
the
expulsion type
in
which
the
gases generated
by
the
melting
fuse
and the
heated casing during
an
over-
current condition cause
a
mechanical indicating
device
to
function.
The
typical dropout fuseholder
unlatches
and the
blown
fuse
remains with
the
hinged
portion
of the
holder.
The
fuses
are
generally selected
by the
utility,
and
so it is
necessary
to
obtain
the
exact
fuse
data
and
available
fault
currents
from
the
utility before starting
a
coordination check.
Other
fuses
commonly used
in
pumping stations
include
medium-
voltage
transformer primary
fuses
on
large installations,
medium-
voltage
current-limiting
fuses
on
medium-voltage (2400
and
4160
V)
motor
starters,
low-
voltage
current-limiting
fuses
in
low-
voltage
switchgear
(to
reduce
the
fault
current avail-
able
to the
loads),
and
low-voltage transformer
pri-
mary
fuses.
In
the
pumping station design examples
in
this
chapter,
fuses
are
used only
to
protect control trans-
formers,
and
they
are
used
in
both
the
transformer
primary
and
secondary sides.
The
electric power
utility
is to
furnish
the
primary
fuse
at
their service
pole.
Circuit
Breakers
Circuit breakers
are
electromechanical switching units
that
are
designed
to
open
a
circuit when specified
overload
or
fault
conditions occur. Either thermal-
magnetic
or
magnetic-only
molded-case
circuit break-
ers are
normally used
in
pumping stations.
The
power
circuit breaker type
may be
preferred
for
large
or
high-
voltage
motors.
The
type
of
circuit breaker
cho-
sen is
based
on
current withstand, interrupting,
and
coordination ratings
and on
economic factors.
The
thermal-magnetic circuit breaker
is
tripped
by a
magnetically operated element
for
severe
faults
and by a
thermal element
for
long-duration overloads. Thermal-
magnetic circuit breakers
may be
adjustable
and
nonad-
justable
and are
used
as
main
and
branch circuit break-
ers and are
used
to
protect almost
all
types
of
loads.
An
adjustable-trip
form
of the
magnetic-only
cir-
cuit breaker
is
most
often
used
in
conjunction with
a
motor starter
to
protect motor branch circuits. This
unit,
called
a
"motor
circuit protector
(MCP),"
is
sized
and
adjusted
to
pass
the
starting current
of a
particular
motor
and
will
not
trip when
the
sustained current
is
less than
the
setting (which
can be as
much
as 13
times
the
full-load current
of the
motor).
It
will trip,
however, under severe
fault
conditions.
The
overload
relays required
on the
motor starter provide
the
required motor overload protection.
Example
9-2
Branch
Circuit
Calculations
for
Pump
Motors
Problem: Size
the
branch circuit breaker, starter,
and the
branch circuit conductors
for
each
of
the
main pumps
of
Example
9-1.
Solution: From Example
9-1,
the
motor full-load current,
/
FL
,
is 34 A.
Size
the
branch circuit breaker.
NEC
430-152
allows
the
circuit breaker
to be
sized
to a
max-
imum
of
250%
of
full-load
current, which
is 34 x 2.5 = 85 A. Use a
70-A
circuit breaker
for a
normal inertia system.
If the
inrush current (approximately
6x7^
=
204)
does
not
persist
more
than
about
10s,
the
circuit breaker will
not
trip.
Be
careful
to
check
the
time-current
curves
of
the
circuit breaker.
If
the
drive inertia
of the
centrifugal pumps were higher than normal,
the
circuit breaker
should
be
sized above
the
250%
value
in
accordance with
NEC
430-52
(which limits
the
maxi-
mum
setting
to
400%)
to
accommodate
the
necessary longer acceleration time
of the
drive.
Size
the
motor circuit protector.
For
adjustable-trip, magnetic-only circuit breakers,
NEC
430-152
allows
the MCP to be set at
700%
of
full-load current,
so 34 x 7 = 238 A. NEC
430-52
limits
the
maximum setting
to
1300%
of the
full-load current,
or 34 x 13 = 442 A.
