If you have overhead electrical service to your house, you almost certainly have a weatherhead. It's a rounded cap that sits atop the metal pole rising from your roof. The connection between the utility's service lines and your home's service lines is made near the weatherhead. The weatherhead's main job is to keep water out of the pipe, and it's an essential part of the transfer of power from the utility to your home.
The Service Entrance
The assembly of structural parts and wiring that connects your home to utility power is known collectively as the service entrance. The service entrance starts at the transformer on the utility's power pole. The heavy insulated wires and metal cable strung overhead from the transformer to your house are called the service drop. The service drop is anchored to the metal pole rising through the roof. The pole is called the mast or riser and it contains a large cable or individual wires known as the service entrance conductors (or service entrance cable). These conductors exit the mast through the weatherhead, which is shaped so that the conductors point downward as they come out.
The Service Point
The shape of the weatherhead helps keep rain and snow out of the mast in more ways than you might expect. First, it simply covers the mast to prevent rain and snow from falling in.
Second, by routing the service entrance conductors downward, the weatherhead allows for a little dip in the service wires, known as a drip loop. This prevents rain from sliding down the service drop wires and into the mast. The service entrance conductors (the wires coming from the weatherhead) form a drip loop, then are connected to the wires in the service drop.
This connection is called the service point and usually represents the point of transfer between the utility provider and the utility customer. Everything on the house side of the service point (with the exception of the utility meter) typically is installed by an electrician. Everything on the utility side of the service point is installed and maintained by utility personnel.
More About Weatherheads
Given the terminology explained above, you can see why weatherheads often are called mastheads or service heads. But not all weatherheads mount to masts. Some types are anchored to the wall of a building, and some have a special clamp for securing the service entrance conductors (usually a cable) directly to the weatherhead. In the latter case, there is no mast. Weatherheads for use with masts may be designed to clamp or thread onto the end of the mast pipe. The mast is usually a rigid metal electrical conduit, either rigid metal conduit (type RMC) or intermediate metal conduit (type IMC). Some weatherheads can be used with nonmetallic (usually PVC plastic) pipe. All of these specifics are governed by the local building authority and/or the utility company, and rules vary from location to location.