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SUBCOURSE EDITION EN5142 B US ARMY ENGINEER SCHOOL INSTALL ELECTRICAL BOXES INSTALL ELECTRICAL BOXES Subcourse EN 5142 EDITION B United States Army Engineer School Fort Leonard Wood, Missouri 65473 1 Credit Hour Edition Date: August 1999 SUBCOURSE OVERVIEW This subcourse is part of the Electrician, Military Occupational Specialty (MOS) 51R, Skill Levels 1 and 2 course. It is designed to provide the knowledge necessary to identify and install various common types of electrical boxes, cover plates, and special devices. It also addresses how to calculate the number of conductors allowable in an electrical box. This subcourse is presented in one lesson that corresponds to the terminal learning objective as indicated below. Work must be accomplished in a manner consistent with environmental laws and regulations. There are no prerequisites for this subcourse. This subcourse reflects the doctrine that was current when this subcourse was prepared. In your work, always refer to the latest official publications. Unless otherwise stated, the masculine gender of singular pronouns is used to refer to both men and women. TERMINAL LEARNING OBJECTIVE: ACTION: You will learn to identify and install various common types of electrical boxes, cover plates, and special devices. You will also learn how to calculate the number of conductors allowable in an electrical box. CONDITION: You will be given the material in this subcourse and an Army Correspondence Course Program (ACCP) examination response sheet. STANDARD: To demonstrate competency of this task, you must achieve a minimum of 70 percent on the subcourse examination. i EN5142 TABLE OF CONTENTS Section Page Subcourse Overview....................................................................................................................................i Lesson: Electrical-Box Installation ............................................................................................................1 Part A - Electrical Boxes................................................................................................................2 Part B - Electrical-Box Uses and Electrical Connectors ...............................................................6 Part C - Electrical-Box Conductors ............................................................................................11 Part D - Electrical-Box Covers ...................................................................................................15 Practice Exercise .....................................................................................................................................17 Answer Key and Feedback......................................................................................................................22 Appendix A: List of Common Acronyms.............................................................................................A-1 Appendix B: Recommended Reading List ............................................................................................B-1 Appendix C: Metric Conversion Chart ................................................................................................C-1 EN5142 ii THIS PAGE IS INTENTIONALLY LEFT BLANK. EN5142 iv LESSON ELECTRICAL-BOX INSTALLATION Critical Task: 051-246-1111 OVERVIEW LESSON DESCRIPTION: In this lesson, you will learn to identify and install various electrical boxes, cover plates, and special devices. You will also learn how to calculate the number of conductors allowable in an electrical box. TERMINAL LEARNING OBJECTIVE: ACTION: You will learn to identify and install electrical boxes, cover plates, and special devices. You will learn their installation procedures according to the NEC. You will also learn how to calculate the number of conductors allowable in an electrical box. CONDITION: You will be given the material contained in this lesson. STANDARD: You will correctly answer all practice-exercise questions at the end of this lesson. REFERENCES: The material contained in this lesson was derived from STP 5-51R12-SM-TG, FM 5- 424, and the NEC Handbook. INTRODUCTION When an electrical circuit is installed, it will begin and end in an electrical box. Electrical boxes are constructed of various types of materials and come in a variety of sizes and shapes. Electrical boxes can be installed in many different ways and places for a variety of uses. After the boxes are installed, conductors are installed in the boxes. All electrical boxes must be covered when the work is completed. 1 EN5142 PART A - ELECTRICAL BOXES 1. Types. a. There are many types of electrical boxes. They are made of either metallic or nonmetallic (NM) material. The metallic box is widely used by the Army. The NM box is becoming more popular because it is less expensive and the installation of conductors is easier and faster than with the metallic box. Figure 1 shows a NM, two-gang box. Figure 1. Nonmetallic, two-gang box b. Electrical boxes come in many shapes, sizes, and depths (Figure 2). Box depths range from 1/2 inch to 6 inches. Boxes of a special size and depth can be ordered. Figure 2. Common metallic boxes c. When required, common metallic boxes can be ganged together to make the box wider (Figure 3). Boxes to be ganged must be of the same type and have a removable side. If a box is not deep enough, extension rings can be added to increase the cubic-inch capacity as needed (Figure 4. Extension rings are attached to a box by tightening the screws. 