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8/9/2019 Transformer Sizing Presentation.pdf
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2013 ElectriCitiesElectric Utility
Webinar Series
Sizing of
Transformers
Calculationof Loads
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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.
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Demand and Demand Factor
Demand Factor The Peak Demand of the whole
system compared to the connected Demand of thesystem.
Expressed as a percentage or a ratio less than 1.
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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
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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 thetransformer.
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.
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Residential Transformer Loading
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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
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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 1 2 3 4 5 6 7 8 9 10
Diversity Factor 100% 90% 75% 65% 63% 62% 61% 61% 61% 61%
Number of Customers 11 12 13 14 15 16 17 18 19 20
Diversity Factor 60% 59% 58% 58% 57% 57% 56% 54% 54% 54%
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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
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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 kVALoading 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
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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.
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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 thetransformer.
Apply the appropriate Coincidence Factor to the connected PeakDemand.
Use the calculated Demand to determine the size of the transformerbased on the Maximum kVA Loading.
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Residential Transformer Loading
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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
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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
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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
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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 4 Determine the Coincidence Factor for the number of customersfrom 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
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Residential Transformer Loading
Example Coincidence Factor Method
Step 6 The answer yields the peak 15-minute demand on thetransformer.
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
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Residential Transformer Loading
Example Coincidence Factor Method
Determine the Transformer Size based on the Maximum kVA LoadingTable.
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
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Residential Transformer Loading
Example Adding a New Customer
Step 1 - Determine the Additional Load on the Transformer based onthe 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
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Residential Transformer Loading
Example Adding a New Customer
Step 2 - Add kW for the New Customer to the Total Connected Load ofthe 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
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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
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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 4 Determine the Coincidence Factor for the number of customersfrom 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
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Residential Transformer Loading
Example Adding a New Customer
Step 6 The answer yields the peak 15-minute demand on thetransformer.
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
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Residential Transformer Loading
Square Footage Table Method
Very Useful in Sizing and Laying out Transformers in NewSubdivisions
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.
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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
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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.
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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.
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Residential Transformer Loading
Diversity Method - Useful when sizing transformers fornew customer load.
Can be used with a variety of sizes of homes located within thesame subdivision.
Useful when there is a mix of Total Electric and Gas Heathomes.
Square Footage Table Method - Useful when sizingtransformers 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 Heathomes.
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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.
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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.
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Commercial Transformer Loading
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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.
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Commercial Transformer Loading
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Commercial Transformer Loading
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Commercial Transformer Loading
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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
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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
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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 bearound 400 kW.
These calculations would then be used to determine the
Transformer Size.
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Commercial Transformer Loading
Application of Maximum kVA Loading on Three PhaseTransformers for Commercial Applications
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
% of Connected Load Method
Winter = 414 kW
Summer = 409 kW
Watts per Square Foot Method
Winter = 404 kW
Summer = 416 kW
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Commercial Transformer Loading
Determine the Transformer Size based on the Maximum kVALoading 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
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Commercial Transformer Loading
Another Method Engineering Loading Data
Use the supplied electrical loading data supplied by the buildingelectrical plans supplied by the buildings 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.
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Commercial Transformer Loading
Another Method Engineering Loading Data
TOTAL CONNECTED ELECTRICAL LOAD
DESCRIPTIONAMPS AT 277/480, 3 PHASE, 4 WIRE
KVAL-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
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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
I d t i l / L C i l
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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, Co-Op, Investor Owned, and other similar facilities on the same system)
I d t i l / L C i l
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Industrial / Large Commercial
Transformer Loading
I d t i l / L C i l
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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
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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
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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
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Delta Secondary
(High-Leg) Transformer SizingTwo-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 isGrounded.
Three-Transformer Banks
Three-Transformer Banks are well suited for Single Phase Loads
and Large Three Phase Loads.
Delta Secondary
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Delta Secondary
(High-Leg) Transformer Sizing
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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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Future Webinar Discussion
Future Webinar Topics
Future Webinar Frequency
Webinar Instructors
2013 El t iCiti
8/9/2019 Transformer Sizing Presentation.pdf
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2013 ElectriCities
Electric Utility
Webinar Series
Sizing of
Transformers
Calculationof Loads
Recommended