The wires from the distribution transformer to the customer's service point are called the "service drop". They are commonly installed overhead, however, they can be installed underground. (It is possible to have an underground service drop from an overhead or underground distribution system). All conductors must be insulated except the neutral, which may be bare in overhead installations. Single phase service drops will have either 2 or 3 wires while three phase service drops will have either 3 or 4 wires.
The National Electrical Code specifies installation requirements on service-drop conductors in terms of wire size and minimum clearances above the ground.