Services
• Primary Service vs
Secondary Service
• Overhead vs Underground
• Common Service Types
– Three Phase
• Wye
– Closed
– Open
• Delta
– Corner Grounded
– Center Tapped
– Open
– Single Phase
Three Phase Service
• Wye
• Delta
– Corner Grounded
– Center Tapped
– Open
1
Wye Service
• Customers needing three
phase for some equipment
and also having large
amounts of single phase load
• Many small commercial &
industrial buildings
• 208/120 Wye
– 208 3 phase
– 120 single phase
• 480/277 Wye
– 480 three phase
– 277 single phase
3 Phase Wye System
• 3 wires connect all at a
common point which
is grounded.
• End points become the
connection points for
the 3 phases.
• Y system – wye
• 2 separate voltages
– Phase to Phase
– Phase to Ground
2
208/120 Wye
• Smaller customers
requiring mostly
single phase and
some 3 phase.
• 480 volt services
are more
economical for
larger motor loads.
480/277 Wye
• Voltages
– 480 volts phase to phase
– 277 volts phase to ground
• Requires step down
transformer to produce
120 volts for receptacles
• 277 volt lighting fixtures
are available
3
3 Phase Delta Systems
• Generally built to areas with
mostly 3 phase load and not
much single phase load. (oil
pumping, irrigation, etc.)
– Advantages: Expense of one less
wire to build and maintain.
– Disadvantage: Unbalanced
voltages from amounts of single
phase load.
• Phases connected end to end
– Each phase relies on the sharing
of one wire as a supply and return
path for the others.
– Voltage the same between any 2
wires
• Popular for ungrounded
systems
Center-Tapped Delta
• Customers with mostly
three phase equipment and
very little single phase
equipment.
A
– Manufacturing &
processing facilities
• 240/120 CT Delta
– Smaller 3 Phase Services
with mostly 3 phase load.
C
– 240 three phase
– 120 single phase
• 480/240 CT Delta
– Larger 3 Phase Services
with mostly 3 phase load.
B
G
Neutral Wire
• 480 three phase
– 240 single phase
• High or Wild Leg
4
Corner Grounded Delta
• 240 or 480 volts.
• Usually only for 3
Phase Motor/Pumping
Load.
– Oil Pumps
– Irrigation Pumps
– Municipal Water
A
G
B
C
Open Delta
• Uses only 2
transformers
• Often from a VPhase Distribution
Line.
Open Wye Primary to Open Delta Secondary
Open Wye (V Phase) Primary
A
C
N
H1
– Wye with 2
Phases and a
Neutral.
• Less Expensive
• Light 3 Phase
H2
X1
X3
X2
H2
H1
X1
X3
X2
a
b
c
n
3 Phase 4 Wire Open Delta Secondary: 120/240, 240/480, etc for lights and power
5
Single Phase Service
• 120/240 volt service.
• Neutral attached to
center bushing in
transformer.
– 120 volts Hot to
Neutral
– 240 volts Hot to Hot
Single Phase from
3 Phase Systems
• Individual single phase
loads can be connected and
balanced to resemble a 3
phase load.
• Voltages will be different
in Wye & Delta systems
• If not balanced, voltage
unbalances will result.
6
Secondary System
•
•
•
•
•
•
•
•
Transformer
Service Drop
Meter Loop
Meter
Ground Rod
Main
Breaker Box
Circuits
Service Drop
• Overhead:
– Service Drop
• Underground:
– Service Lateral
• Three Phase:
– 3 or 4 wires
• Single Phase:
– 2 or 3 wires
• Triplex
• Quadraplex
7
Meter Loop
• A Meter Loop is
comprised of:
– Weatherhead
– Service Mast/Riser
– Meter Base/Can
Meter
• The Cash Register
• Owned by Utility
• Installed in customer’s
equipment.
8
Main Disconnect
• Today, electrical
codes require some
type of disconnect
device (switch) to shut
off all the circuits
supplied by the
disconnect.
• Usually a “Main
Breaker” in the
breaker box.
• Older or larger
facilities may have
“Main Disconnects”
rather than main
breakers.
Ground Wire/Rod
• Most electrical systems in North
America are grounded for safety
and lightning protection.
• Their is generally a ground
wire/rod at the:
– Service transformers.
– Meter Base
– Breaker Box
• Formal Names:
– Grounding System Conductor (wire)
– Grounding System Electrode (rod)
9
Breaker/Fuse Box
• Formally called the
“Service Entrance Panel”
• Provides a location for
breakers or fuses to
protect the individual
circuits in the facility.
• Some larger customers
have “Sub-panels” which
contain breakers and
fuses.
Circuits
• Feeders
– circuits supplying services
or sub-panels.
– the service drop is a
“feeder”.
• Branch Circuits
– Individual Branch Circuit
• Circuit to one piece of
equipment
– Appliance Branch Circuit.
• Circuit to several fixed
appliances
– Lighting Branch Circuit
• Circuit supplying only
lighting fixtures.
– General Purpose Branch
Circuit
• Circuit to receptacles and
lights.
10