$25. 002009 ieee 52 ieee

Will We Face Distribution 

Circuit Congestion?

The load of a customer or a group of customers on the distribu-

tion system constantly changes. Often, planners size and con-

fi gure distribution equipment based on statistical load surveys 

and historical load profi les, while taking the load  diversity 

into consideration. Data—such as average, maximum, and 

diversifi ed demand; maximum noncoincidental demand; load 

factor; and diversity factor—are used to design and confi gure 

Average Residential Load (Southern California)

0

0.5

1

1.5

2

2.5

3

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

Hours

kW Load

Shoulder

Summer

Winter

figure 4.

 Typical residential home load profile in Southern California with superimposed PHEV charging load.

Level I  

Level II

Level II

Level III

120V/15 A 

1.4 kW

120V/30 A

3.3 kW

208V/32 A

6.6 kW

208V/80 A 

16 kW

Sedan

40

16

5

8

3

Compact

150

35

4–5

22

8

4

Roadster

220

53

5–7

33

12

6

3

Single-Phase

Three-Phase

Battery 

Energy

kWh

Mile/kWh

Type

Driving

Range on

Batteries

(Miles) 

Time (Hours) for Full Charge

figure 3.

 Plug-in vehicle charging time.

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march/april 2009 

IEEE

 

power & energy magazine  

57

distribution circuits. Most distribution systems in the United 

States were designed decades ago based on the loading anal-

ysis performed at the time. Major changes in load levels and 

load patterns may require upgrades to the transformers and 

other equipment or changes to the switching confi guration 

shifting loads between transformers.  Furthermore, loading 

of a distribution feeder is inherently unbalanced because of 

the large number of unequal single-phase and double-phase 

loads that must be served. Unbalanced conditions, resulting 

from an increasing number of plug-in vehicles, could result 

in degradation of power quality, increased harmonics and 

voltage problems, and increasing line losses, and they also 

could potentially damage utility and customer equipment. In 

addition, signifi cant changes in load patterns can impact line 

voltages, especially over long feeders. 

On a typical distribution circuit, shown in Figure 5, most 

residential and commercial customers are served from radial 

feeders and secondary distribution networks. More than 80% 

of all distribution circuits within the United States are in the 

15-kV class voltage level; the primary voltages are 12.47, 13.2, 

or 13.8 kV. The sizing of such circuits varies greatly; however, 

under typical operating conditions, 4–6 MVA is representa-

tive of the peak loads on most 15-kV class feeders. These 

circuits typically have a main three-phase feeder with various 

three-phase and single-phase lateral branches. Also typically, 

the larger commercial or industrial loads are served from the 

main feeder and metered at the primary voltage. For most 

other customers, the primary voltage is stepped down with 

distribution transformers to the “secondary” or low-voltage 

level, 480Y/277 volts or 208Y/120 volts for the three-phase 

voltages serving most commercial  

buildings and 240/120 

volts for single-phase service, which serves most residential 

customers. Typical pole mount or overhead transformers are 

sized at 25 or 50 kVA for single-phase applications serving 

several single-family residences. Three-phase transformers 

are standardized at 75, 150, 300, or higher kVA levels.

Substation

138 kV

12.47 kV

21 MVA

138/12.47 

5 MVA

12.47 kV

50 kVA

12.47 kV/ 220 V

Single-Phase Residential Lateral – 1.5 MVA

50 kVA

12.47 kV/208Y/120

Three-Phase Residential Lateral – 1.5 MVA

Recloser

Three-Phase Commercial Lateral – 2 MVA

1 MVA

12.47 kV/4160/480 V

1.5 MVA

12.47 kV/4160/2.4 kV

Open Tie

120 A fuse

400-A Peak

600-A Emergency

Feeder Rating

Voltage Regulator

25 kVA

Secondary Distribution

Primary Feeder

Single-Phase Residential

Three-Phase Residential

Single-Phase Residential

Residential Circuit – Secondary Voltage

Commercial Customer – Primary Feeder Voltage

Distribution Congestion

6–9 p.m.

Distribution Congestion 8–11 a.m

Figures Courtesy of Google

figure 5.

 A typical radial distribution feeder—potential circuit congestion conditions.

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