UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Utilities Service

BULLETIN 1726C-115

SUBJECT: Checking Sag in a Conductor by the Return Wave Method

TO: RUS Electric Borrowers and RUS Electric Staff

EFFECTIVE DATE: Date of Approval

EXPIRATION DATE: Indefinite

OFFICE OF PRIMARY INTEREST: Electric Staff Division

PREVIOUS INSTRUCTIONS: This revised bulletin replaces RUS

Bulletin 1726C-115, “Checking Sag in a Conductor by the Return

Wave Method,” issued October 20, 1992.

FILING INSTRUCTIONS: Discard RUS Bulletin 1726C-115, dated

October 20, 1992, and replace with this revised issue. This

bulletin is available on the Internet via the RUS electric web

site: www.usda.gov/rus/electric.

PURPOSE: This guide publication presents a convenient and

practical method for checking sag in a conductor regardless of

span length, tension, size or type of the conductor.

/s/ BDS, Jr. 09/24/98

_____________________________ _____________________

 Assistant Administrator Date

 Electric Program
Bulletin 1726C-115

Page 2
Bulletin 1726C-115

Page 3

TABLE OF CONTENTS

1. Introduction 3

2. Discussion 3-4

ABBREVIATIONS

RUS Rural Utilities Service

INDEX:

 CONDUCTORS:

 Sag Checking by Return Wave Method
Bulletin 1726C-115

Page 4

1. INTRODUCTION

1.1 This bulletin describes a convenient method for checking sag

in a conductor. This method has been used extensively in

checking sag in conductors on RUS borrowers' systems.

2. DISCUSSION

2.1 The return wave method of checking the sag in a conductor is

applicable regardless of the span length, tension, size or type

of conductor. The time required for a wave initiated on a

conductor suspended in air between two fixed supports to traverse

between the supports is dependent on the amount of conductor sag.

A conductor wave originating at one support will travel to the

next support where it will be reflected back to the point of

origin where it will again be reflected back to the adjacent

support and the cycle repeats. This cyclic action continues

until the wave is eventually damped out.

2.2 The relationship between the time required for a conductor

wave to travel a number of times between supports and the

conductor’s sag is given by the equation:

2

2N

T

D 48.3 











= (1)

Where:

 D = conductor sag in inches

 T = time in seconds

 N = number of return waves counted

The Time-Sag Table on pages 5 & 6 was calculated from the above

equation.

To convert from inches to millimeters use Formula (2) as given by

the equation:

X = 25.4D (2)

Where:

 D = conductor sag in inches

 X = conductor sag in millimeters

2.3 A wave may be initiated in a conductor close to a support by

striking the conductor or by throwing a light, dry, nonmetallic
Bulletin 1726C-115

Page 5

rope or cord over the conductor and pulling the conductor down

strongly and quickly releasing the conductor (cord). The return

wave arrival may be felt by placing a finger on the conductor if

the striking method is used, or if a cord is used, the return

wave arrival may be felt in the cord by maintaining light tension

on the conductor.

2.4 For an accurate determination of sag, it is important that

the correct number of return waves be selected on the Time-Sag

Table. The table was prepared for sags corresponding to 3, 5,

10, and 15 return waves. The choice of the number of return

waves depends principally upon the span length and size of

conductor. For long spans and large conductors, the number of

return waves that can be accurately counted is less than for

short spans and small conductors. The largest number of return

waves should be used consistent with the conditions encountered

as this minimizes errors in recording time.

2.5 The wave initiating impulse is not counted as a return wave.

A stop watch is started when the wave initiating impulse is

applied and stopped immediately as the last of the number of

return waves of 3, 5, 10, or 15 as previously selected is

received.

Satisfactory results cannot be obtained when the conductor is in

motion as a result of other stimuli. Conductor movement, due to

wind or craftspeople working on the line, makes it difficult to

count the number of return waves. Additionally, satisfactory

results cannot be obtained if any objects, such as tree branches,

are in contact with the conductor in the span being checked.

2.6 A sufficient number of tests should be made in each span

until at least three equal readings are obtained.

Example:

A wave is initiated on a conductor span and a time of 14.1

seconds is measured upon receipt of the 10th return wave. From

formula (1) or from the Time Sag Table, it can be seen that the

conductor sag is 24 in. (610 mm).
Bulletin 1726C-115

Page 6

TIME SAG TABLE

Return of Wave Return of Wave

Sag

(Inches)

3rd

Time

(Sec.)

5th

Time

(Sec.)

10th

Time

(Sec.)

15th

Time

(Sec.)

Sag

(Inches)

3rd

Time

(Sec.)

5th

Time

(Sec.)

10th

Time

(Sec.)

15th

Time

(Sec.)

