Conductors. Two main kinds of conductors can be used for the transmission run—aluminum and copper. Aluminum has more than 1.6 times the resistance per gauge (diameter) than copper. Because of the higher resistance compared to copper, aluminum requires a larger wire size to handle a given amount of current. Even so, aluminum conductors can still be less expensive than copper depending upon current metal prices.
If the wiring will be run in conduit, larger wire size means larger conduit and increased overall cost. Aluminum wire is stiff and more difficult to pull through conduit and route into electrical boxes, and is not flexible enough for direct connection to the power terminals of most hydro plants. Finally, moisture has a greater corrosive effect on aluminum than copper. If conductors rated for direct burial are used, aluminum wiring will be more susceptible to damage if a wire is nicked. For all these reasons, copper is the preferred choice. For direct burial, the wire type should be USE, RHW, or UF. In conduit, THHN/THWN is typically used.
Conduit. Using wire rated for direct burial is tempting for hydro systems with long transmission runs, since skipping the conduit means less expense up-front, and makes installation a bit faster. However, protecting the conductors in conduit is recommended. In the case of direct burial, unless care is taken to bed the conductors in sand and remove rocks that might come in contact with the cable, damage can occur during backfilling. Trust us—trying to locate a broken conductor in a 1,000-foot wire run buried 30 inches deep isn’t something you want to experience. In addition, it’s desirable to use conduit for high-voltage transmission runs so there’s one more layer of protection for both the conductors, and for anyone that might inadvertently come into contact with them.
Connections. For making electrical connections at the turbine, soldered copper-lug connectors are ideal. If the transmission run is aluminum, use a short length of copper wire from the turbine’s output terminals to a junction box, where split bolts can be used to join to the aluminum and copper conductors. Apply a corrosion inhibitor wherever dissimilar metals are joined. Wound-field hydro turbines (which use modified vehicle alternators) have electrical terminals that are somewhat exposed, so caution is advised when servicing them.