Design of a Power Distribution System
1. Objective
The objective of this project is to design an overhead residential electrical power distribution
system.
2. Background
Alabama Power Company continually balances the needs of its customers with the financial goals
of its investors. To do so effectively, company engineers must work long and hard to ensure that
the company's distribution system is designed in an efficient manner. The majority of Alabama
Power Company's customers are in residential neighborhoods, and thus the majority of its
distribution system is built to serve residences.
Figure 1 - Example of information for a new addition.
One of the most challenging facets of a distribution engineer's job occurs when a new subdivision
is to be built. The engineer meets with the developer and discusses the number and types of homes
that will be built in the development. Generally, the developer provides a schematic of the
subdivision that identifies the types of homes, the right-of-way, and any special structures such as a
clubhouse. An example of such a drawing appears in Figure 1. However, a good rule-of-thumb is
to assume that half of the homes are all electric and half are all gas. From that point, the engineer is
responsible for designing an efficient and cost effective electrical distribution system that guarantees
each customer will be served with electricity.
3. Design Guidelines
Alabama Power Company engineers have a set of guidelines to help them design efficient
systems. But before we describe those guidelines, it is important to note a few basic facts about
electric distribution systems in general. First, the distribution engineer is concerned with delivering
electricity from electrical substations to the customer, not from the power generating plants. This
electricity is delivered from the substation and carried in conductors (or as you probably know
them, wires) at 7.2 kV. The conductors are sometimes laid out on the tops of power poles and
sometimes buried underground in insulated cables. Once at the customer's residence, the voltage is
reduced from 7.2 kV down to 240 V via a transformer. These transformers are sized (25 kilovoltamp(KVA), 37.5 KVA, 50 KVA, 100 KVA, etc.) according to the number of customers (and thus
the "load") they will be serving. The conductor running from the transformer to the customer is of a
different size from the conductor used to carry the electricity from the substation to the transformers
because the voltage is so much smaller. The engineer is responsible for ensuring that the
distribution system he or she designs gets the right amount of electricity to the right place, at the
right time. Fortunately, in this design problem, we have dramatically simplified the process.
3.a. Conductors
Your guidelines for the conductors are relatively simple. You use two basic conductors. First,
you will use a #2 ACSR conductor for carrying the electricity from the substation (or in this case
from an existing power line) to the transformers. This conductor is such that you can run spans (a
span is the distance between two power poles) of up to 400 feet. However, please be advised that
the lot lines in the development will more than likely dictate that your spans will be much shorter.
Second, you will use a #1/0 triplex to carry the electricity from the transformer to the customer.
This conductor will allow spans of up to 200 feet. However, flicker and load drop calculations may
well dictate shorter span lengths.
3.b. Flicker and Load Drop Calculations
As the electricity is conducted through the wires of your triplex, a loss in voltage or load drop
occurs. Alabama Power Company has established standards for both flicker and load loss. Flicker
is not to exceed 5% while the maximum allowable load drop is 3%. The two equations below can
be used for these calculations.
Voltage Drop: Voltage drop is defined as a reduction in voltage due to a steady state current
flow at any specified point on the circuit from the source to the load. It is often expressed as an
actual value in volts or as a percentage of some nominal value. The nominal value for most
voltage drop calculations in the distribution voltage classes is 120 volts. A 5% voltage drop
would be a drop of 5% of 120 volts, or a 6 volt drop, resulting in a voltage of 114 volts.
Generally, a substation bus is considered a regulated voltage source, and the total voltage drop
seen by the customer would include the drop through the primary conductors, distribution
transformers, secondary conductors and service conductors. The secondary voltage drop would
include only the drop through the secondary and service conductors. An engineering design
limit of 3% has been established for secondary voltage drops. The percent secondary voltage
drop can be calculated using the formula below:
%VOLTAGE DROP = KVA x Distance x Kvd
1000
where Kvd = 0.6729 for # 1/0 triplex at 0.90 pf (power factor).
Flicker: Flicker is a special case of voltage drop and is associated with voltage disturbances
that are of a transient nature. In a residential setting, flicker would primarily be caused by the
starting of a large electrical motor such as found in the compressor of a central air conditioner or
heat pump. Since these disturbances are generally of a highly inductive nature, it is necessary to
also include the effects of passing a large inductive load through the transformer windings. An
engineering design limit of 5% has been established for flicker. The % flicker can be calculated
using the formula below:
%FLICKER = KVAlr x Distance x Kfd + Vtrans
1000
where Kfd = 0.3744 for # 1/0 triplex at 0.40 pf.
