HouseTuttle CurrentCarryingCapacityofACSR 1959ID633VER39
HouseTuttle CurrentCarryingCapacityofACSR 1959ID633VER39
HouseTuttle CurrentCarryingCapacityofACSR 1959ID633VER39
H. E. HOUSE
MEMBER AIEE
MEMBER AIEE
of ACSR
and electric current, the following equation is valid (1) qc+qr =I2r+q.
or
P. D. TUTTLE
I-qc +qr-qs
r
(1A)
Synopsis: Current-temperature characteristics of stranded-aluminum conductor steel reinforced, known throughout the industry as ACSR, have been investigated. The effects of surface conditions, wind velocity, altitude, and solar radiation are illustrated for a widely used size of conductor; curves of current-carrying capacity versus conductor outside diameter are given for design conditions of 75 C (degrees centigrade) conductor temperature and 25 C ambient temperature at 2-fps (feet per second) wind velocity. Necessary formulas and tables to permit computation of current values for any set of operating conditions are included. Computed values of current are in close agreement with test data which have obtained by Aluminum Company of America (Alcoa) and other investigators.
SINCE first introduced by Alcoa in 1909, the use of ACSR for overhead electric power transmission lines has grown steadily until it has almost replaced copper for such use. In most new construction, aluminum instead of copper is being used for overhead distribution conductors. Because of the presence of the steel core in ACSR and its consequent effect on the electrical characteristics of the conductor, considerable test work has been carried on throughout the years to evaluate effective resistance. This is needed to compute the current-carrying capacity of the conductor. Early investigations were carried out by Work for Alcoa at the Carnegie Institute of Technology, Pittsburgh, Pa.l The well-known publications of Luke2 and Schurig and Frick' were followed periodically by others,4-8 indicating a strong and continued interest in the subject.
Results of tests for the determination of the emissivity of stranded-aluminum conductors for surface conditions of both new and weathered conductors were reported
in 1956.'
Tests to determine the effective 60-cycle resistance of a great variety of sizes and strandings of ACSR have been carried
Paper 58-41, recommended by the AIEE Trans. miion and Distribution Committee and approved by the AIEE Technical Operations Department for presentation at the AIEE Winter General Meeting, New York, N. Y., February 2-7, 1958. Manuscript submitted October 16, 1957; made available for printing November 6, 1957.
H. E. HoUsE and P. D. TUTTLE are with Alcoa Research Laboratories, Massena, N. Y.
out at the Alcoa Research Laboratories at Massena, N. Y. Conductors were strung under tension on a 120-ft (foot) test span. Values of 60-cycle resistance were measured up to a conductor temperature of 200 C or 3,000 amperes/square inch if 200 C temperature was not reached. The method which was used in these tests is described by Tompkins, Jones, and Tuttle.'0 A co-operative research program between the Illinois Institute of Technology, Chicago, Ill., and Alcoa Research Laboratories has been completed.11,12 The results of this work provide a means of accurate computations of reactance and resistance for ACSR of any combination of aluminum and steel stranding. Because of the tremendous growth of the electrical utility industry, there remain very few long transmission lines in the eastern part of the United States. Lines that were once long have been looped into newly constructed substations. The load on these short transmission lines is limited by the heating of the conductors rather than by stability and voltage regulation, as was the case as late as the 1930's. For this reason, an accurate understanding of the thermal capabilities of the conductors is more important than ever before. The formula developed by McAdams" for convected-heat loss of single horizontal tubes and wires has been found to give accurate convected-heat loss for stranded conductors. This formula has been combined with the results of emissivity tests9 and data on solar radiation,14"' and fieldtest data on absorption of solar and sky radiation on outdoor test spans of stranded conductors, in order to evaluate the current-carrying capacity of ACSR. With accurate values of a-c resistance for a variety of strandings, it is now possible to compute the current a conductor will carry for any given set of conditions of temperature, wind velocity, surface condition, and altitude above sea level, both with and without the effect of the sun.
where q8 is convected-heat loss, q. is radiated-heat loss, I is the current in amperes, r is the effective a-c resistance in ohms/ft of conductor, and qs is the amount of heat received from solar and sky radiation. Each heat quantity in the equation is expressed in watts/lineal ft of conductor.
