Further, one cannot build pylons arbitrarily high at either end of the crossing section, and there is a usually a considerable minimum height because of ships crossing under the line, so there is often a high mechanical tension in the conductors at long spans. This tension requires conductors made largely of steel, which have a worse electrical conductivity than the common overhead line conductors consisting of copper, Aldrey or aluminum-encased steel, and also limits the amount of transmittable electrical power. For this reason, for crossings with a span width of more than approximately 2 km, those in charge of construction should consider laying an underwater cable as the more practicable solution.