Select
an
MCP
with
a
50-A
rating
and an
adjustable-trip range
of 150 to 580 A. The
initial setting should
be
approximately
200 A
(see,
for
example, literature
from
the
Square
D
Company;
the
ratings
of
other manufacturers
are
similar).
Size
the
disconnect
switch
at
motor.
If a
disconnect switch
is
required
at the
motor,
NEC
430-
110
applies,
and
this requires
the
switch
to be
rated
at
least
at
115%
of the
motor's
full
load
cur-
rent,
or 34 x
1.15
=
39
A.
Checking
Branch
Circuit
Voltage
Drop
NEC
210-19,
footnote
a,
recommends that
the
branch
circuit
voltage drop
be
less
than
3%, and the
total drop
in
feeders
and
branch circuits must
not
exceed
5%.
Good design limits
the
voltage drop
to
less
than
2%
each
for
feeders
and
branch circuits.
Another
facet
of
voltage drop (which
is not
gov-
erned
specifically
by the NEC but is
essential
to
good
operation)
is the
motor-
starting
voltage
drop
in the
branch
circuits
and
feeders.
An
induction
or
synchro-
nous
motor draws about
six
times
the
full-load
current
at
the
moment
of
starting,
and
this decreases
to the
current
necessary
to
drive
the
load during
the
remain-
der of the
acceleration time
and the
normal running
time. Ordinarily,
a 13 to 15%
voltage drop
is
accept-
able
and
provides
sufficient
voltage
for
accelerating
the
drive.
The
starting voltage drop with
No. 6 AWG
copper conductors
is
approximately 1.4%,
and the
running
drop
is
about 0.3%.
The
starting voltage drop
is a
problem only
on
long runs.
The
method
for
calcu-
lating
the
voltage drop
is
given
in
Example
13-1.
Miscellaneous
Drives
and
Loads
Two
important drives,
the
pump room sump pump
and
the
instrument
air
compressor,
are
rated
at
1
I
2
hp
each.
These drives should
be
served
by
branch circuits
of
the
motor control center
at 460 V
(three phase).
The
exhaust
fans
and the
duct heaters could also
be
served
by
individual motor
circuits.
Because
of the
inter-
locking requirements
and to
facilitate
safe
maintenance
practices,
two
heating
and
ventilating
panels
should
be
provided.
The
arrangement
of the
panels
is
shown
in
sin-
gle-line diagram
form
in
Figures
9-2 and
9-3.
Each heating
and
ventilating control panel includes
all of the
power
and
control
functions
for the
duct
heater
and its
interlocked
fans.
The
manufacturer
of
the
panel should
be
given
the
option
of
providing
a
transformer
large enough
to
serve
the
fans
at 120 V or
208 V
(single phase)
from
a
208Y/120-V
panelboard
in
the
H&V
control panel.
Interlocks
from
the
station's automatic transfer
switch
cut off the
duct heater when power
is
being
taken
from
the
station standby generator. Fans
and
lou-
vers
are
still required
to
operate under these conditions.
The
next larger
standard-size
disconnect switch
is
rated
at 60 A, so use a
60-A
disconnect
switch
(in a
housing suitable
for the
environment) with
an
auxiliary control contact that opens
when
the
switch
is in the
open
position.
The
contact should open before
the
main contacts open
in
order
to
drop
out the
motor starter.
Size
the
motor starter.
The
motor starter manufacturer's data shows that
an
NEMA size
2
starter
is
rated
for
25-hp,
460-V,
three-phase
motor-starting
duty,
so use a
NEMA size
2
starter.
A
size
3
starter would
be
specified
if it is
likely that
a
30-hp motor will
be
substituted
in the
near
future
or the
motor
is of a
type that draws more current than
is
shown
in NEC
tables.
Low-speed
motors
often
require larger starters; check with
the
starter manufacturer.
Size
the
branch circuit conductors. Branch circuit conductors must
be
rated
at
least
at
125%
of
the
motor's
full-load
current,
or 34 x
1.25
=
42.5
A.
Using
a
maximum conductor temperature
of
6O
0
C,
NEC
Table
310-16
lists
No. 8 AWG
copper
at 40 A and No. 6 AWG
copper
at 55 A.