CONVECTED-HBAT LOSS The fundamental relationship for convected-heat loss of single horizontal tubes and wires is given by McAdams (see reference 13, p. 220). This is expressed by
the dimensionless equation 0 hDk = 0.32+0.43
(2)
where
set of conditions. This formula is recommended for Reynolds numbers ranging from 0.1 to 1,000 which include air velocities up to 2 fps for conductors up to 1.3inch diameter. The units used in electrical engineering are watts, degrees centigrade, and feet. Accordingly, h, the surface coefficient of heat transfer, is expressed in watts/sq (square) ft/C; Do is conductor outside diameter in ft; kf is the thermal conductivity of air, (watts) (ft)/(sq ft) (C); G is the mass velocity of air in lb (pounds)/ hr (hour) (sq ft) cross section, or the product of air density pf in lb/ft' times the velocity V in ft/hr. The quantity uf is the absolute viscosity of air in lb-mass/ft-hr. Density, viscosity, and thermal conductivity are at the temperature of the air film given by the relationship
hDo/kf is the Nusselt number, and DoGl,of is the Reynolds number for any
if
tc+t.
By simplifying and expressing conductor diameter D in inches, the following equation is obtained
1169
FIEBRUARY 1959
Table 1. Viscosity, Density at Sea Level to 15,000 Ft, and Thermal Conductivity of Air
Temperature
F*
32 ... 41.... 50.... 59 ... 68 .... 77 .... 86 .... 95 ....
C
(_
\100
14
Viscosity,
Af
Absolute
Density, pf
Sea Level 5,000 Ft 10,000 Ft
1S,000 Ft
Conductivity, kI
Thermal
104.... 40... .313 .... 95.98 ...O.0461.....0.0704 .... .0.0586.... .0.0484.... .0.0397.....0.00830 113 .... 45 .. 318 .... 102.26 ... 0.0467.....0.0693 .... 0.0577 ... 0.0476 .... 0.0391 ..... 0.00841 122 .... 50... .323....108.85 .....00473.....0.0683....0.0568.... 0.0469.... .0.0385.....0.00852 131 .... 5.... 328 ....115.74 ....0.0478.....0.0672....0.0559 .... 0.0462 .... 0.0379 ..... 0.00864 140 .... 60... 333....122.96 ..0.0484 ..... 0.0661 .... .0.0550 .... .0.0454....0.0373 ..... 0.00875 149 .... 65... 338....130.52 ...0.0489 ..... 0.0652.... 0.0542.... .0.0448.... .0.0367 ..... 0.00886 158 .... 70... 343.... 138.41l..0.0494.....0.0643... .0.0535... .0.0442 .... .0.0363 ..... 0.00898 167. ... 75... .348 ..146.66 .....00500 ..... 0.0634....0.0527 .... 0.0436.... 0.0358.....0.00909 176.... 80.. ..353 .... 155.27 . ....0505.....0.0627 .... 0.0522....0 0431... .0 0354.... 0.00921 185 ... 85... 358.... 164.26 ...O.0510 ..... 0.0616 .0.0513 .... .0.0423.... .0.0347.....0.00932 194 .... 90.. ..363.... 173.63 ...O.0515.....0.0608 .....0.0506 .... .0.0418 .... .0.0343.....0.00943 203.... 95... 368.... 183.40 ... 0.0521 ......0599 .... .0.0498....0.0412... .0.0338.....0.00952 212.... 100....373. ... .19357..0.0526 ..... 0.0591....0.0492.... .0.0406 .... .0.0333 ..... O.00O66
O.. .273 .... 55.55. ...0.0415 ..... 0.0807. ..0 .0671....0.0554 .... .0.0455 .... 0.00739 5... 278 .... 59.73...0.0421.....0.0793. ..0.0660....0.0545.... 0.0447 .... 0.00750 10... 283.... 64.14 . 0.0427 . 0.0779. .. .0.0648. .0.0535 ... 0.0439 . 0.00762 16. 288.... 68.80. .....00433 ..... .0765... .0.0636.... 0.0526.... .0.0431.....0.00773 20... 293.... 73.70 ... 0.0439.....0.0752.... 0.0626 .... 0.0517. ...0.0424.... 0.00784 25 .298 .... 78.86 ... 0.0444.....0.0740.... 0.016. ..0.0508 .... 0.0417.....0.00795 30... 303 84.29 0.0450.....0.0728.... 0.0606.... 0.0500....0.0411 ..... 0.00807 35.... 308 .... 89.99 ... 0.0456 ..... 0.0716 .... 0.0596.... 0.0492.... 0.0404.....0.00818
....
le conductor temperature C.
