The Mast and Main Service Panel

As a home electrician you will probably never have occasion to install a complete service panel yourself. However at some time you may need to wire a new branch circuit into an existing panel. To accomplish this successfully you need to know how to evaluate the adequacy of the panel and , if necessary, how to locate and install a sub panel or how to replace an existing but outmoded panel even when the rest of the wiring for the home is all right. You also need to know how to calculate the shortest, most efficient branch-circuit runs from the panel. For any of these tasks it pays to know how service panels are installed, at least in theory, and for general electrical safety it pays to know how they operate in relation to the entire home electrical system. This section will guide you through this phase of the work, emphasizing the techniques, materials, and tools needed to do the job.

Installing the wires in the service panel in a neat and orderly fashion helps to keep all the circuits organized.

THE MAIN CONDUCTORS

In wiring an entire house, the next step after rough wiring is installing the main service panel. This ubiquitous metal box is the place where the electricity coming into the house connects with the branch circuits. It is the common denominator in any wiring job, whether it's adding a single new circuit to an existing service or wiring a whole house.

The Service Drop and Service Lateral

Power from the electric utility company’s distribution system is brought to a house in one of two ways: either by overhead wires through what is called the service drop, or by under ground cables through what is called the service lateral. The service drop connects to the service-entrance conductors outside the house at a point near the top of the service conduit. The other ends of the service-entrance conductors connect to the top terminals in the meter socket. The service lateral conductors, although they enter the meter box from the bottom, nevertheless connect directly to the top terminals in the meter socket, except the neutral conductor, which runs unbroken to the neutral bus. From this point on, the two services are usually alike.

The terminals at the bottom of the meter socket connect to the service disconnecting means, and from there to the service overcurrent-protection device. In modern panels the disconnecting means and the overcurrent-protection de vice are usually combined in the main service breaker. This breaker supplies power to the hot buses in the panel to which the branch-circuit breakers or fuses are connected.

Overhead Service

Service-drop conductors, including the required drip loops, must be at least 10 feet above grade at the building and in areas accessible only to pedestrians, and 15 feet above residential driveways. Also, service conductors must be installed out of reach of windows, doors, and porches, which according to the Cede means beyond 3 feet. The area directly above a window is considered to be out of reach, so there is no restriction to wires above the top of a window.

The point at which the ser vice drop attaches to the building should be below the top of the service conduit. The 10- foot-clearance rule is a major factor used to determine how the service conduit will be in stalled. It’s often not possible to maintain this clearance on one- story houses without passing the service conduit through the roof. This type of installation is known as a low-roof service.

Low-roof service masts are al ways made of 2-inch rigid metal conduit because it has the strength necessary to sup port the service drop that attaches to it.

EMT (electrical metallic tubing) and rigid nonmetallic conduit, as well as rigid metal conduit, can be used when the service conduit doesn’t have to pass through the roof to pro vide the necessary clearance and instead the service drop attaches directly to the house. In this case, the conduit can be smaller than 2 inches; 1 conduit is large enough for 100- amp services.

Check with your power company before installing the service equipment to make certain its service drop will attach to your house where you want it to. This usually isn’t a problem unless the distance from the utility pole to the point on the house selected for the service is too great, or if for some reason the required vertical clearance can’t be maintained.

Usually the service conduit rises straight up from the meter socket, but sometimes it must be bent 90 degrees to reach the point at which the service drop attaches to the house. In rare cases, because of the presence of porches, windows, and other obstacles, the route the conduit must take from the meter to the service drop requires a number of bends. Remember that the total bends between conduit openings can’t exceed 360 degrees. If the installation re quires more bends than permit ted, use a pull box or C-conduit body in the middle of the run, if necessary, to provide a mid-run conduit opening and conform to the 360-degree requirement. Use the openings for pulling wire only; service- entrance conductors can't be spliced in a pull box.

The exact location of the ser vice conduit and meter socket is usually determined by the placement of the service panel. When the main circuit breaker is located in the service panel, the panel must be installed as near as possible to the point at which the service conductors enter the building. Service panels are commonly installed in basements or attached garages and , sometimes, on the outside of the house at a place where there will be sufficient working space around the panel and where it won’t be necessary to move or climb over obstacles in order to reach it. If the panel is to be located well inside the building, the main breaker, or fused disconnect, must be located outside the house so that all the wiring inside will be properly protected from overcurrents and is accessible for emergency shutoff.

Read The Meter and Main Circuit Breaker in the next section to help you decide where the main breaker and perhaps the service-entrance panel itself will be installed. The meter box will need to be in place before conduit is installed to ensure that the two connect properly.

