Electric Service

The Service Panel

The typical panel shown in Fig. 1 is rated to handle 200A and it has 48 circuit positions.

Fig. 1. A typical 200 Amp panel has 48 circuit positions. This one has seventeen 120V breakers, eleven 240V double-pole breakers, and nine circuits spare.

On the panel, you can see that there is an index label pasted and numbered 1 through 48. It is a good practice to fill in the circuit description on the label. In that case, if you need to disconnect any circuit during troubleshooting or maintenance, you know where the breaker is located in the panel.

Each branch circuit has its own circuit breaker in the panel. To be exact, the "hot" wire at a potential of 120V, which is typically black, connects to the terminal alongside the breaker. The white wire in the cable connects to a neutral strip (bus) located inside the panel, as illustrated in Fig. 2.

Fig. 2. The service panel gets three wires from the electric company. Each circuit breaker feeds a hot wire to a branch circuit.

When you need 240V for a branch circuit, use three-wire cable. The third wire is usually red. The 240V breakers have two "hot" terminals, one for the black wire and the other for the red wire. White still goes to the neutral strip, along with all the other white wires from the other branch circuits.

In addition to the hot wires and the neutrals, you will also find a green or bare wire with most branch circuits. This wire is used solely for safety purposes; it does not carry any current. It is used to place the metal enclosures of electrical equipment at a true ground potential for the prevention of electrical shocks.

The primary function of the service panel is to disconnect automatically, by means of the breaker or fuse, any defective circuit that could draw too much current, such as the appliance in Fig. 3.

Fig. 3. Should a short circuit develop in one of the branches, a fuse blows or a circuit breaker trips for that branch circuit.

Between the electric meter and the branch circuits is found the main disconnect. Its current rating is the total sum of all the branch circuits. The main disconnect has the function of shutting off all power in case of a major or total breakdown in the wiring system.

The main disconnect gets the 3-wire input from the meter. Typically, the black wire of the cable connects to the left terminal and the red wire connects to the right terminal. The third wire, which is bare, attaches to the neutral strip in the box and it is grounded securely by connecting it to an approved ground, such as a cold-water pipe.

Electric Meters

The electric meter is supplied by the electric company and is plugged into a meter socket that the electrical contractor supplies and installs. The meter is sealed by the electric company. When there is meter trouble, the power company will handle it.

The meter socket is simply a connecting device that receives a 3-wire input from the entrance head. The black wire is attached on the left and the red wire on the right. Neutral goes right through unbroken, as shown in Fig. 4.

Fig. 4. The electric meter itself is supplied by the power company, but the electrical contractor is responsible for installing the meter socket.

The meter device is a very slow moving electric motor. It turns faster as more current flows, and slows down as current consumption is reduced.

The meter is simple to read. You can record each number from left to right. The meter is read periodically, usually once a month.

Entrance Head

The entrance head, or weatherhead, in Fig 5, is a connecting device for the service entrance conductors after they are spliced to the service drop from the electric company.

Fig. 5. The entrance (weatherhead) is a part of the service entrance. It connects between the service drop and the meter by cable or raceway.

The service entrance conductors extend from the end of the electric company service drop, through the entrance head, through the meter, and into the service panel, ending at the main disconnect.

When installing the entrance head, a great deal of thought is given to keeping water and weather out off the conduit.

The entrance cable from the weatherhead to the service drop must be at least 36 in. long. The black wire should be on the the left and the red wire on the right. The neutral is in the center of the head.

The weatherhead has to be above the top insulator of the incoming service drop. That way, water running down the service drop cannot enter the weatherhead.

Service Drop

The service drop is so called because the wires from the electric company usually take a downward drop from the electric pole to the weatherhead. 

The service drop belongs to the electric company and is installed by them. The National Electrical Code (NEC) recommends how and where a service drop is to be installed.

For example, service drops have to have a clearance of not less than 3 ft from the highest points of roofs under certain circumstances.

Service drops can be installed on the sides of buildings or to a mast holding the entrance head  as in Fig. 6.

Fig. 6. The service drop is designed to be safely installed above different kinds of areas. A mast-type riser is often required to achieve minimum mounting heights.

While most illustrations show the drop ending in three separate wires, the three wires can be wrapped together across the drop to give a neat appearance and to make the wires less exposed to weather damage. The resulting cable is called triplex.

The service entrance wires are attached to the service drop wires with strong, bolted connections. At the bottom of the drip loop, a tiny notch is sometimes made in the insulation. That way, when there is heavy rain, the drip loop cannot possibly develop a siphon effect and cause water to run upward into the weatherhead and conduit.

Service Grounding

You've probably heard the term ground, neutral, and common. The term "ground" means that the ground we live on is a reference potential for almost every electric circuit.

All conductors in electric cables have their insulation color coded. Black and red insulation is used on the "hot" wires, and white insulation is used on the neutral wire. The neutral is usually attached to ground at the service panel. By "ground" is meant the true earth ground.

The electrical code also requires that one current-carrying wire of the electrical system must make a good, permanent electrical connection to ground. This, of course, is the neutral or common wire.

Ground is at a potential of zero volts for every circuit.

Exposed metal in an electrical system that is correctly grounded is safe to touch.

Grounding must take place at the service entrance, as illustrated in Fig. 7. The neutral strip in the service panel must be grounded.

Fig. 7. The service-entrance neutral conductor must be grounded correctly to a true earth ground, usually a water pipe or a special grounding electrode.

There are two general methods of grounding the service entrance:

1. One method involves the use of a No.4 wire, safely hidden from mechanical injury, which is attached to a cold-water pipe in your plumbing system. (See Fig. 8.)
2. The second method of grounding, may be typical for a farm, where lightning strikes are more common and the ground has to function as a lightning arrester. (See Fig. 9). A grounding conductor is then attached to the rod with the correct ground clamp and run to the neutral overhead wire on the drip loop.

Fig. 8. In an urban area the ground must lead to the street side of the water meter and be protected carefully from any mechanical injury.

 Fig. 9. On a farm, due to its isolation and the greater possibility of lightning strikes, the ground system usually has separate grounding rods.

*To sum up, the electric service entrance consists of the circuits that handle the power company's entry into the home. It includes the entrance cables from the service drop splice, the weatherhead, the kilowatt-hour meter, the service entrance disconnect, and the grounding components.