Use
three
No. 6 AWG TW or THW
insulated copper conductors plus
No. 6 or No. 8 AWG
ground
conductor (NEC Table
250-95
lists minimum-size grounding conductors).
THW
insula-
tion
is a
thermal plastic material that softens when heated. Many
designers
prefer XHHW insu-
lation,
a
cross-linked synthetic polymer, which
is
thermal-setting
and
hardens when heated.
Example
9-3
Branch
Circuit
Calculations
for
Sump Pump
Problem: Size
the
circuit breaker,
the
starter,
and the
branch circuit conductors
for the
sump
pump
(and
all
other loads
of
identical horsepower rating)
in
Example
9-1.
Solution:
One
manufacturer's
1150-rev/min,
460-V,
1/2-hp
motor
is
listed with
a
full-load
current
of 1 A and a
locked-rotor
current
of
12.5
A.
Size
the
motor circuit protector. Size
the MCP at
700%
of
full-load
current
per NEC
430-
152,
or 1.0 x 7.0 = 1 A.
Select
an MCP
rated
at 3 A
with
an
adjustable-trip range
of 8 to 22
A—slightly
above
the 7 A
required. Depending
on the
exact characteristics
of the
motor,
the
setting should
be
very
close
to
(or
slightly exceed)
NEC
limits.
Size
the
starter. Although
the
manufacturer's data show
a
size
O
starter,
the
starter should
be
an
NEMA size
1,
which
is the
minimum recommended
by the
authors.
Size
the
branch circuit conductors. These
must
be
rated
at
125%
of the
full-load
current,
or
1.0 x
1.25
=
1.25
A. The use of
minimum-size
wire,
No. 12 AWG
stranded copper conductor,
is
recommended;
No. 14 AWG
conductor
is
adequate.
The
pumping station lighting
and
small ancillary
loads operate
at 120 V
(single phase)
or at 208 V
(sin-
gle
and/or three phase).
A
step-down transformer
and
circuit breaker panelboard
are
used
to
feed
this load.
The
calculations
for
feeding such equipment
are
illus-
trated
in
Example
9-4.
Figure
9-2. Pump room heating
and
ventilating panel.
Figure
9-3.
Wet
well
heating
and
ventilating panel.
Example
9-4
Lighting
and
Small
Power
Transformers
Problem: Calculate
the
size
of the
lighting
and
small power transformers
in
Example
9-1.
Solution:
The
principal loads
on the
lighting panelboard
are the
pumping
station's
light-
ing
and
receptacle circuits
and the
battery charger. Allow 1000
W for the
battery charger.
Allow
3000
VA for
lighting
in the
pump building
and 500 VA in the wet
well. Allow
1500
W
for
receptacles.
The
calculated load
is 600 VA or 6
kVA.
The
next larger
size
transformer
is
9
kVA.
A
9-kVA,
three-phase,
480-V
delta primary,
208Y/120-V
secondary transformer should
be
used.
Size
the
transformer
primary circuit
breaker.
NEC
450-3b governs
the
sizing
of
transformer
overcurrent
protection. Where circuit breaker protection
is
provided
in
both
the
primary
and
sec-
ondary,
the
maximum primary overcurrent protection
is
250%
of
full-load
current.
The
maxi-
mum
secondary overcurrent protection
is
125%
of the
full-load
current. From Equation
8-6
(^
=
^/3
V
L
x/
L
x
P/),
/PL
=
P^/(V
L
x
*/3Pf)
=
90007(48073
x
0.95)
=
11.4A
if
a
power
factor
of
0.95
for the
lighting
and
small loads
on
this panelboard
is
assumed (usual
for
fluorescent
lighting).
250%
of/FL
=
11.4x2.5
=
28A
This maximum
must
not be
exceeded,
so use the
next lower size, which
is a
20-A primary
circuit breaker.
Size
the
secondary
circuit breaker
(panelboard
main
breaker).
The
full-load
current
is
/
FL
=
90007(208
x
73)
= 25A
125%
of
/PL
=
25x1.25
=
31A
Use
a
30-A secondary circuit breaker. Secondary protection should always
be
provided.
Size
the
primary
conductors.
The
primary
is
protected
by a
20-A circuit breaker (refer
to NEC
Table
310-16).