Installing the Mast

Once you have installed the meter box and main breaker, measure from the meter socket to a point a little more than 13 feet above grade. Cut a piece of 1¼” rigid metal conduit to this length. (For through-the- roof installations that require 2-inch rigid conduit, keep reading.) Ream or file the inside of the cut end of the conduit. Threading isn’t necessary be cause the service-entrance head will simply slip over the conduit and will be held in place with setscrews or a clamp. Screw the weathertight hub onto the other end of the conduit.

Hold the conduit and hub centered over the top conduit opening of the meter enclosure. Plumb the conduit and make light pencil marks on the wall on each side of the conduit about 2 feet up from the socket and about 1 foot down from the top. Remove the conduit and fasten conduit hangers to the wall between the pencil marks, both top and bottom. Put the conduit back in place, making certain the gasket is in place between the hub and the enclosure, and bolt the hub to the meter box. Secure the conduit in the hangers with screws and nuts. Slip the service head onto the conduit and tighten it in place. Mount a wire hanger or rack to the side and 1 foot below the service head. If triplex cable will be used for the service drop, install a single triplex wire hanger. If the drop will use three separate conductors, you will need three separate insulated wire hangers. Install the top wire hanger 1 foot from the top of the mast and the others approximately 6 inches apart.

A low-roof service is in stalled in much the same way except 2-inch rigid metal conduit is used and the holes through which the conduit will pass must be accurately cut in the roof and soffit. Special roof flashing and a conduit boot are also necessary to do this job. Mark a plumb line that extends from the center of the socket hub to the soffit. Using a square, extend this line out to the edge of the soffit and make a pencil mark there. Then calculate the distance from the wall to the centerline of the conduit and mark this on the soffit with a line about 3 inches long that crosses the plumb-line extension. There is an easy way to determine how far out this will be. Simply measure from the back to the center of the conduit opening in the conduit hangers you are using.

71 A Typical Service drop (to your home)

Lining up the hole in the roof with the hole in the soffit can be done in several ways. An 18-inch-long 1 drill bit can be used to drill a pilot hole up through the soffit and roof. The drill must be held plumb side to side and front to back when this is done. Another method requires two people, very careful measuring, and a square and level. With this method, bring the plumb- line extension out to the edge of the roof by placing a square against the wall so that its other leg crosses the conduit center line. Mark the edge of the fascia here. Measure the distance from the centerline to the fascia. Hold a 2-inch-long piece of 2 by 4 with square-cut ends flat against the fascia so that the bottom edge lines up perfectly with bottom of the fascia and one end is lined up with the centerline mark. This puts the open face of the 2 by 4 beyond the shingles. Add 1½ inches to the measure you just made and mark this dimension on the level (or on a short piece of wood if the level isn't long enough). Next hold the square flat against the face of the 2 by 4 so that one leg is sticking straight up; eye it with the end of the 2 by 4 to make certain it's straight. Then hold the dimension mark on your level (or your board) against the upright leg of the square and level it where the far end touches the roof and the dimension mark touches the square. Hold the level or board as squarely as possible to the edge of the roof. Mark this spot by driving a nail partway into the roof. Drill 1½-inch pilot holes at the marks, both up and down.

Once the pilot holes have been drilled, the conduit holes can be cut out with a 2½-inch hole saw or dial saw. Don’t be too concerned if the two holes don’t line up perfectly; the top hole can be opened up with a saw if necessary.

Install one conduit hanger about 1 foot above the meter socket and one 1 foot below the soffit. Then slip the flashing up under the shingle above the hole (the bottom of the shingle may have to be cut out). Let the bottom of the flashing lap the next shingle down. Center the flashing over the conduit centerline. Measure the length of conduit needed to put the top of the conduit at least 13 feet above grade and about 3 feet above the roof. Cut the conduit and ream or file the cut end. Then pass it down, threaded end first, through the holes and hangers, and thread it tightly into the meter-socket hub. Install and tighten the hanger screws and then nail the flashing in place. Slip the boot over the mast and push it down as far as it will go. Install wire hangers as previously de scribed. Slip the service- entrance head onto the mast and lock it in place with the setscrews or clamp.

72 Mast Installation

Service Drop components: Weatherhead; Insulated 2” conduit mast through roof; 120v wires; Neutral wire rope; Drip loops; Hashing; Soffit; Upper hanger; Insulator; Meter; Meter socket; A center of conduit hole to edge of soffit; B = 1½” ; C=A+B ; D = upright leg of square ; E = dimension C, measured on level

Installing the Conductors

For connection to the service drop, 3 feet of conductor must extend out of the service head. Add this to the distance measured from the top of the mast, through the meter socket, to the main lugs of the panel. Add 3 feet for good measure, and buy two pieces of black wire and one piece of white wire cut to this length.

