A Field Guide to Power Transmission Lines

The number of energy-carrying wires on them is three or fewer (per circuit, some distribution poles carry more than one three-phase circuit) and they tend to hold other equipment on them as well, such as transformers, fuses, switches, and even telephone and cable lines. For the transmission lines, though, which handle bulk power, the voltages can be as high as 500 kV and still carry thousands of amps of current. Another advantage of steel is that it can be easier to produce structures with more than one pole (supporting the wires via a crossarm of some sort) for the largest of transmission lines.

Source: hackaday.com