2 Figure 3. Box ganging Figure 4. Extension ring 2. Mounting. a. Electrical boxes are mounted in several different ways. Some boxes come with the mounting brackets attached. The side-mounting box is attached to a stud with nails or screws (Figure 5). Figure 5. Side-mounting boxes b. The front-mounting box is attached to a stud by hammering the spikes of the mounting bracket into the stud (Figure 6, page 4). This procedure is known as setting the box. Nails and screws are also driven through the mounting bracket to ensure that the box is securely mounted to the stud to prevent movement. 3 EN5142 Figure 6. Front-mounting boxes c. An adjustable bar hanger is designed for installation between wall studs, ceiling joists, or floor joists. The bar hanger is nailed or screwed to studs or joists, then a box is mounted to the bar hanger (Figure 7). NOTE: Do not use a bar hanger if anything heavier than a light fixture is to be applied; for example, do not use a bar hanger for a ceiling fan. Figure 7. Adjustable bar hanger d. Electrical boxes must be mounted so that the outer edge of the box will be flush with the finished surface of the wall. Some boxes come with thickness gauges stamped on them (Figures 8 and 9). Align the gauge against the side of the stud (Figure 9). Move the box in or out, depending on the thickness of the proposed finished surface. Nail or screw the box to the stud. EN5142 4 Figure 8. Gauged boxes Figure 9. Gauge alignment e. Electrical boxes installed in walls constructed of noncombustible material can be mounted up to 1/4 inch back from the finished surface. Electrical boxes mounted in walls containing combustible materials must be mounted flush with the finished surface. f. All electrical boxes (including junction boxes) should be mounted so that the conductors are accessible. Wall covering should never cover boxes. g. Electrical switch boxes may be mounted 46 to 52 inches from the top of the box to the finished surface of the floor. Outlet boxes may be mounted 12 to 18 inches from the bottom of the box to the finished surface of the floor. 5 EN5142 PART B - ELECTRICAL-BOX USES AND ELECTRICAL CONNECTORS 3. Uses. Electrical boxes are used to mount a variety of devices such as switches, light fixtures, and receptacle outlets (Figure 10). Special-purpose rings (Figure 11) can be added to a box to give it additional volume and versatility as to what type of device can be installed inside the box. Electrical boxes are also used as junction boxes (Figure 12). A junction box is used when a circuit splits into two or more directions and when splicing of wires is necessary. Figure 10. Devices Figure 11. Special-purpose rings EN5142 6 Figure 12. Junction box 4. Connectors. a. Electrical conductors/wires are connected to a box with connectors. Connectors come in different sizes and shapes and are used to secure all types of cable or conduit to the box (Figures 13 through 15, pages 7 and 8). NOTE: All boxes have a limit to the number of conductors that can be installed in them. These limits are explained in Part C, page 11. Figure 13. Nonmetallic-sheathed cable connectors 7 EN5142 Figure 14. Flexible metal conduit and armored cable connectors Figure 15. Conduit connectors b. Always use the appropriate connector for the type of cable or conduit being used. The design of the box also dictates the type of connector to be used. When using a round ceiling pan (round electrical box), never use conduit or connectors that require a locknut or bushing on the round surface of the box (Figure 16). EN5142 8 Figure 16. Round ceiling pans c. Use connectors on the flat back part of the box, not on the round sides of the box. Conduit and connectors are not used on the round surface because there would not be a good, tight connection between the bushing and the box or between the locknut and the box (Figure 17). Figure 17. Incorrect conduit connection d. Never leave knockout holes open in an electrical box; seal them with a knockout plug (Figure 18). The item marked A is pushed into the hole. The item marked B is installed by placing it half inside the box and half outside the box and then securing the two pieces together with a screw through the middle of the fixture. Figure 18. Knockout plugs 9 EN5142 e. Box-hanging straps are mounted to building studs to support electrical boxes that accommodate lighting devices (Figure 19). If the box is too deep, you may have to use a fixture extension. Figure 19. Lighting outlet box EN5142 10 PART C - ELECTRICAL-BOX CONDUCTORS 5. Conductor Count a. When all boxes are installed, installation of conductors can begin. A common problem that arises is how to determine the number of conductors that can be installed in a box. This problem is easily solved by consulting the NEC Handbook. A sample NEC table is shown in Table 1. This table shows the number of conductors that can be installed in a box, based on the American Wire Gauge (AWG) classification of the wire. Table 1. Sample NEC table b. Other factors also affect the number of conductors that can be mounted in a box. Different size conductors require different amounts of free space. This free space is measured in cubic inches. Table 2 is a sample NEC table showing the amount of free space required per conductor, based on the largest size conductor in the box. The number of conductors permitted is reduced by one for each fixture stud, fixture extension, internal connector, or clamp (Figure 20, page 12). Reduce the number of conductors by two for each switch or receptacle. No allowance is required for a connector with a connecting device outside of the box. Locknuts and bushings do not count. Table 2. Sample NEC table showing the volume required per conductor 11 EN5142