2013 ElectriCities
Electric Utility
Webinar Series
Sizing of
Transformers
Calculation
of Loads
Demand and Demand Factor
• Demand – The value of electrical power required for a
particular load.
– Expressed in kW.
– Usually averaged over a 15-minute or 30-minute period for billing
purposes.
• Peak Demand – The maximum Demand of the whole
system at any one time.
Demand and Demand Factor
• Demand Factor – The Peak Demand of the whole
system compared to the connected Demand of the
system.
– Expressed as a percentage or a ratio less than 1.
Demand and Demand Factor
• Example - Average Home
– Typical Electrical Loads:
•
•
•
•
•
•
Water Heater – 4,500 watts (4.5 kW)
Range / Oven – 8,000 watts (8.0 kW)
Central Air Conditioner – 6,000 watts (6.0 kW)
Clothes Dryer – 5,000 watts (5.0 kW)
Dishwasher – 2,000 watts (2.0 kW)
Lighting, Fans, Appliances, Other – 7,500 watts (7.5 kW)
– Connected Demand = 33 kW
– Peak Demand = 18 kW
– Demand Factor = 18 kW / 33 kW = 0.545
Residential Transformer Loading
Diversity Method
•
Information Required
– Average Square Footage of the Homes
– Number of Homes with Gas and Electric Heat
•
Information from Tables
– Peak kW Demand for the size of the home.
– Add all of the Peak Demands for the number of homes connected to the
transformer.
– Apply the appropriate Diversity Factor to the connected Peak Demand
for the number of homes.
– Use the calculated Demand to determine the size of the transformer
based on the Maximum kVA Loading.
Residential Transformer Loading
Residential Transformer Loading
•
Example – Diversity Method
– 5 Homes - All Electric; between 1,500 and 1,800 square feet.
– Step 1 – Determine the Connected Peak kW for each home.
Electric
Summer
Electric
Winter
Gas
Summer
Gas
Winter
kW for 1,200 s.f.
13
15
8
8
kW for 1,500 s.f.
15
18
10
10
kW for 1,800 s.f.
16
20
11
11
kW for 2,400 s.f.
18
21
12
11
kW for 3,000 s.f.
21
26
14
11
Residential Transformer Loading
•
Example – Diversity Method
– Peak kW per Home = 20 kW winter; 16 kW summer
– Step 2 – Add the Peak Demands for all of the homes.
• 20 kW x 5 homes = 100 kW Winter Peak Demand
• 16 kW x 5 homes = 80 kW Summer Peak Demand
– Step 3 – Apply the Diversity Factor to the Connected Peak Demand.
Number of Customers
Diversity Factor
Number of Customers
Diversity Factor
1
2
3
4
5
6
7
8
9
10
100%
90%
75%
65%
63%
62%
61%
61%
61%
61%
11
12
13
14
15
16
17
18
19
20
60%
59%
58%
58%
57%
57%
56%
54%
54%
54%
Residential Transformer Loading
•
Example – Diversity Method
– Diversity Factor for 5 homes = 63%
– Step 4 – Calculate the Peak Load on the Transformer.
• 100 kW x 63% = 63 kW; 80 kW x 63% = 51 kW
– Step 5 – Determine the Transformer Size based on the Maximum kVA
Loading Table.
Transformer
Size
Summer
140%
Winter
160%
Transformer
Size
Summer
140%
Winter
160%
10
14
16
50
70
80
15
21
24
75
105
120
25
35
40
100
140
160
37.5
53
60
167
234
267
Residential Transformer Loading
•
Example – Diversity Method
– Step 4 – Calculate the Peak Load on the Transformer.
• Winter Demand = 100 kW x 63% = 63 kW
• Summer Demand = 80 kW x 63% = 51 kW
– Step 5 – Determine the Transformer Size based on the Maximum kVA
Loading Table.
