Transmission Circuits

The heart of any utility power-distribution system is the cable used to tie distant parts of the network together. Conductors are rated by the American Wire Gauge (AWG) scale. The smallest is no. 36, and the largest is no. 0000. There are 40 sizes in between. Sizes larger than no. 0000 AWG are specified in thousand circular
mil units, referred to as “MCM” units (M is the Roman numeral expression for 1000). The cross-sectional area of a conductor doubles with each increase of three AWG sizes. The diameter doubles with every six AWG sizes.

Most conductors used for power transmission are made of copper or aluminum. Copper is the most common. Stranded conductors are used where flexibility is required. Stranded cables usually are more durable than solid conductor cables of the same AWG size. For long distances, utilities typically use uninsulated aluminum conductors or aluminum conductor steel-reinforced cables. For shorter distances, insulated copper wire normally is used.

Ampacity  is the measure of the ability of a conductor to carry electric current. Although all metals will conduct current to some extent, certain metals are more efficient than others. The three most common high-conductivity conductors are:

• Silver, with a resistivity of 9.8 Ω /circular mil-ft  
• Copper, with a resistivity of 10.4 Ω /circular mil-ft 
• Aluminum, with a resistivity of 17.0 Ω /circular mil-ft 

The ampacity of a conductor is determined by the type of material used, the cross-

sectional area, and the heat-dissipation effects of the operating environment. Conductors operating in free air will dissipate heat more readily than conductors placed in a larger cable or in a raceway with other conductors will. Table 1.5 lists the princi pal parameters of common wire sizes.