KVAlr should be selected from the chart below:
Compressor Size
2.5 Tons
3.0 Tons
4.0 Tons
KVAlr
17.8
23.5
31.7
Vtrans should be selected from the chart below:
Transformer
KVA Rating
10
15
25
37.5
50
75
100
%Voltage Drop Through Transformer
(Vtrans)
Compressor Size
2.5 Ton
2.49
1.54
1.28
0.66
0.57
0.40
0.32
3.0 Ton
3.29
2.04
1.69
0.88
0.75
0.53
0.42
4.0 Ton
4.40
2.75
2.28
1.18
1.01
0.72
0.57
3.c. Load Sizing
The engineer starts with the developer's best guess as to the types of homes he or she is going to
build in the development. Naturally, not all of the homes will be the same, so they may have very
different electricity needs. For instance, an "all electric" home will have a higher electricity demand
than a home that is heated with gas. The engineer uses a table much like Table 1 to "size the load"
in the development. Table 1 is used to estimate the load requirements of each home. This step in
the design is very important when you are deciding where and what size transformers to put in
place. In some instances it is best to put four customers on a single 50 KVA transformer, when
other instances deem it most efficient to put two separate 25 KVA transformers each serving two
customers. These decisions are made based on a number of factors including distances between
customers, the layout of lot lines, expenses, and flicker/load drop calculations.
Load sizing information is vital in transformer sizing. Transformers are sized to meet 110% of the
total load, or 1.10 times its rating. For example, a 25 KVA transformer can handle 25 x 1.1 = 27.5
KVA. The engineer's approximations concerning load sizing are extremely important and can make
or break the design.
Transformer Loading: As mentioned previously, transformers are utilized to reduce the 7.2 kV
primary voltage to 120/240 volt service for customer utilization. They come in a variety of KVA
ratings (15, 25, 37.5, 50, 75, 100) and should be sized using the data below. Transformers
serving residential customers can handle a 10% overload.
Table 1 - Load sizing information
NUMBER OF APPLIANCES
DESCRIPTION
LIGHTS, REF., & MISC.
WATER HEATER
RANGE & OVEN
AIR CONDITIONING
(3 TON CENTRAL UNIT)
HEAT PUMP (2.5 TON
WITH 10KW STRIP HEAT)
HEAT PUMP (3.0 TON
WITH 12KW STRIP HEAT)
HEAT PUMP (4.0 TON
WITH 15KW STRIP HEAT)
RESISTANCE HEAT
(15KW STRIP)
DRYER
1
2
3
4
5
6
7
8
9
10
2.5
4.5
2.5
4.5
5.0
5.6
4.4
7.4
7.5
6.6
6.0
10.2
9.5
7.6
7.6
13.2
11.2
9.3
9.1
16.0
12.8
10.3
10.5
18.6
14.1
11.0
11.8
21.0
15.3
11.2
13.0
25.8
16.5
11.4
14.0
26.6
17.6
11.6
15.0
29.5
4.4
6.6
8.7
11.0
13.0
15.0
16.8
18.4
20.7
22.0
5.3
8.0
10.7
12.8
15.5
18.0
20.3
22.4
24.8
27.5
7.0
10.4
14.0
17.0
20.2
23.2
26.3
29.5
32.7
36.0
8.7
14.3
21.8
25.5
30.0
34.5
40.5
45.0
49.5
54.0
1.7
2.4
3.1
3.8
4.5
5.2
5.9
6.6
7.3
8.0
3.d. Power poles
There are numerous sizes of power poles available to the engineer. Pole selection is based on a
number of factors including elevation changes within the subdivision. To simplify things
tremendously, you will use only 40 foot poles for the primary conductor (since the telephone and
cable companies will have joint use of the poles). You do have 35 foot poles available to you for
guying situations (more on this later). Each pole must be fitted with its own special hardware
depending on its use. However, to again simplify things, you can assume that each pole will come
equipped with the proper hardware and that the costs provided later in this assignment include all
hardware costs. The only distinction that will be made between 40 foot poles are those that are used
in spans making an angle of less than 30o and those used in spans making angles of more than 30o.
These two uses require different hardware that is hard to ignore. The first situation requires a pole
top pin insulator on which the conductor can be lain. The second requires a special insulator that
can not sit on top of the pole. This second piece of hardware is more expensive.