-
fabsolute viscosity, lb/(hr) (ft), computed from formula in reference 17. pf -density. lb of air/fts, computed from data given in reference 18. kf -thermal conductivity of air, watts/(sq ft)(C) at tf- (Sc+ia)/2, reference 13, Table XI. tg- ambient temperature C.
* Degrees Fahrenheit.
I Dp1 V\o~~~.521
watts/lineal ft of conductor (3A) For Reynolds numbers from 1,000 to 50,000 the following empirical formula is recommended by McAdams
-a
1 kf
0.24(
o 0)
A
which expressed in electrical engineering units is 0.5275 X 10-8 watts/sq ft/K4, where K is temperature in degrees Kelvin or C+273.'6 The quantity e is the thermal-emissivity constant which for new conductor is 0.23 and for flat-black well-weathered conductor 0.91 or possibly higher. The area of a circumscribing cylinder A is expressed in sq ft. Converting to conductor outside diameter in inches with temperature in K gives
0.5275X10 iKKDe qr ~~(K 4 - Ka4) 12
=-
conditioning systems.","5
qs=a(QD Sill 9+Qd)A'
December and January. The effect of solar radiation on conductor temperature is more important than before because its maximum intensity now occurs at the same time as the peak load. The amount of heat received by a flat surface perpendicular to the sun's rays and located outside the earth's atmosphere is approximately 123 watts/sq ft of surface. However, because of the earth's atmosphere, part of this energy is absorbed before reaching the earth. Points of high altitude of, e.g., 10,000 ft, such as exist in the Rocky Mountain area, receive about 25% more solar energy than sealevel areas; see Table II. The amount of solar heat received by a conductor also depends on the altitude of the sun above the horizon and the effective angle of incidence between the direct rays of the sun and the exposed surface. In addition to direct radiation, heat is radiated from the sky to the object. This quantity also varies with the sun's altitude. Atmospheric contamination has a marked effect on the solar heat received. Considerable work has been done in the field of solar-energy studies, in connection with the heating of buildings, as a source of power, and relative to the solar-heat gain required to be absorbed by airThe amount of heat received from the
(6)
sphere are having the yearly peak loads during July and August rather than
(SA)
kft-a)
watts/ft of conductor (4A) Values for pf, pf, and kf are given in Table I.
For convected-heat loss in still air the following formula checks closely with test data obtained at Alcoa Research Laboratories in a room free from drafts.
qc - 0.072D"( 7(- ta)l "
(100
where QD is direct solar radiation and Qd is sky radiation, both in watts/sq ft; A' is the projected area of the conductor, and a is the solar-absorption coefficient. Outdoor tests at Massena indicate this is 0.23 for new conductor and 0.97 for black conductor. For simplicity in computaTable Ill. Total Heat Received by Surface at Sea Level Normal to Sun's Rays
Altitude, He, Degrees
5.... 10 15 20 25 30 35 40 45 50 60
Solar
Q,, Watts/Sq Pt
Clear Atmosphere
Atmosphere
Industrial
where D is conductor diameter in inches, tc conductor temperature in C, and la is the temperature of the surrounding air in C.
RADITED-HiEAT Loss oF CONDUCTOR The radiated-heat loss of a conductor is given by the expression
(5) -eA(Kc'-Ka') qr, where a is the Stefan-Boltzmann constant,
Table
for
5,000 15,.000
*
10,000
... ...
80 90
21 ............. 12.6 22.3 . 4 0.2 ..... .54.2 ..... 30.6 .64.4 .. 39.2 .71.5 ..... 46.6 .77.0 ..... 553.0 .81.5 ..... 57.5 .84.8......f .61.5 .87.4 ..... 64.6 .90.0 ..... 67.5 .92.9 ..... 71.6 70 .95.0 ......,......75.2 .95.8 ..... 77.4 78.9 . 96.4.
.....
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FEBRUARY 1959
Table IV. Altitude and Azimuth in Degrees of Sun at Various Latitudes at Declination of 23.0 Degrees, Northem Hemisphere, June 10 and July 3*
Local Sun Time
qc =20.95 watts/ft
qr =0.138X 1.108X0.23(146.66-78.86)
(8A)
12 Noon
2:00 P.M.
