44
Electric Power Distribution Handbook
The dc resistance is inversely proportional to the area of a conductor;
doubling the area halves the resistance. Several units are used to describe a
conductor’s resistance. Conductivity is often given as %IACS, the percent
conductivity relative to the International Annealed Copper Standard, which
has the following volume resistivities:
0.08145
W
-in.
2
/1000 ft = 17.241
W
-mm
2
/km = 10.37
W
-cmil/ft
And with a defined density of 8.89 g/cm
3
at 20
∞
C, the copper standard has
the following weight resistivities:
875.2
W
-lb/mi
2
= 0.15328
W
-g/m
2
Hard-drawn copper has 97.3%IACS. Aluminum varies, depending on type;
alloy 1350-H19 has 61.2% conductivity.
Temperature and frequency — these change the resistance of a conductor.
A hotter conductor provides more resistance to the flow of current. A higher
TABLE 2.5
Characteristics of Steel Conductors
Size
Diameter,
in.
Conductor
Area, in.
2
Weight,
lb/1000 ft
Strength,
lb
Resistance,
WW
WW
/1000 ft
dc
25˚C
60-Hz ac at the
given current level
10A 40A 70A 100A
High-Strength Steel — Class A Galvanizing
5/8 0.621 0.2356 813 29,600 0.41 0.42 0.43 0.46 0.49
1/2 0.495 0.1497 517 18,800 0.65 0.66 0.68 0.73 0.77
7/16 0.435 0.1156 399 14,500 0.84 0.85 0.88 0.94 1.00
3/8 0.360 0.0792 273 10,800 1.23 1.25 1.28 1.38 1.46
Utilities Grade Steel
7/16 0.435 0.1156 399 18,000 0.87 0.88 0.90 0.95 1.02
3/8 0.380 0.0882 273 11,500 1.23 1.25 1.28 1.38 1.46
Extra-High-Strength Steel — Class A Galvanizing
5/8 0.621 0.2356 813 42,400 0.43 0.43 0.44 0.47 0.50
1/2 0.495 0.1497 517 26,900 0.67 0.68 0.69 0.73 0.78
7/16 0.435 0.1156 399 20,800 0.87 0.88 0.90 0.95 1.02
3/8 0.360 0.0792 273 15,400 1.28 1.29 1.31 1.39 1.48
Extra-High-Strength Steel — Class C Galvanizing
7/16 0.435 0.1156 399 20,800 0.70 0.70 0.71 0.71
3/8 0.360 0.0792 273 15,400 1.03 1.03 1.03 1.04
5/16 0.312 0.0595 205 11,200 1.20 1.30 1.30 1.30
Source:
EPRI,
Transmission Line Reference Book: 345 kV and Above
, 2nd ed., Electric Power
Research Institute, Palo Alto, CA, 1982.
1791_book.fm Page 44 Monday, August 4, 2003 3:20 PM
(C) 2004 by CRC Press LLC