• For a 37.5 kVA Transformer:
– Winter 160% Loading = 60 kW
– Summer 140% Loading = 53 kW
• For a 50 kVA Transformer:
– Winter 160% Loading = 80 kW
– Summer 140% Loading = 70 kW
Demand and Coincidence Factor
• Coincidence Factor – A ratio of the average running kW
load to the connected kW load on a utility transformer
based on the number of customers connected to the
transformer.
– Expressed as a percentage or a ratio less than 1.
– Usually will decrease as the number of connected customers
increases.
– Applied by the use of tables.
Residential Transformer Loading
Coincidence Factor Method
•
Information Required
– Highest Summer kWh and Winter kWh Usage (from billing records).
– Number of Homes Connected to the Transformer.
•
Information from Tables
– Determine the kW Demand for the home based on kWh usage.
– Add all of the Peak Demands for the customers connected to the
transformer.
– Apply the appropriate Coincidence Factor to the connected Peak
Demand.
– Use the calculated Demand to determine the size of the transformer
based on the Maximum kVA Loading.
Residential Transformer Loading
Residential Transformer Loading
•
Example – Coincidence Factor Method
– 3 Homes
– Step 1 – Determine the Summer and Winter Peak kWh Usage for each
home (listed in table below)
Summer kWh
Winter kWh
Customer # 1
2,224
853
Customer # 2
2,734
1,274
Customer # 3
1,849
1,283
Residential Transformer Loading
•
Example – Coincidence Factor Method
– Step 2 – Determine the kW Demand for each home.
– Step 3 – Add the kW Demands.
Summer
Winter
kWh
kW Demand
kWh
kW Demand
Customer #1
2,224
11.57
853
7.47
Customer #2
2,734
13.16
1,274
9.97
Customer #3
1,849
10.29
1,283
10.27
Totals
35.0
27.7
Residential Transformer Loading
•
Example – Coincidence Factor Method
– Step 3 – Add the kW Demands.
• Summer Demand = 35.0 kW
• Winter Demand = 27.7 kW
– Step 4 – Determine the Coincidence Factor for the number of customers
from the table.
Number of
Customers
Coincidence
Factor
1
1.00
2
0.85
3
0.74
4
0.66
5
0.61
Residential Transformer Loading
•
Example – Coincidence Factor Method
– Step 4 – Determine the Coincidence Factor for the number of customers
from the table.
• Coincidence Factor = 0.74
– Step 5 – Multiply the total kW Demand by the Coincidence Factor for
Summer and Winter loads.
• Summer Transformer Demand = 35.0 kW x 0.74 = 25.9 kW
• Winter Transformer Demand = 27.7 kW x 0.74 = 20.5 kW
Summer
Winter
Coincidence
Factor
Total kW
Demand
Transformer
kW Demand
Coincidence
Factor
Total kW
Demand
Transformer
kW Demand
0.74
35.0 kW
25.9 kW
0.74
27.7 kW
20.5 kW
Residential Transformer Loading
•
Example – Coincidence Factor Method
– Step 6 – The answer yields the peak 15-minute demand on the
transformer.
• Summer Transformer Demand = 35.0 kW x 0.74 = 25.9 kW
• Winter Transformer Demand = 27.7 kW x 0.74 = 20.5 kW
• Apply the two kW demands in step 5 to the Maximum kVA Loading table for
transformers.
Transformer
Size
Summer
140%
Winter
160%
Transformer
Size
Summer
140%
Winter
160%
10
14
16
50
70
80
15
21
24
75
105
120
25
35
40
100
140
160
37.5
53
60
167
234
267
Residential Transformer Loading
•
Example – Coincidence Factor Method
– Determine the Transformer Size based on the Maximum kVA Loading
Table.