Ho
ZO
He
Zo
20... .62 .... 78. ...87... 0. 62... 282 25... .62 .... 88. 88 ... 180. 62. ...272 30... .62.... 98.. 83 .. 180. 62... .262 35... .61... 107 . 78...180. 61... 253 115.. 40....60... 73 .. 180 60 ."45 45... 57.... 122 . 68 .. 180. 57.. .238 50... .54... .128. f3.. .180 ..54... 232 60... .47....137 .. 3. 80.47 .23 70... .40... 143. 43.. 180.40... 217
*
V.]kf( t,
(Ka4
100)
)
'
0.138D1 K_ [(~~100)
(7)
SAMPLE COMPUTATION In the sample computation the following conditious apply:
Drake conductor, 795 MCM (thousand circular mils), 26/7 ACSR, (new) wind velocity=2 fps at sea level air ternperature-25 C =I conductor temperature-=75 C=1, conductor outside diameter=1.108 inches conductor a-c resistance=0.0265 ohm/ 1,000 ft
=2.37 watts/ft (9) Assume the following: azimuth of line 135 degrees, latitude 35 degrees north, clear atmosphere, 12 noon. H, = 78 Zc 1800
-
Z,=1350
tion, Table III shows total heat received from both direct and sky radiation for both clear and industrial atmosphere. This introduces a small amount of error as sky radiation does not depend on the angle of incidence. However, this error cannot be detected in the final value of conductor current. In the case of a round, horizontally placed conductor, the angle 9 is given by O=cos-1[cos He cos (Z,-Zl)] (6A)
(10)
q.=0.23X0.986X95.6X 112
2.01 watts/ft (12)
if=(75+25)/2 =50 where H, is the altitude of the sun above pf=0.0683 (Table I) the horizon, Zc is the azimuth of the sun, yf=0.0473 (Table I) (Table I) and Z1 is the azimuth of the conductor kf=0.00852 V= 3,600 X2 =7,200 ft/hr (north-south line Z& = 1800). See Table
e =0.23
40
w
0.
w -J
cn
w
-J 0
-j
30
NEW CONDUCTOR 10.000 F T. EL -SUN NEW CONDUCTOR - SUN NEW CONDUCTOR - NO SUN BLACK CONDUCTOR -SUN BLACK CONDUCTOR -NO SUN
25-C AMBIENT TEMPERATURE
z
75C CONDUCTOR TEMPERATURE
2 20 0
I--
0. w
z
10
COMPUTED VALUES
0
US2F EC
600
l_
800
1000
1200
1400
M0
1800 2000
2200
2400
2600
2600
*s
as
40
#a ' t
was0
0
-.s s
OC
aj
N o
N N C4 C4 coN
e
l
ll lpi
on
N t
la
"0
Is Is>
I
:v
-:
1171
FEBRUARY 1959
0T
0.50
W
maximum.
a value based on considerable test data,' has been consistently used throughout the industry because it represents a safe
L&i
0.30
0.20
0 0 0
a.
co
tc
0.10
.1oo*c
-
t=5000
75 C
C.) z 4
I.-
0.07
0.05 0.03
__ __ __
New Drake conductor has been selected to illustrate the effect of changing ambient temperature, with the sun effect neglected, at a constant current -of 1,000 amperes. This is shown in the following.
Ambient Temperature, C
0 25 40
.
ra
0
C)
C)
Conductor Temperature, C
57 . 83. 101 .
Temperature Rise, C
0.02
.
.
67 58
61
0.01
0.005
0.2
0.4
0.6
2.0 1.8 1.4 1.2 1.6 1.0 0.8 CONDUCTOR OUTSIDE DIAMETER -INCHES
2.2
2.42d2.6
I1111
11
11 I I I1
to
0
I
la
D
Co0
.,
to
000C I,,tQf
.t
* COe 4
_,
C0
It is evident that the effect of selecting ambient only slightly different from a standard design value will have little effect on the actual temperature rise as illustrated by the foregoing example in which a 40 C change in ambient only increased the temperature rise 4 C. This increase is still less when sun effect is taken into account, because this tends to cancel the effect of radiated-heat loss, leaving only convected-heat loss, which varies approximately with temperature rise, to balance to Pr loss.
an
In computing sun effect, a value of 85 watts/sq ft was used for total radiation and 750, giving an effective heat from the sun of 82 watts/sq ft. It is significant that there is a definite discontinuity in the curves between the sizes 4/0 ACSR 6/1 and 226.8 MCM ACSR 26/7. This is explained by the increased magnetizing effect on the steel core; the current in the single layer of aluminum strands gives rise to eddycurrent and hysteresis losses in the steel core which in turn cause a marked
e=
cqncelled.
