voltage but are always used at 345 kV and above to limit corona. To maintain the distance between
bundle conductors along the line, spacers made of steel or aluminum bars are used. Figure 13.5 shows
some typical arrangement of stranded bundle configurations.
13.3 Current-Carrying Capacity (Ampacity)
In overhead transmission lines, the current-carrying capacity is determined mostly by the conductor
resistance and the heat dissipated from its surface [8]. The heat generated in a conductor (Joule’s effect)
is dissipated from its surface area by convection and radiation given by
I^2 R¼S(wcþwr)WðÞ (13:6)whereR¼conductor resistance (V)
I ¼conductor current-carrying (A)
S ¼conductor surface area (sq. in.)
wc¼convection heat loss (W=sq. in.)
wr¼radiation heat loss (W=sq. in.)
Heat dissipation by convection is defined as
wc¼0 : 0128
ffiffiffiffiffi
pv
pTair^0 :^123ffiffiffiffiffiffiffiffiffiffi
dcondp DtðÞW (13:7)wherep ¼atmospheric pressure (atm)
v ¼wind velocity (ft=s)
dcond¼conductor diameter (in.)
Tair ¼air temperature (kelvin)
Dt ¼TcTair¼temperature rise of the conductor ( 8 C)
Heat dissipation by radiation is obtained from Stefan–Boltzmann law and is defined as
wr¼ 36 : 8 E
Tc
1000 4
Tair
1000"# 4
ðÞW=sq:in: (13:8)wherewr¼radiation heat loss (W=sq. in.)
E ¼emissivity constant (1 for the absolute black body and 0.5 for oxidized copper)
Tc ¼conductor temperature ( 8 C)
Tair¼ambient temperature ( 8 C)
dddddd(a) (b) (c)FIGURE 13.5 Stranded conductors arranged in bundles per phase of (a) two, (b) three, and (c) four.