• Summer Transformer Demand = 25.9 kW
• Winter Transformer Demand = 20.5 kW
• For a 15 kVA Transformer:
– Summer 140% Loading = 21 kW
– Winter 160% Loading = 24 kW
• For a 25 kVA Transformer:
– Summer 140% Loading = 35 kW
– Winter 160% Loading = 40 kW
Residential Transformer Loading
•
Example – Adding a New Customer
– Step 1 - Determine the Additional Load on the Transformer based on
the Diversity Method Tables.
• New 2,200 square foot Home, Total Electric.
– Summer Peak = 18 kW
– Winter Peak = 21 kW
– Step 2 - Add kW for the New Customer to the Total Connected Load of
the Existing Customers
Residential Transformer Loading
•
Example – Adding a New Customer
– Step 2 - Add kW for the New Customer to the Total Connected Load of
the Existing Customers
Summer
Winter
kWh
kW Demand
kWh
kW Demand
Customer #1
2,224
11.57
853
7.47
Customer #2
2,734
13.16
1,274
9.97
Customer #3
1,849
10.29
1,283
10.27
New Customer
---
18
---
21
Totals
53.0
48.7
Residential Transformer Loading
•
Example – Adding a New Customer
– Step 3 – Add the kW Demands.
• Summer Demand = 53.0 kW
• Winter Demand = 48.7 kW
– Step 4 – Determine the Coincidence Factor for the number of customers
from the table.
Number of
Customers
Coincidence
Factor
1
1.00
2
0.85
3
0.74
4
0.66
5
0.61
Residential Transformer Loading
•
Example – Adding a New Customer
– Step 4 – Determine the Coincidence Factor for the number of customers
from the table.
• Coincidence Factor = 0.66
– Step 5 – Multiply the total kW Demand by the Coincidence Factor for
Summer and Winter loads.
• Summer Transformer Demand = 53.0 kW x 0.66 = 35.0 kW
• Winter Transformer Demand = 48.7 kW x 0.66 = 32.1 kW
Summer
Winter
Coincidence
Factor
Total kW
Demand
Transformer
kW Demand
Coincidence
Factor
Total kW
Demand
Transformer
kW Demand
0.66
53.0 kW
35.0 kW
0.66
48.7 kW
32.1 kW
Residential Transformer Loading
•
Example – Adding a New Customer
– Step 6 – The answer yields the peak 15-minute demand on the
transformer.
• Summer Transformer Demand = 53.0 kW x 0.66 = 35.0 kW
• Winter Transformer Demand = 48.7 kW x 0.66 = 32.0 kW
• Apply the two kW demands in step 5 to the Maximum kVA Loading table for
transformers.
Transformer
Size
Summer
140%
Winter
160%
Transformer
Size
Summer
140%
Winter
160%
10
14
16
50
70
80
15
21
24
75
105
120
25
35
40
100
140
160
37.5
53
60
167
234
267
Residential Transformer Loading
Square Footage Table Method
•
Very Useful in Sizing and Laying out Transformers in New
Subdivisions
•
Information Required
– Average Square Footage of Homes in the Section of the Subdivision
– Number of Homes that are Total Electric and that have Gas Heat.
– Number of Homes proposed to connect to each transformer location.
Residential Transformer Loading
Square Footage Table Method
•
Information from Tables
– The Number of Total Electric Customers.
– The Number of Gas Heat Customers
– The Number of Homes that will be connected at the Transformer
Location.
– Use the Table to Determine the Transformer Size for the particular
Location
Residential Transformer Loading
•
Example – Square Footage Table Method
– 3 Homes - Gas Heat; between 1,500 and 1,800 square feet.
– 4 Homes - Total Electric; between 1,500 and 1,800 square feet.
Residential Transformer Loading
•
Example – Square Footage Table Method
– 3 Homes; Gas Heat; go down from “3” Gas Customers
– 4 Homes; Total Electric; go right from “4” Electric Customers
– Table recommends 50 kVA transformer.
Residential Transformer Loading
Diversity Method - Useful when sizing transformers for
new customer load.
– Can be used with a variety of sizes of homes located within the
same subdivision.