1172
minum strands with the spiraling in the opposite direction in each successive layer, the magnetizing effect is almost entirely
increase in effective a-c resistance. In the case of more than one layer of alu-
Condition
No Sun
Black Condition
Sun
1,564 .. 1,020 .
ACSR
Sun
1,590 MCM 54/19............1,430 . ...... 941 . 795 MCM 54/7 ........,,,,. No. 4 6/1 ................... 149 .......
1,482 973
151 .........
.. ..
155 .......
.500 960
148
FIEBRUARY 1959
resistance of the conductor with an increase in conductor temperature. An increase in conductor temperature may be caused by either increased ambient temperature or increased current. Eddycurrent and hysteresis losses in the core increase the effective a-c resistance noticeably for single-aluminum-layer conductors, as previously explained. The magnetic loss component of a-c resistance increases with an increase in current until the point of magnetic saturation has been reached, after which there is no further increase in this component. This particular behavior of ACSR is dealt with fully by Lewis and Tuttle."
ment with test data obtained by the Alcoa Research Laboratories and those observed by other organizations.
References
1. ELECTRICAL CHARACTERISTICS OP ACSR (a pamphlet). Aluminum Company of America. Pittsburgh, Pa., May 1946. 2. CURRE3NT CARRYING CAPACITY OF WIRBS AND CABLES, George E. Luke. Westinghouse Electric Journal, Pittsburgh, Pa., Apr. 1923.
Journal, Franklin Institute, Philadelphia, Pa., vol. 23, no. 5, Nov. 1940, pp. 583-617. 25. HEAT TRANSMISSION AS INFLUENCED BY HRAT CAPACITY AND SOLAR RADIATION, P. C. Houghton, J. L. Blackshaw, B. M. Pugh, P. McDermott. Paper no. 923, Transactions, American Society of Heating and Ventilating Engineers, New York, N. Y., Jan. 1932. 26. A RATIONAL HE[AT GAIN METHOD FOR ITE DBTERaINATION OF ALa CONDITIONING COOLINGO LOADS, F. H. Faust, L. Levine, F. 0. Urban. Journal, Heating, Piping and Air Conditioning
Section, Ibid., Aug. 1935.
Chicago, Ill., Dec. 1943. 5. SAFB RATINGS FOR OVERHEAD LINE CONDUCTORS, Leonard M. Olmsted. AIEE Transactions, vol. 62, 1943, pp. 845-53. 6. ELECTRICAL HEATING CRACTERISTICS OF OVERHE:AD CONDUCTORS, PARTS I-IV, E. B. George. Electric Light and Power, Dec. 1944; Jan. 1945; Apr. 1945; Dec. 1945. 7. CURRENT CARRYING CAPACITY OF OV3RHEAD CONDUCTORS, H. A. Enos. Electrical World, New York, N. Y., May 15, 1943.
8.
oP BARB CONDUCTORS FOR OUTDOOR SERVICE, 0. R. Schurig, C. W. Frick. General Electric Review, Schenectady, N. Y., vol. 33, Mar. 1930. 4. DBTBRMINING CURtBNT RATINGS OF OVERHEAD CONDUCTORS, PARTS I AND II, H. P. Seelye, A. L. Malmstrom. Electric Light and Power,
Discussion
W. A.
CURRENT CARRYING CAPACITY OF ACSR CONDUCTORS, J. H. Waghorne, V. E. Ogorodnikov. AIEE Transactions, vol. 70, pt. II, 1951, pp.
1159-62.
537-39.