Take the cap off the service head. Tape the three wires together at one end arid push them up the mast until about 3 feet are sticking out of the top of the mast. Use a fish tape to pull the taped wires into the conduit when there are bends in the service conduit. Go to the top of the mast and bend the wires over and temporarily tape them to the mast. Return to the socket and cut the black wires—but not the white—so they’ll fit into the top set of lugs in the meter socket. Strip just enough insulation from each wire so that the bare conductors will fit entirely into the lugs. Insert the wires and tighten the lugs. After tightening, work the wires back and forth a bit and then tighten the lugs some more. The white wire fits into a “lay-in” neutral lug in the center of the socket. Remove a band of insulation from the wire that's just a bit wider than the lug is long, but do not cut the wire.

Tape the two remaining black wires, which were cut off the wires pushed up the service conduit, to the free end of the white wire and push all three through the nipple and into the panel. Strip the black wires and connect them to the bottom set of lugs in the meter socket. Then lay the white wire in its lug and tighten it. Push all the excess wire out of the socket enclosure into the panel. Route the wires into place in the panel, the black wires to the main breaker lugs, the white to the neutral bus. Strip and connect them, working them back and forth after tightening them, and then tightening them some more.

The cap of the service head has a plastic-like insert with knockouts in it. Break out three of these that are as far from one another as possible. Insert the white wire in the center opening and the black wires in the remaining openings. Slide the cap up the wires and over the service head, and tighten the two screws that hold it in place.

The service conduit must be both weathertight and arranged to drain. The service head is designed to be weather- tight, and a means of conduit drainage is accomplished by providing a 0.25-inch weep hole in the bottom of the meter s or SLB (service el conduit body) if one is used. Weathertight fittings are necessary when EMT is used as ser vice conduit. There’s always the possibility that condensation may occur in a conduit that runs between the warm indoors and cold outdoors, such as where a service conduit enters a house. Pack sealing com pound into the nipple, or SLB, to prevent the movement of air through the conduit.

Underground Service

Everything after the meter socket is the same for an under ground service as for an over head service, but installation of the conductors up to the meter socket is different. In many areas the power company brings the service-lateral cable across your property and connects it to the lugs at the top of the meter socket. Their trenching and cable installation is done mechanically and leaves almost no damage in the yard. It’s your responsibility to pro vide a piece of conduit for protecting the cable where it emerges from the ground. This conduit must be long enough to reach from the bottom of the meter socket to a point 24 inches below grade -- typically, 2-inch Schedule 80 PVC conduit is used here, but rigid metal conduit or IMC (intermediate metal conduit) may also be used. The pvc should have a threaded adapter solvent-cemented to both ends. A non metallic bushing is screwed onto the end that terminates in the ground, and a locknut and nonmetallic bushing must be left on the top end for attachment to the meter socket. This assembly is usually left next to the meter socket for eventual installation by power company employees. When metal conduit is used to provide this protection it, too, must have a nonmetallic bushing installed on the trench end and a lock nut and an insulated grounding bushing at the meter end.

Consider renting a trencher if it’s necessary for you to do your own trenching in order to reach the power company’s mains or transformer, contact the power company before be ginning. A representative will ask where you will be digging, and will tell you how far to go and what wiring method to use. Also, clear where you in tend to dig with other utilities such as the gas company, the telephone company, and the water department, in case they have pipes or wires buried in your pathway.

Service-lateral trenches dug across residential properties need be only 6 inches deep for rigid metal conduit and IMC, 18 inches deep for pvc conduit, and 24 inches deep for direct-burial conductors and cables. All of these wiring methods require an 18-inch-deep trench where they pass under a residential driveway. After the power company and other utilities have approved your trench route, roll up the sod, set it aside, and start digging. Keep the trench as straight as possible, maintain a uniform depth, and place the excavated dirt within a few feet of the trench. Make the trench the width of a shovel except at the ends, where it's best to make it a bit wider and deeper.

There’s nothing particularly difficult about installing conduit in a trench. The most important thing to remember is that no rocks or dirt can be present in the conduit. This means you should look through each length before installing it; and , when installing, be certain to keep the open end up out of the trench. Let the open end rest on a board or shovel laid across the trench as you go along. If for any reason an open conduit end must be left in the trench, cover the end with plastic and tape it up. You can keep groundwater from accumulating in your conduit by installing a drain tee at the low point of the conduit. A 4-inch- long nipple should be threaded into the tee and pointed down into a 1-foot-deep hole that has been filled with gravel.