Branch Circuit, Feeder, and Service Calculations; Overcurrent Protection; Grounding; Wiring Methods; and More

CODE CITATIONS Article 220––Branch Circuit, Feeder, and Service Calculations Article 240––Overcurrent Protection Article 250––Grounding Article 300––Wiring Methods Article 310––Conductors for General Wiring Article 373––Cabinets, Cutout Boxes, and Meter Socket Enclosures Article 380––Switches Article 410––Lighting Fixtures, Lampholders, Lamps, and Receptacles Appendix D, Example D3 Installing lighting track Q: As part of our electrical job for a new art museum, we are installing various lengths of lighting track in different locations. The maximum continuous length of track is 40 feet. What is the maximum length permitted on a 120-volt, 20-ampere branch circuit? How much load must be added to the service calculations for 280 feet of 20-ampere lighting track? A: A unit load of 150-volt amperes for every 2 feet of track must be added to the feeder and service calculations. Therefore, (280 x 150 divided by 2) 21,000 volt-amperes must be included in the calculations to determine the feeder and service capacity. Requirements for calculating lighting-track load for feeders and service equipment are mentioned in Section 220-12(b). Although Example D3 shows adding 25 percent to the unit lighting loads in Table 220-3(a) to obtain the load current and conductor ampacities for feeder and service for a store building, I do not believe the unit loads in Table 220-3(a) (that do not exist) should be considered as continuous loads. For this reason, I did not add 25 percent to the lighting-track calculations. It also should be pointed out that the examples appear in Appendix D, which is not an enforceable part of the National Electrical Code (NEC). There is no restriction on the length of track that can be supplied from a 20-ampere, 120-volt branch circuit. Where it is known that 40 feet of track will not be loaded to more than 16 amperes (track lighting in a museum is considered to be a continuous load), a single 20-ampere branch circuit is acceptable. However, to take full advantage of the feeder and service capacity, limit the length to about 25 feet on each branch circuit. With this arrangement, 11 20-ampere branch circuits would be necessary. It is my understanding that some electrical inspectors interpret the 150-volt-ampere for every two feet of track calculation as limiting the length of lighting track on a 20-ampere branch circuit to 25 feet or less. For this reason, you should find out in advance if there is a restriction on the length of track permitted on a branch circuit. Tap conductors Q: I have an 800-ampere, 208Y/120-volt fused service switch that is supplied by four—250kcmil aluminum-type THWN service-entrance conductors. There are four conductors per phase. This service switch feeds four 200-ampere-fused disconnects. May these feeder conductors originate in the service disconnect or must a wireway be provided? Do the rules for parallel conductors apply to service-entrance conductors? A: Section 373-8 has requirements for wiring space inside of switches. This section allows the switch enclosure to be used as a junction box or wireway where the contained conductors do not occupy more than 40 percent of the cross-sectioned area of the wiring space, and the conductors, splices, or taps do not take up more than 75 percent of that space. I assume the 200-ampere tap conductors will be 250kcmil aluminum with Type THWN insulation. The 800-ampere switch must be marked “Suitable for Use as Service Equipment” or “Suitable Only for Use as Service Equipment.” The wiring diagram must indicate which terminals in the switch are suitable for connection of aluminum conductors. This marking must be independent of any markings on the terminals or lugs. Four barrel lugs must be provided at all line and load terminals of the 800-ampere switch. Pressure-wire connectors (lugs) must be marked “AL” or “AL-CU” and have a range that includes 250kcmil. Each set of feeder conductors connected to the load side of the switch cannot exceed 10 feet in length. These conductors qualify under the 10-foot tap rule mentioned in Section 240-21(b)(1). These conductors must also satisfy additional requirements, such as: minimum conductor ampacity and installation in a raceway. See Section 240-21(b)(1) for all of the rules. Service-entrance conductors in parallel are treated the same as any other parallel conductor feeder or branch circuit. Rules for conductors in parallel are in Section 300-3(b) and Section 310-4. Most of the basic information needed for paralleling conductors is in Section 310-4. Expansion joints Q: Are expansion fittings required where electrical metallic tubing is run horizontally on a concrete block wall that has expansion joints? A: Allowance has to be made for expansion and contraction. Does an expansion fitting listed for the purpose have to be used? The answer is “No,” but means must be provided to allow for movement of the raceway system without pulling it apart at the couplings and connectors. One way to accomplish this is to use a short length of flexible metal conduit or liquidtight flexible metal conduit at the building expansion joint, allowing enough slack in the flexible