Because
the
6O
0
C
rating
of a
copper conductor
at 20 A
requires
No.
12
AWG
size,
use
three
No. 12 AWG
copper conductors plus
one No.
12
AWG
ground
conductor.
Size
the
secondary
conductors.
The
secondary
is
protected
by a
30-A circuit breaker.
Because
the
6O
0
C
copper temperature rating
at 30 A
requires
a No. 10 AWG
conductor,
use
four
No. 10 AWG
copper conductors (three phase
and one
neutral conductor), plus
one No. 10 AWG
ground
conductor.
Size
the
panelboard circuit breakers.
The
panelboard circuits should
all be
rated
at 20 A, and
all
of the
conductors should
be No. 12 AWG
copper.
The H&V
panels require
460-V,
three-phase
power.
The
sizing
of the
feeder circuit breaker
and
feeder
conductors
is
illustrated
in
Example 9-5.
Example
9-5
Heating
and
Ventilating
Electrical
Equipment
Problem: Determine
the
panel loads,
the
size
of the
circuit breakers,
and the
size
of the
conduc-
tors
for
(1)
the
pump building heating
and
ventilating
and (2) the wet
well heating
and
ventilat-
ing
in
Example
9-1.
Solution:
(1)
Pump building
H&Vpanel.
Per NEC
code,
for the
largest motor
add 25% of 1
A
(=
0.25A)
for
minimum conductor size.
The
duct heater
is
rated
at
16.5
kW, so the
rated cur-
rent
is
16,5007(460x73)
= 21 A
Four
/
2
-hp
motors
at 1 A
each
= 4 A
Transformer,
1
kVA,
single phase
= 2 A
Largest motor
x
25% =
0.25
A
Total
H&V
panel load
=
27.25
A
Choose
a
circuit breaker
for at
least
125%
of
total calculated load:
27.25
x
1.25
=
34
A
Use
a
40-A
circuit breaker
in the
MCC.
Conductors must have
an
ampacity (current-carrying capacity)
of no
less than
40 A. In the
75
0
C
column
in NEC
Table
310-16,
No. 8 AWG
conductors
are
rated
at 40 A and No. 6 AWG
conductors
are
rated
at 55 A. Use
three
No. 8 AWG
copper conductors
and one No. 8 AWG
ground
conductor.
(2)
Wet
well
heating
and
ventilating
panel
The wet
well
H&V
panel
is
calculated
in a
similar
manner.
A
circuit rated
at 60 A
minimum
is
required.
Use a
70-A circuit breaker
in the
MCC,
and
use
three
No. 4 AWG
copper conductors
and a No. 8 AWG
ground conductor.
Service
Load
Now
that
the
branch circuits
for
motors
and for
heat- load
can be
determined,
and the
main transformer,
its
ing
and
ventilating
are
sized,
the
total station (service) protection,
and the
conductors
can be
sized.
Example
9-6
Service
Circuit
Transformer,
Protection,
and
Conductors
Problem: Find
the
sizes
of the
service circuit breakers,
the
main transformer,
the
primary
and
secondary protection
for the
transformer,
and the
service conductors
for the
pumping station
in
Examples
9-1
to
9-5.
Solution: Summarize
all the
pump station loads:
Pumps:
3 x 34 A = 102 A
Air
compressor
and
sump pump:
2 A
Lighting load:
11
A
Pump building
H&V:
27 A
Wet
well H&V:
48 A
Total station load:
190
A
Treat
all
loads
as
"continuous duty."
Per NEC
430-62(b),
the
rating
of the
main circuit
breaker
may be
sized
for
possible
future
larger loads.
Size
of
main
transformer.
The
transformer's kilovolt-ampere rating should exceed
the
calcu-
lated load
of the
station
by
15
to 20% or
more depending
on
several factors.
The
lack
of a
quickly
obtainable replacement transformer
may
dictate operation
at a
somewhat lower total temperature
9-7.
Lighting
Use the IES [2]
lumen method
for
lighting calcula-
tions. Manufacturers'
selection
charts
are too
inaccu-
rate
for
small buildings because they
are
based
on
somewhat
different
criteria.