5 1.9 3.2 6.4 9.7 55 6.4 10.7 21.3 32.0

6 2.1 3.5 7.0 10.6 56 6.5 10.8 21.5 32.3

7 2.3 3.8 7.6 11.4 57 6.5 10.9 21.7 32.6

8 2.4 4.1 8.1 12.2 58 6.6 11.0 21.9 32.9

9 2.6 4.3 8.6 13.0 59 6.6 11.1 22.1 33.2

10 2.7 4.6 9.1 13.7 60 6.7 11.1 22.3 33.4

11 2.9 4.8 9.5 14.3 61 6.7 11.2 22.5 33.7

12 3.0 5.0 10.0 15.0 62 6.8 11.3 22.7 34.0

13 3.1 5.2 10.4 15.6 63 6.9 11.4 22.8 34.3

14 3.2 5.4 10.8 16.2 64 6.9 11.5 23.0 34.5

15 3.3 5.6 11.1 16.7 65 7.0 11.6 23.2 34.8

16 3.5 5.8 11.5 17.3 66 7.0 11.7 23.4 35.1

17 3.6 5.9 11.9 17.8 67 7.1 11.8 23.6 35.3

18 3.7 6.1 12.2 18.3 68 7.1 11.9 23.7 35.6

19 3.8 6.3 12.5 18.8 69 7.2 12.0 23.9 35.9

20 3.9 6.4 12.9 19.3 70 7.2 12.0 24.1 36.1

21 4.0 6.6 13.2 19.8 71 7.3 12.1 24.2 36.4

22 4.0 6.7 13.5 20.2 72 7.3 12.2 24.4 36.6

23 4.1 6.9 13.8 20.7 73 7.4 12.3 24.6 36.9

24 4.2 7.0 14.1 21.1 74 7.4 12.4 24.8 37.1

25 4.3 7.2 14.4 21.6 75 7.5 12.5 24.9 37.4

26 4.4 7.3 14.7 22.0 76 7.5 12.5 25.1 37.6

27 4.5 7.5 15.0 22.4 77 7.6 12.6 25.3 37.9

28 4.6 7.6 15.2 22.8 78 7.6 12.7 25.4 38.1

29 4.6 7.7 15.5 23.2 79 7.7 12.8 25.6 38.4

30 4.7 7.9 15.8 23.6 80 7.7 12.9 25.7 38.6

31 4.8 8.0 16.0 24.0 81 7.8 13.0 25.9 38.9

32 4.9 8.1 16.3 24.4 82 7.8 13.0 26.1 39.1

33 5.0 8.3 16.5 24.8 83 7.9 13.1 26.2 39.3

34 5.0 8.4 16.8 25.2 84 7.9 13.2 26.4 39.6

35 5.1 8.5 17.0 25.5 85 8.0 13.3 26.5 39.8

36 5.2 8.6 17.3 25.9 86 8.0 13.3 26.7 40.0

37 5.3 8.8 17.5 26.3 87 8.1 13.4 26.8 40.3

38 5.3 8.9 17.7 26.6 88 8.1 13.5 27.0 40.5

39 5.4 9.0 18.0 27.0 89 8.1 13.6 27.1 40.7

40 5.5 9.1 18.2 27.3 90 8.2 13.7 27.3 41.0

41 5.5 9.2 18.4 27.6 91 8.2 13.7 27.5 41.2

42 5.6 9.3 18.7 28.0 92 8.3 13.8 27.6 41.4

43 5.7 9.4 18.9 28.3 93 8.3 13.9 27.8 41.6

44 5.7 9.5 19.1 28.6 94 8.4 14.0 27.9 41.9

45 5.8 9.7 19.3 29.0 95 8.4 14.0 28.0 42.1

46 5.9 9.8 19.5 29.3 96 8.5 14.1 28.2 42.3

47 5.9 9.9 19.7 29.6 97 8.5 14.2 28.3 42.5

48 6.0 10.0 19.9 29.9 98 8.5 14.2 28.5 42.7

49 6.0 10.1 20.1 30.2 99 8.6 14.3 28.6 43.0

50 6.1 10.2 20.3 30.5 100 8.6 14.4 28.8 43.2

51 6.2 10.3 20.6 30.8 101 8.7 14.5 28.9 43.4

52 6.2 10.4 20.8 31.1 102 8.7 14.5 29.1 43.6

53 6.3 10.5 21.0 31.4 103 8.8 14.6 29.2 43.8

54 6.3 10.6 21.1 31.7 104 8.8 14.7 29.3 44.0
Bulletin 1726C-115

Page 7

TIME SAG TABLE (con’t.)

Return of Wave Return of Wave

Sag

(Inches)

3rd

Time

(Sec.)

5th

Time

(Sec.)

10th

Time

(Sec.)

