Electric Power Generation, Transmission, and Distribution

.In regions of heavy ice loads, the maximum sags and the permanent increase in sag with time
(difference between initial and final sags) may be due to ice loadings.
Ice loads for use in designing lines are normally derived on the basis of past experience, code
requirements, state regulations, and analysis of historical weather data. Mean recurrence intervals for
heavy ice loadings are a function of local conditions along various routings. The impact of varying
assumptions concerning ice loading can be investigated with line design software.

TABLE 14.1 Definitions of Ice and Wind Load for NESC Loading Areas

Loading Districts

Heavy Medium Light Extreme Wind Loading

Radial thickness of ice

(in.) 0.50 0.25 0 0

(mm) 12.5 6.5 0 0

Horizontal wind pressure

(lb=ft^2 ) 4 4 9 See Fig. 14.4

(Pa) 190 190 430

Temperature

( 8 F) 0 þ 15 þ 30 þ 60

( 8 C)  20  10  1 þ 15

Constant to be added to the

resultant for all conductors

(lb=ft) 0.30 0.20 0.05 0.0

(N=m) 4.40 2.50 0.70 0.0

NOTES: BASIC WIND SPEED 70 MPH

GULF OF MEXICO

90 SPECIAL WIND REGION

90

(^8080)
70
(^7070)
80
80
70
70 70
Tacoma
Cheyenne
Lincoln
Des Moines
Rapid City
Billings
Bismarck Fargo Duluth
Minneapolis
Davenport
Chicago
Kansas City
Columbus
Detroit
Lansing Buffalo
Pittsburgh
Richmond
Knoxville
Birmingham
Shreveport
Little Rock
St. Louis
JacksonJackson
Atlanta
Raleigh
Norfolk
Columbia
Tampa
Miami
New Orleans
Phoenix Amarillo
PACIFIC OCEAN
ATLANTIC OCEAN
80
(^8080)
80 100 110
110
110
110110
100
(^8090)
70
100
110
ALASKA0 50 100
110
110
90
(^7080)
70
70
70
90
90
90
100
0 100200
SCALE 1: 20,000,000
300 400500 MILES

1. VALUES ARE FASTEST-MILE SPEEDS AT 33 FT (10 M) ABOVE GROUND FOR EXPOSURE CATEGORY C AND ARE ASSOCIATED WITH AN ANNUAL PROBABILITY OF 0.02.


2. LINEAR INTERPOLATION BETWEEN WIND SPEED CONTOURS IS ACCEPTABLE.3. CAUTION IN THE USE OF WIND SPEED CONTOURS IN MOUNTAINOUS REGIONS OF
   ALASKA IS ADVISED.

110

Seattle

Salt Lake City

Salem

Denver

Las Vegas

San Diego

San Francisco

Fresno

Los Angeles

(^9080)
70
Albuquerque
Fort Worth
Oklahoma City
Dodge City
FIGURE 14.4 Wind pressure design values in the United States. Maximum recorded wind speed in miles/hour.
(From Overend, P.R. and Smith, S.,Impulse Time Method of Sag Measurement, American Society of Civil Engineers.
With permission.)