– Useful when there is a mix of Total Electric and Gas Heat
homes.
Square Footage Table Method - Useful when sizing
transformers for new customer load.
– Can only be used when the sizes of homes to be served from
the service transformer are relatively the same size.
– Useful when there is a mix of Total Electric and Gas Heat
homes.
Residential Transformer Loading
Coincidence Factor Method - Useful when sizing
transformers for existing loads or adding new load to
existing loads.
– Can be used with only available customer billing information.
– Easily accommodates the addition of new residential loads to
existing service transformer locations.
Commercial Transformer Loading
Watts per Square Foot Method
•
Information Required
– Type of Business or Institution.
– Total square footage of the facility.
•
Information from Tables
– By using the Average Watts per Square Foot tables the average kW
Demand of the facility can be estimated.
Commercial Transformer Loading
Commercial Transformer Loading
% of Connected Load Method
•
Information Required
– Type of Business or Institution.
– Total connected electrical load of the facility.
•
Information from Tables
– By using the % of Connected Load tables the average kW Demand of
the facility can be estimated.
Commercial Transformer Loading
Commercial Transformer Loading
Commercial Transformer Loading
Commercial Transformer Loading
• Example – Watts per Square Foot Method
– Grocery Store
• Winter = 10.1 watts per square foot
– For structures with electric heat.
• Summer = 10.4 watts per square foot
• Size of the Facility = 40,000 square feet
• Average Demand:
– Winter = 40,000 x 10.1 = 404,000 watts = 404 kW
– Summer = 40,000 x 10.4 = 416,000 watts = 416 kW
Commercial Transformer Loading
• Example – % of Connected Load Method
– Grocery Store
• Total Connected Load = 920 kW
• Of the total 920 kW, 250 kW is Winter Heating.
• Average Demand:
– Winter = 920 kW x 45% = 414 kW
– Summer = (920 kW – 250 kW) = 670 kW x 61% = 409 kW
Commercial Transformer Loading
• Comparison of Two Methods
– Grocery Store
• Watts per Square Foot Method
– Winter = 404 kW
– Summer = 416 kW
• % of Connected Load Method
– Winter = 414 kW
– Summer = 409 kW
• The calculated Peak kW Demand for the Grocery Store should be
around 400 kW.
– These calculations would then be used to determine the
Transformer Size.
Commercial Transformer Loading
• Application of Maximum kVA Loading on Three Phase
Transformers for Commercial Applications
Watts per Square Foot Method
Winter = 404 kW
Summer = 416 kW
% of Connected Load Method
Winter = 414 kW
Summer = 409 kW
Transformer
Size
Summer
120%
Winter
140%
Transformer
Size
Summer
120%
Winter
140%
150
180
190
750
900
1,050
225
270
315
1,000
1,200
1,400
300
360
420
1,500
1,800
2,100
500
600
700
2,500
3,000
3,500
Commercial Transformer Loading
Determine the Transformer Size based on the Maximum kVA
Loading Table.
– Maximum Average Transformer Demand
• Summer = 416 kW
• Winter = 414 kW
– For a 225 kVA Transformer:
• Summer 120% Loading = 270 kW
• Winter 140% Loading = 315 kW
– For a 300 kVA Transformer:
• Summer 120% Loading = 360 kW
• Winter 140% Loading = 420 kW
– For a 500 kVA Transformer:
• Summer 120% Loading = 600 kW
• Winter 140% Loading = 700 kW
Commercial Transformer Loading
• Another Method – Engineering Loading Data
– Use the supplied electrical loading data supplied by the building
electrical plans supplied by the building’s engineer.
• Information will include total connected and diversified load
information.
• This information and calculations used in the engineers
determination are based on National Electric Code load tables, not
on Utility practices and history.
• Be Cautious – Check the calculations for yourself to satisfy yourself
that you are installing the correct size transformer so serve the load.
