8/11/2019 Tension and Sag 07-25-06

1/7

Tension and Sag

The 2007 National Electrical Safety Code (NESC) addresses Tension and Sag in Sections

235. 251. 252. 253, 21, 23, and 277

Table of Contents:

!. "efinitions of Tension and Sag#. Engineering "esign "escri$tion

C. %ethods of Tensioning

". &ed'ced Tension Constr'ction

E. 'y Tension. *ine of Site

. Sag Charts+. *oading one-. rades of Constr'ction

. &eso'rces

A1. Definition of Tension:Tension is defined in the NESC as /The longit'dinal force

eerted on a cale d'ring installation. -n other ords tension can e e$lained as force$'lling the cales or ires at either end y hat they are attached to or the eight of the

cale itself. Tension is also a$$lied to ins'lators

There are to 4ain ty$es of tension

-nitial is defined in the NESC as / The longit'dinal tension in the cond'ctor $rior

to the a$$lication of any eternal load -n other ords the characteristics of the

cale efore ti4e, te4$erat're, eight etc. of the cond'ctor stretch or shrin theire

inal is defined in the NESC as /The longit'dinal tension in a cond'ctor after it

has een s'6ected for an a$$reciale $eriod of the loading $rescried for theloading district in hich it is sit'ated, or e'i8alent loading, and the loading

re4o8ed. -n other ords hat the characteristics of the cale are e$ected to e

after ti4e, te4$erat're, eight and other factors ha8e effected it.

A2. Definition of Sag :Sag is defined in the NESC as /The distance 4eas'red 8ertically

fro4 a cond'ctor to the straight line 6oining its to $oints of s'$$ort. -n other ords

sag is 's'ally the loest $oint on a cale or ire eteen to $oints. This lo $oint is4eas'red as tho'gh there ere a straight line eteen the attach4ent $oints on either end

and the distance eteen this lo $oint and the straight line is the sag.

B. Engineering Design:The a$$ro$riate sag and tension is deter4ined y

8/11/2019 Tension and Sag 07-25-06

2/7

S$an lengths

Strand si9e

*oad

Stor4 loading area

Te4$erat're

:ertical clearances ao8e grade

:ertical clearances fro4 other 'tilities

;ole lengths

Class of $ole

C. Methods of Tensioning:

Ty$es of Tensioning

"yna4o4eter are the $referred4ethod for testing tensioning

Electronic

%echanical

"yna4o4eter

A light rope tossed over the conductor near one end ofa span and give it a good hard jerk down. At the sameinstant press the button on the watch to start it. Youthen feel for return pulses in the rope as the shock waveyou created runs up and down the conductor. At theinstant you feel the third or fifth return you stop thewatch. Read the number on the appropriate scale on theface of the watch and you have your sag in feet. Thescales are direct reading and no math is needed.

Third a8e ret'rn

8/11/2019 Tension and Sag 07-25-06

3/7

The s4aller cales re$resent the

difference in sag afteren8iron4ental changes< notice ho

the sag eteen cond'ctors differs.

NESC clearances 4ay not e 4et

d'ring different te4$erat'res, ice,ind, etc. or o8er ti4e.

This 4ethod is not reco44ended

%atching sag

D. Reduced Tension Construction (Slack Span: &ed'ced tension is hen traditional

g'ying is not $ractical. This sho'ld e a8oided if $ossile. &ed'ced tension 's'ally

re'ires g'ying in the sa4e direction as the red'ced tension s$an. 'ying can ea8oided ith stronger $ole

'ying in sa4e direction =sing larger class $ole

"o'le Slac S$an to $ro8ide corner $oles and street clearance

E. !u" Tension: ;er NESC 21C2 (in lay4an>s ter4s) the g'y sho'ld e considered a

$art of the str'ct're and designed and installed ith the $ro$er tension to s'$$ort the

tension of the attach4ents it s'$$orts. The g'y can create an i4alance in tension if

installed to tight. The note for this section also clarifies that g'ys 4'st not e loose.

8/11/2019 Tension and Sag 07-25-06

4/7

#. $ine of Site: ;er NESC 235C3 (in lay4an>s ter4s) $ri4ary $oer cales cannot sag

elo the attach4ent $oints of the highest co44'nication cale in s$ans o8er 150 feet.

Secondary, ier etc is ?@ ;ri4ary elo line of site< :iolation

!. Sag Charts: Sag Charts are 'sed y 4ost aerial 'tilities in one for4 or another to

deter4ine ho 4'ch tension to 'se to $'ll the ire '$ to the a$$ro$riate sag. They co4ein 4any for4s ased on a 8ariety of for4'las. So4e are co44ercially $ro8ided lie

those 4entioned in &eso'rces, so4e are created in ho'se. They all consist of so4e asic

infor4ationA

Bire "ia4eter

S$an *ength

Bire Beight

S'$$orting Cale Characteristics

Te4$erat're

&ated #reaing Strength

Tension

8/11/2019 Tension and Sag 07-25-06

5/7

%. $oading &one::arying en8iron4ental conditions create ha9ards that effect aerial

cales differently. Bhen looing at the =nited States %a$ the loading 9one a$$ears that

all of ?regon has a /%edi'4 loading 9one. S$ecial Bind &egions change the standard/%edi'4 *oading one to /Etre4e loading. There are 3 ty$es of loading 9ones

+ea8y, %edi'4, *ight and Etre4e.

?regon a$$ears to e only %edi'4

.

Notice the S$ecial Bind &egions

Nor4ally ?regon can e$ect D5 4ile inds< hoe8er in /Etre4e areas those inds

4ay increase to 120 4iles or 4ore. The *oading one or district deter4ine so4e of the

o8erload factors 'sed for engineering.

8/11/2019 Tension and Sag 07-25-06

6/7

%. $oading &one (cont.)

%ea'"*oading "istricts are generally in the central and northeast =.S. states.

There is an ass'4$tion of loer te4$erat'res and greater ice 'ild'$ on cales

and cond'ctors. This 4ay re'ire ad6'st4ents 4ade s'ch as o$en

8/11/2019 Tension and Sag 07-25-06

7/7

8oltage of the s'$$ly cond'ctor ill not e de