A
lighting level
of 20
foot-candles
(ft • cd) is
usually
sufficient
for
pumping
stations
if
there
are
adequate GFCI receptacles
for
work
lights needed
for
repairs.
The
following lighting
levels
are
recommended:
•
Offices:
50-100
ft - cd
•
Control areas:
30-75
ft • cd
•
Equipment areas
15-50
ft • cd
•
Outdoor areas:
1-5 ft • cd.
Many
states
now
have
a
lighting limitation based
on
watts
per
square
foot,
and
this limitation should
be
investigated prior
to
design.
Obtain
fixture
data
from
manufacturers'
catalogs.
These
data
are
classified
by
several parameters based
on
Equations
9-1 and
9-2.
The
room cavity ratio,
RCR,
is
defined
as
RCR =
5H(L
+
W)/
A
(9-1)
where
H is the
distance
from
the
work plane (usually
2 ft 6 in.
above
the floor in
offices
and
control areas,
3 ft
above
the floor in
equipment areas,
and the floor
itself elsewhere)
to the
bottom
of the
light
fixtures, in
feet,
L is
room length
in
feet,
W is
room width
in
feet,
and
A is floor
area
in
square feet.
From
the IES
handbook
[2],
the
relationship
between illumination
and the
number
of fixtures is
Number
of
fixtures
ft
-cd
(number
of
lamps/fixture)
(lumens/lamp)
area
(CU)(LLD)(LDD)(RSD)
,^
where area
is in
square feet,
CU is a
coefficient
of
uti-
lization depreciation
and
varies
from
0.01
to
0.99
(Table
9-1),
LLD is
lamp lumen depreciation,
LDD is
luminaire dirt depreciation (dimensionless),
and RSD
is
room surface depreciation (dimensionless).
than
the
full-load
rating would produce. Predictions
of
substantial load growth within
10 yr
make
oversizing
worth studying.
A
slight oversizing, such
as
15%, allows
for
some adjustment
in
main
pump
sizes
or in H&V or
other auxiliary equipment without
the
necessity
of
rebuilding
the
entire
station
electrical system.
For a
station load
of 190 A, 190 x 480 x 73 = 158 kVA and 15%
oversizing
adds
23.7
kVA for a
total
of
181.7 kVA,
so use the
next larger standard size unit, which
has
a
rating
of
225
kVA.
Size
of
transformer
primary
and
secondary
protection. Assume that
the
fuse
will
be
provided
by
the
utility. Although
the
utility
is not
required
to
design
to NEC
standards,
the
fuse
size
is
likely
be the
same because
it is
based
on
load size,
and
utilities
use
conservative conductor cur-
rent
ratings
for
their equipment.
For
transformers with less than
6%
impedance
and a
primary voltage greater than
600,
NEC
allows
a
maximum
fuse
rating
of
300%
of the
full-load current.
The
480-V secondary
circuit
protection cannot
be
more than 250%
of the
full-load current.
For a
utility voltage
of
12,500,
the
primary
is
225,000/(12,50073)
=
10.39
A , so the
maximum
fuse
per the NEC
is
10.4x3
=
31
A.
A
30-A
fuse
could
be
used,
but a
20-A
fuse
designated
2OT
type provides transformer pro-
tection closer
to the
overloading point
of the
motor.
The
secondary
is
225,000/(48O73)
=
270.6
A. The
maximum circuit breaker (main
MCC
circuit
breaker)
is
271
x 2.5 = 677 A; the
minimum circuit breaker (the total connected load plus
50%
of
largest motor)
is 207 A.
Use
a
400-A
frame
circuit breaker with
a
300-A trip.
Sizing
the
service conductors.
The
service conductors should
be
sized
for any
foreseeable
load growth that
can be
accommodated without
the
necessity
of
replacement.
The
minimum size
is
that which
will
be
protected
by the
main circuit breaker, namely,
300 A at
75
0
C
(refer
to NEC
Table
310-16).
Use
three
350-kcmil
copper phase conductors plus
one No. 2
neutral (grounded) conductor
in
a
concrete-encased duct
4 in. in
diameter.
The
neutral conductor must
be
sized according
to
NEC
Table 250-94
as a
minimum.