EISSIVITY AND ITS EFFECT ON TEB CURRENTCARRYING. CAPACITY OF STRANDED ALUMINUM CONDUCTORS, C. S. Taylor, H. E. House. Ibid., vol. 75, pt. III, Oct. 1956, pp. 970-76. 10. MEASUREMBNTS OF RESISTANCB AND REACTANCB OF EXPANDBD ACSR, Joel Tompkins, B. L. Jones, P. D. Tuttle. Ibid., vol. 74, pt. III, June 1955, pp. 368-75. 11. THE RBSISTANCEC AND REACTANCE OF ALUMINUM CONDUCTORS, STEBL REINFORCED, W. A. Lewis, P. D. Tuttle. Ibid., pp. 1189-1215 o this issue. 12. TEE MAGNBTIC PROPBERTIBS OF ACSR CORE WIRE, T. W. Matech, W. A. Lewis. Ibid., pp. 1178-89 of thi issue. 13. HBAT TRANSMISSION (book), W. H. McAdams. McGraw-Hill Book Company, Inc., New York, N. Y., second edition, 1942. 14. HBATING, VENTILATING AND AiR CONDITIONINO GuIDE 1956. American Society of Heating and Air Conditioning Engineers, New York, N. Y., 1956. 15. POWBR FROM SOLAR ENBRGY, J. r. Yollot. Transactions, American Society of Mechanical Engineers, New York, N. Y., vol. 79, no. 6, Aug. 1957, pp. 1349-57. 16. A REVIBW Op TERMAL RADIATION CONSTANTS, 1N. W. Snyder. Ibid., vol. 76, 1954, pp.
9.
Conclusions
The necessary formulas, curves, and tables have been presented which will enable transmission engineers to select the size of ACSR most suitable for their requirements. It is believed that the data given to illustrate the effect of the sun are of importance in light of the fact that many system peak loads are now occurring in the daytime during the summer months, because of air-conditioning and pumping-equipment loads. Computed values of current-carrying capacity at sea level are im close agree-
981. 18. FAN ENGINEERING, Richard D. Mason, editor. Buffalo Forge Company, Buffalo, N. Y., fifth edition, 1948. 19. THE AMERICAN NAUTICAL ALMANAC 1957. U. S. Naval Observatory, Washington, D. C., 1957. 20. SIGHT REDUCTION TABLBS FOR AIR NAVIGATION, VOLS. II, III. Publication no. 249, U. S. Navy Hydrographic Office, Washington, D. C., 1957. 21. BARLow's TABLBS, L. J. Comrie, editor. Chemical Publishing Company, New York, N. Y., fourth edition, 1944. 22. THERMAL RADIATION TABLBS AND APPLIcATIONS, R. V. Dunkle. Transactions, American Society of Mechanical Engineers, vol. 76, 1954, pp. 549-52. 23. GAs TABLECS (book), J. H. Keenan, J. Kaye. John Wiley & Sons, Inc., New York, N. Y., 1948. 24. PROPOsED STANDARD SOLAR RADIATION CURtVS FOR ENOINEBRING USE, Parry Moon.
17. Tim VISCOaIrY, THERMAL CONDUCTIVITY AND P&ANDTL NUMBER FOR ANR mm OTHER GSEs,
which may affect the heat balance of a conductor that is carrying alternating electric current with the usual prescribed limits of conductor temperature and ambient temperature. Particularly, the effect of sunshine is noted. However, the application and operating engineer is in need of published data or guides which should be forthcoming from manufacturers of ACSR and all-aluminum conductors as to the effects of loading above the currents which give the usual temperature rises. Obviously there is a time-current relationship for such overloads, i.e., the shorter the time the greater is the amount of current that may be allowed to flow above that which would just give the desired temperature rise. Specifically, there is probably a temperature somewhat above 75 C where continuous operation would cause a reduction in the tensile strength, another temperature where the tensile strength would be reduced 5% if operated at that temperature a specific time, etc. Or, are we to assume that aluminum has not agreed upon temperature limit and will lose some percentage of its tensile strength if operated continuously at even 75 C? There are data available for determining how much a transformer may be overloaded under emergency conditions without jeopardizing its life, or, in some cases a calculated loss-of-life expectancy may be calculated and is acceptable. Similarly, it is desirable to know how much a conductor may be overloaded during an emergency and for how long. For example, assume that one of two parallel circuits is out of service and it is desired to carry an overload current (say 25% above the rated value which would give 75 C conductor temperature) over the daily peak rather than to cut off
Morgan (Washington Water Power Company, Spokane, Wash.): The authors are to be commended for the thoroughness with which they have considered the factors
Perhaps the steel reinforcing will provide for most of the loss of margin of tensile strength in ACSR conductors. But, allaluminum conductor may be particularly vulnerable to overload currents, and, if it is, perhaps we should know its critical conductor temperatures or time-current overload
characteristics.
E. E. George (Ebasco Servces Inc., Little Rock, Ark.): The authors have done an excellent job in utilizing pre-
customers.
FEBRUARY 1959
1173