Electric Power Generation, Transmission, and Distribution

elongation will reduce along a curve parallel to the final curve, but the conductor will never return to its
original length.
For example, refer to Fig. 14.8 and assume that a newly strung 795 kcmil-37 strand AAC ‘‘Arbutus’’
conductor has an everyday tension of 2780 lb. The conductor area is 0.6245 in.^2 , so the everyday stress is
4450 psi and the elongation is 0.062%. Following an extremely heavy ice and wind load event, assume
that the conductor stress reaches 18,000 psi. When the conductor tension decreases back to everyday
levels, the conductor elongation will be permanently increased by more than 0.2%. Also the sag under
everyday conditions will be correspondingly higher, and the tension will be less. In most numerical sag-
tension methods, final sag-tensions are calculated for such permanent elongation due to heavy loading
conditions.

14.3.1.3 Permanent Elongation at Everyday Tensions (Creep Elongation)

Conductors permanently elongate under tension even if the tension level never exceeds everyday levels.
This permanent elongation caused by everyday tension levels is called creep (Aluminum Company of
America, 1961). Creep can be determined by long-term laboratory creep tests, the results of which are
used to generate creep curves. On stress-strain graphs, creep curves are usually shown for 6-mo, 1-yr, and
10-yr periods. Figure 14.8 shows these typical creep curves for a 37 strand 250.0 through 1033.5 kcmil
AAC. In Fig. 14.8 assume that the conductor tension remains constant at the initial stress of 4450 psi. At
the intersection of this stress level and the initial elongation curve, 6-month, 1-year, and 10-year creep

35,000

30,000

25,000

20,000

15,000

10,000

15,000

0

.1 .2

Unit Strain, %

Stress, psi

6 Month Creep

1 Year Creep10 Year Creep

Initial Aluminum

Final Aluminum

Equations for Curves (X = unit strain in %; Y = stress in psi):

Test Temperature 70 8 F to 75 8 F

X = −5.31 × 10 −^3 + (1.74 × 10 −^5 ) Y−(6.17 × 10 −^10 ) Y^2 + (5.05 × 10 −^14 ) Y^3

Y = 136 + (7.46 × 104 ) X− (8.51 × 104 )X^2 + (2.33 × 104 )X^3

Y = (85.20 X −16.14) × 103

Y = (42.30 × 103 )X

Y = (38.20 × 103 )X

Y = (30.60 × 103 )X

Initial Aluminum:

Final Aluminum:

6 Month Creep:

1 Year Creep:

10 Year Creep:

.3 .4 .5

FIGURE 14.8 Stress-strain curves for 37-strand AAC.