Pole Class: Pole class refers to the diameter of the pole at certain specified points along its
length. As the pole class number decreases, the pole diameter increases. A larger diameter pole
has greater ability to support vertical loading and resist bending moments associated with
horizontal and eccentric forces acting on the pole. For this exercise, you can assume that a class
5 pole is sufficient to support the forces created by any configuration of primary and secondary
you
choose to utilize. You will be required to select the minimum pole class necessary to support the
required transformer once you have determined its size. Pole class should be selected from the
table below:
Size of Transformer
15 KVA
25 KVA
37.5KVA
50 KVA
75 KVA
100 KVA
Minimum
Pole Class
6
6
5
5
4
4
3.e. Guying
The laws of physics (specifically statics) require that you guy some of your poles. Any time a
pole marks the end of an electric line, it must have a down guy to balance the forces on the pole.
The down guy stretches from near the top of the pole down to a metal rod that is driven in the
ground. This down guy generally stretches a distance of 20 feet (specifically, half of the pole
height) from the pole. If you, as the design engineer, forget to call for such a guy, the pole will
eventually fall down. Often times, you are able to attach a single down guy to a pole. Other times,
however, your design is such that a down guy would lie outside the easement we are provided. In
these situations, you must run a line to another pole that can have a down guy attached to it.
There is another situation that requires a down guy. When a span makes an angle of more than 2o,
you need a down guy on the back of the pole to balance the forces on the pole. In general, the down
guy should "split the angle" of the conductor. Figure 2 shows a schematic of a typical down guy.
Figure 2 - Typical down guy.
Line Angles: The pole hardware utilized to attach conductors to the pole has a maximum line
angle offset limitation. This limit should not be exceeded and is identified below:
POLE HARDWARE
Single Phase Primary Tangent
Single Phase Primary Suspension
Single Phase Primary Deadend
Secondary Spool Tangent
Secondary Rack Tangent
Secondary Deadend
MAXIMUM LINE
ANGLE
0 - 30 degrees
30 - 60 degrees
60+ degrees
0 - 2 degrees
0 - 60 degrees
60+ degrees
3.f. Easement
Alabama Power Company, like all power companies, obtains an easement which allows them to
place and maintain facilities on a customer's property. This easement can vary dramatically from
place to place, from company to company. For the purposes of this design problem, you may
assume an easement that will reach 15 feet off the right-of-way (the edge of the lot). Note any
locations that require additional easement.
4. Costs
Alabama Power Company has standard costs for all of its property. Table 2 (shown on this page
and the next) is a cost sheet you are to use for the various material (labor costs are included) you
may need for this design. You are to produce a cost efficient design.
Table 2- Overtime Meadows Material Cost Data
POLE DESCRIPTION
35 FOOT CLASS 5 CCA POLE
40 FOOT CLASS 5 CCA POLE WITH GROUND
40 FOOT CLASS 4 CCA POLE WITH GROUND
45 FOOT CLASS 5 CCA POLE WITH GROUND
45 FOOT CLASS 4 CCA POLE WITH GROUND
COST ($)
250
403
442
457
485
POLE HARDWARE
SINGLE PHASE PRIMARY TANGENT
SINGLE PHASE PRIMARY SUSPENSION
SINGLE PHASE PRIMARY DEADEND
SECONDARY SPOOL TANGENT
SECONDARY RACK TANGENT
PRIMARY GUY
SECONDARY GUY
STUB GUY
COST ($)
79
60
57
38
40
218
189
104
TRANSFORMER DESCRIPTION
15 KVA 7200/120/240 VOLT CONVENTIONAL
25 KVA 7200/120/240 VOLT CONVENTIONAL
37.5KVA 7200/120/240 VOLT CONVENTIONAL
50 KVA 7200/120/240 VOLT CONVENTIONAL
75 KVA 7200/120/240 VOLT CONVENTIONAL
100 KVA 7200/120/240 VOLT CONVENTIONAL
COST ($)
857
965
1130
1328
1884
2406
CONDUCTOR DESCRIPTION
#2 ACSR PRIMARY PER 1000 FT.
#1/0 AAC TRIPLEX SECONDARY PER 1000 FT.
COST ($)
699
1630
5. Helpful Hints

Produce a design that is cost effective.

Make sure that you do not stretch secondary conductors so far that flicker/load drop
requirements are not met.

Be careful not to undersize loads.

Do not overload transformers.

Do not run a conductor over a customer's property on which you do not have an easement. In
accordance with Murphy's law, it is this conductor that will have to be replaced, and we will
have to pay the customer to service the failed equipment.