conduit for it to move with the building structure. Because the flexible conduit is being used where flexibility is required, an equipment-grounding conductor must be provided to satisfy Sections 350-14 and 351-9. Since the length of flexible conduit used in this application is less than 6 feet, a bonding jumper may be installed on the outside of the conduit. The bonding jumper must closely follow the surface of the flexible metal conduit. If expansion fittings are used, Section 250-98 requires that they be made electrically continuous by equipment bonding jumpers or other means. Is a neutral conductor necessary? Q: On a 480-volt, three-phase service that supplies three-phase motors, is a neutral bus kit required in the service disconnecting means? What is the purpose of having a neutral where there are no phase-to-neutral loads? A: The utility-owned transformer is probably wye connected on the secondary with the midpoint grounded. If this is the way the secondary is connected, a grounded circuit conductor (neutral) must be included with the service-entrance conductors. This is required by Section 250-24(b). Also, this system grounded conductor cannot be smaller than the grounding electrode conductor required by Table 250-66. Where the ungrounded service conductors are larger than 1,100kcmil copper or 1,750kcmil aluminum, the grounded conductor cannot be smaller than 12.5 percent of the largest service-entrance phase conductor. At the service equipment, the system grounded conductor is connected to the grounding electrode system with a grounding electrode conductor. Where there are no phase-to-neutral loads, it is not necessary to include a neutral conductor in feeders and branch circuits. Should a phase-to-ground fault occur on a feeder or branch circuit, then fault current will flow on the accidentally grounded phase conductor to the grounding conductor back to the service equipment, to the main bonding jumper, then to the neutral conductor, and finally, to the secondary winding of the faulted phase conductor. Without the neutral conductor, the ground return path would be through the earth. Various Sections in the NEC do not recognize the earth as a low-impedence path for phase-to-ground fault current. If the system grounded conductor is not included in the service-entrance conductors on a grounded system, a phase-to-ground fault will probably result in energizing metal enclosures of the electrical systems and will not cause the opening of any overcurrent devices. This could be hazardous! Switches and lighting fixtures in bathrooms Q: Does the NEC require ground-fault circuit-interrupter (GFCI) protection for a switch located next to a shower stall? This switch controls a lighting fixture installed in the shower. A: There is no requirement for GFCI protection for the lighting fixture or switch. However, there are some rules that limit the types of lighting fixtures permitted in shower stalls. First of all, the fixture has to be marked “Suitable for Wet Locations” or, depending on the location of the fixture in the shower, “Suitable for Damp Locations.” These requirements appear in Section 410-4(a). Cord-connected fixtures, hanging fixtures, and lighting track cannot be located within 3 feet horizontally or 8 feet vertically of the shower stall threshold. The reference for these restrictions is part (d) of Section 410-4. The snap switch must be grounded, and it must provide a means to ground a metal faceplate, even though a metal faceplate is not installed. Where the switch is secured to a grounded metal box with metal screws, it is considered to be effectively grounded. Requirements for grounding snap switch faceplates are covered in Section 380-9. Section 380-4 prohibits the installation of a switch in a shower unless it is part of a listed tub or shower assembly. This is the way the second sentence in Section 380-4 reads: “Switches shall not be installed within wet locations in tubs or shower spaces unless installed as part of a listed tub or shower assembly.” Common equipment grounding conductor Q: When installing conductors in a raceway that are supplied from two different voltage systems, such as 208Y/120-volt and 480Y/277-volt, the Code requires neutrals of each system to be identified. One is required to be white or natural gray and the other white with a colored stripe that is not green. Is there any requirement for identification of the equipment grounding conductor for each different voltage system or is green permitted for both systems? May the metal raceway mentioned earlier serve as the equipment-grounding conductor for both voltages? What is the minimum insulation rating on the conductors for this application? A: Rigid metal conduit, intermediate metal conduit, and electrical metallic tubing are all acceptable as an equipment-grounding conductor where installed according to Section 250-120(a). Because the raceway contains conductors operating at over 250 volts to ground, additional bonding as outlined in Section 250-97 is required. The insulation on all of the conductors in the raceway must be rated for 600 volts. This requirement is in Section 300-3(c). FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.

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