15th

Time

(Sec.)

Sag

(Inches)

3rd

Time

(Sec.)

5th

Time

(Sec.)

10th

Time

(Sec.)

15th

Time

(Sec.)

105 8.8 14.7 29.5 44.2 155 10.7 17.9 35.8 53.7

106 8.9 14.8 29.6 44.4 156 10.8 18.0 35.9 53.9

107 8.9 14.9 29.8 44.7 157 10.8 18.0 36.1 54.1

108 9.0 15.0 29.9 44.9 158 10.9 18.1 36.2 54.3

109 9.0 15.0 30.0 45.1 159 10.9 18.1 36.3 54.4

110 9.1 15.1 30.2 45.3 160 10.9 18.2 36.4 54.6

111 9.1 15.2 30.3 45.5 161 11.0 18.3 36.5 54.8

112 9.1 15.2 30.5 45.7 162 11.0 18.3 36.6 54.9

113 9.2 15.3 30.6 45.9 163 11.0 18.4 36.7 55.1

114 9.2 15.4 30.7 46.1 164 11.1 18.4 36.9 55.3

115 9.3 15.4 30.9 46.3 165 11.1 18.5 37.0 55.4

116 9.3 15.5 31.0 46.5 166 11.1 18.5 37.1 55.6

117 9.3 15.6 31.1 46.7 167 11.2 18.6 37.2 55.8

118 9.4 15.6 31.3 46.9 168 11.2 18.7 37.3 56.0

119 9.4 15.7 31.4 47.1 169 11.2 18.7 37.4 56.1

120 9.5 15.8 31.5 47.3 170 11.3 18.8 37.5 56.3

121 9.5 15.8 31.7 47.5 171 11.3 18.8 37.6 56.4

122 9.5 15.9 31.8 47.7 172 11.3 18.9 37.7 56.6

123 9.6 16.0 31.9 47.9 173 11.4 18.9 37.9 56.8

124 9.6 16.0 32.0 48.1 174 11.4 19.0 38.0 56.9

125 9.7 16.1 32.2 48.3 175 11.4 19.0 38.1 57.1

126 9.7 16.2 32.3 48.5 176 11.5 19.1 38.2 57.3

127 9.7 16.2 32.4 48.6 177 11.5 19.1 38.3 57.4

128 9.8 16.3 32.6 48.8 178 11.5 19.2 38.4 57.6

129 9.8 16.3 32.7 49.0 179 11.6 19.3 38.5 57.8

130 9.8 16.4 32.8 49.2 180 11.6 19.3 38.6 57.9

131 9.9 16.5 32.9 49.4 181 11.6 19.4 38.7 58.1

132 9.9 16.5 33.1 49.6 182 11.6 19.4 38.8 58.2

133 10.0 16.6 33.2 49.8 183 11.7 19.5 38.9 58.4

134 10.0 16.7 33.3 50.0 184 11.7 19.5 39.0 58.6

135 10.0 16.7 33.4 50.2 185 11.7 19.6 39.1 58.7

136 10.1 16.8 33.6 50.3 186 11.8 19.6 39.2 58.9

137 10.1 16.8 33.7 50.5 187 11.8 19.7 39.4 59.0

138 10.1 16.9 33.8 50.7 188 11.8 19.7 39.5 59.2

139 10.2 17.0 33.9 50.9 189 11.9 19.8 39.6 59.3

140 10.2 17.0 34.1 51.1 190 11.9 19.8 39.7 59.5

141 10.3 17.1 34.2 51.3 191 11.9 19.9 39.8 59.7

142 10.3 17.1 34.3 51.4 192 12.0 19.9 39.9 59.8

143 10.3 17.2 34.4 51.6 193 12.0 20.0 40.0 60.0

144 10.4 17.3 34.5 51.8 194 12.0 20.0 40.1 60.1

145 10.4 17.3 34.7 52.0 195 12.1 20.1 40.2 60.3

146 10.4 17.4 34.8 52.2 196 12.1 20.1 40.3 60.4

147 10.5 17.4 34.9 52.3 197 12.1 20.2 40.4 60.6

148 10.5 17.5 35.0 52.5 198 12.1 20.2 40.5 60.7

149 10.5 17.6 35.1 52.7 199 12.2 20.3 40.6 60.9

150 10.6 17.6 35.2 52.9 200 12.2 20.3 40.7 61.0

151 10.6 17.7 35.4 53.0 201 12.2 20.4 40.8 61.2

152 10.6 17.7 35.5 53.2 202 12.3 20.5 40.9 61.4

153 10.7 17.8 35.6 53.4 203 12.3 20.5 41.0 61.5

154 10.7 17.9 35.7 53.6 204 12.3 20.6 41.1 61.7