Remember to put in down guys.
6. Example
Consider the sample development shown in Figure 1. Here, the engineer does not have to make
too many decisions because the new development is small and the street is straight. Figure 3 shows
one possible power distribution system for the development. The following ideas are considered:
1. Sizing the loads (and looking at possible future homes) dictates the use of three 25 KVA
transformers.
2. The placement of the poles dictates the use of four down guys.
3. The standard conductor is #2 ACSR. The #1/0 triplex is satisfactory for the secondary
conductors since the homes are so close to the right-of-way.
4. Since the street is straight, there is no need to guy for angles in the line.
Figure 3 - Possible design
7. The Development
You will be designing an electric distribution system for Overtime Meadows. This exciting new
development is on the outskirts of beautiful Brookwood just in Tuscaloosa County. The
development will be home to as many as 160 new Alabamians who will work in the new Mercedes
Benz plant off of Interstate 59. This new development will include 40 homes, some of which are all
electric and some of which are gas. Figure 4 shows the overall characteristics of the development.
Alabama Power Company is excited about Overtime Meadows because it feels that it is simply one
of the first of a number of developments that will be built in the Brookwood area. Alabama Power
Company is committed to providing its customers in Overtime Meadows with an efficient, aesthetic
electric distribution system.
Figure 4 - Layout for the final design
8. Deliverables
Each team is responsible for the following deliverables:
1. A distribution system design.
2. A report summarizing the design project. As a minimum, your report should include
discussions of the following points:
i. What is the problem?
ii. What is your design? Include figures showing the layout and cost of your
distribution system.
iii. What are the advantages and disadvantages of your design compared to
alternatives?
iv. What factors strongly influenced your design?
v. What was your design procedure? (Briefly mention important assumptions,
significant analyses, major decisions, and important results.)
3.
An oral presentation in which you will describe your distribution system.
General Project Notes
Professional engineers are paid to solve this problem every day across the country. Bill
Burroughs, P.E., a supervisory engineer at Alabama Power, was consulted to ensure that the
project’s technical data is accurate. There are two particularly interesting aspects of this project:
(1) It will be possible to have an engineer from Alabama Power come and present information to
assist the students. (2) This is an actual subdivision in Tuscaloosa County. Thus upon
completion of the project, the students may drive out to the subdivision to see how the project
was actually engineered. It should be noted that, in many power companies women engineers
often work in the area of power distribution.
Notes to the Instructor on
Design of a Power Distribution System
1. Background
Given the layout of a new subdivision, the teams are to design a system to distribute power to
each residence. There are numerous guidelines given in the problem statement. The teams
should try to lay out their system in a cost efficient manner, minimizing the number of
components, yet still servicing the needs of the customers.
2. Guidelines
A blue print size copy of the subdivision drawing in Figure 4 can be made available for each
team.
There are 8 poles in the sample solution which are considered Single Phase Primary Deadends.
When determining the type of hardware necessary for each pole, this designation is given to any
line angle change greater than 60 degrees. For instance,
2 lines at a 90
degree turn
3 lines into a
transformer
These particular examples will also require guy lines. The Single Phase Primary Deadend
designation indicates the type and cost of pole hardware.
The teams must decide on the various types of homes that may be in the subdivision. The ruleof-thumb is to assume that half of the homes will be gas homes and the others will be electric.
The teams will need to determine the layout of the various types of homes.
Another point worth noting is that the 2126.03 feet of # 1/0 triplex line drops are the line drops
made to each residence. There are 40 lots in this Subdivision, and an average lot (front side) is
around 80 feet. Using this average and the 35 foot setback requirement, an approximation was
found for the length of # 1/0 triplex needed to service the residences.
53.15’
35’
80/2=40’
3. Sample Solution
The blue print of one proposed distribution system for Overtime Meadows is available from
the report authors. The equipment costs for this solution are given below.
15 25 KVA transformers @ $917
21 Class 5 Poles @ $371
2126.03 feet of #1/0 triplex Secondary line drops @ $1882/1000 feet
485 feet #1/0 triplex Secondary @ $1882/1000 feet
15 Primary Guy Lines Hardware @ $129
9 Secondary Guy Lines Hardware @ $97
1950 feet primary conductor @ $738/1000 feet
Pole hardware
=
=
=
=
=
=
=
=
The total approximate cost for powering this subdivision is $32,577.06.
$13,755.00
$ 7791.00
$ 4001.19
$ 912.77
$ 1935.00
$ 873.00
$ 1439.00
$ 1870.00