Commercial Transformer Loading
• Another Method – Engineering Loading Data
TOTAL CONNECTED ELECTRICAL LOAD
DESCRIPTION
AMPS AT 277/480, 3 PHASE, 4 WIRE
KVA
L-1
L-2
L-3
INTERIOR LIGHTS
4.0
8.0
4.0
4.0
EXTERNAL LIGHTS
4.0
-
-
1.0
RECEPTACLES
16.0
16.0
12.0
12.0
AHU FAN & UNIT HEATER FANS
2.0
2.0
4.0
2.0
EXHAUST FANS
6.0
1.0
2.0
3.0
ELECTRIC HEAT
-
17.0
17.0
10.0
* COOLING
9.0
-
9.0
5.0 *
WATER HEATERS
10.0
10.0
-
5.0
SHOP EQUIPMENT
7.0
7.0
7.0
6.0
MISCELLANEOUS
2.0
-
2.0
1.0
TOTAL
51.0
61.0
48.0
44.0
* NOT CALCULATED IN TOTAL
Commercial Transformer Loading
• Another Method – Engineering Loading Data
– School Maintenance Building
– 44 kVA Connected Load
– 3,000 Square Foot Facility
– Key Electrical Components Derated for Average Demand:
•
•
•
•
•
•
Receptacles: 12 kVA – derate to 5% = 0.6 kVA
Lighting: 5 kVA – derate to 85% = 4.25 kVA
Water Heating: 5 kVA – derate to 25% = 1.25 kVA
Fans: 5 kVA – derate to 40% = 2 kVA
Heating: 10 kVA – derate to 80% = 8 kVA
Shop Equipment & Misc: 7 kVA – derate to 35% = 2.45 kVA
– Average Demand = 18.55 kVA
Industrial / Large Commercial
Transformer Loading
• Engineered Loading Data
– Panel Schedules
– Actual Diversified Load Tables
• Watts per Square Foot Tables
• Usage History from Other Electric Utilities (Other Cities, CoOp, Investor Owned, and other similar facilities on the same system)
Industrial / Large Commercial
Transformer Loading
Industrial / Large Commercial
Transformer Loading
• Areas of Consideration in Sizing Transformers for High
Load Factor Customer
– High load factor customers use a large amount of energy (kWh)
for extended periods of time.
– 24-hours per day, 7-days per week department store is high load
factor usage as compared 10-hour per day, 6-days per week
local merchant which has a lower load factor.
Industrial / Large Commercial
Transformer Loading
• Areas of Consideration in Sizing Transformers for High
Load Factor Customer (continued)
– Due to the extended energy usage service transformers do not
have a chance to cool down from operating at their nameplate
rating.
– Normal distribution class transformers are not designed to
operate at their nameplate rating for extended periods of time.
Industrial / Large Commercial
Transformer Loading
• Areas of Consideration in Sizing Transformers for High
Load Factor Customer (continued)
– To account for the high load factor usage transformers to serve
this class of customer need to be oversized by 20% to 30% or
specified as a substation class transformer.
– Substation class transformers can operate at their nameplate
rating for extended periods without excess heating of the core.
Delta Secondary
(High-Leg) Transformer Sizing
Two-Transformer vs. Three-Transformer Bank Sizing
– Two-Transformer Banks
• Two-Transformers Banks work well for Single Phase Loads and
relatively small Three Phase Loads.
• Two-Transformer Banks are more stable than Three-Transformer
Banks because the Neutral on the Primary Side of the bank is
Grounded.
– Three-Transformer Banks
• Three-Transformer Banks are well suited for Single Phase Loads
and Large Three Phase Loads.
Delta Secondary
(High-Leg) Transformer Sizing
Secondary Conductor Sizing
Example – 4/0 Aluminum Service Drop; 150 feet in length; 25 kVA of
load. kVA x Feet = 3,750
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2013 ElectriCities
Electric Utility
Webinar Series
Sizing of
Transformers
Calculation
of Loads