Branch Circuits, Grounding, Flexible Metal Conduit, Motors, and Transformers

CODE CITATIONS Article 210—Branch Circuits; Article 220—Branch-Circuit, Feeder, and Service Calculations; Article 250—Grounding; Article 350—Flexible Metal Conduit; Article 410—Lighting Fixtures, Lampholders, Lamps, and Receptacles; Article 430—Motors, Motor Circuits, and Controllers; and Article 450—Transformers and Transformer Vaults (Including Secondary Ties) Number of outlets on a branch circuit Q: Is there any limit to the number of receptacle outlets, or the number of lighting outlets, permitted on a 20-ampere general-purpose branch circuit? Is there a difference in the requirements between dwelling units and commercial applications? A: To the answer the second question first, the answer is yes, there is a difference in requirements for dwellings and commercial buildings. Let’s look at some of the differences. The minimum lighting loads for various occupancies are shown in Table 220-3(a), and vary from a minimum of 0.25 volt-ampere per square foot for storage warehouses to a maximum of 3.5 volt-amperes per square foot for banks and office buildings. After the total lighting load is determined by Table 220-3(a), Lighting Load Demand Factors for some occupancies are permitted by Table 220-11. Also, Section 220-3(b) points out that the minimum load for each outlet for general-use receptacles and outlets not used for general illumination must be computed in accordance with items (1) through (11) of Section 220-3(b). For receptacle outlets in other than dwelling units, a load of 180 volt-amperes must be applied for every receptacle outlet with three or fewer receptacles mounted on a single strap. Where there are four or more receptacles installed on a single outlet box, each receptacle is assigned a load of 90 volt-amperes. For example, four receptacles are considered to be a 360 volt-ampere load. This requirement has the effect of limiting the number of 125-volt, 15- or 20-ampere duplex receptacles to 13 on a 20-ampere branch circuit or 10 duplex receptacles on a 15-ampere branch circuit. Although there is no restriction in the National Electrical Code (NEC) on the number of receptacle outlets that may be connected to a 20-ampere, or 15-ampere general-purpose branch circuit in a dwelling unit, there are some municipalities that limit the number of openings on general-purpose branch circuits in dwellings. The numbers are usually 13 for 20-ampere branch circuits and 10 for 15-ampere branch circuits. These are some of the significant differences in the requirements for 20-ampere general-purpose branch circuits installed in dwellings and for those installed in commercial occupancies. Transformer secondary overcurrent protection Q: The primary conductors for an indoor transformer are the service conductors for a building. I installed six circuit breakers on the secondary of the transformer to supply six feeders—one feeder for each floor. The inspector said I need a single main disconnect on the secondary side of the transformer. Is he correct in this interpretation? A: The inspector could be right or wrong, depending on other facts that are missing from the question. We need to establish the primary voltage, kVA rating of the transformer, ampere rating of the overcurrent protection, and percent impedance of the transformer. Let’s assume we have a three-phase dry-type transformer rated 150 kVA with 2.4kV primary and a 480Y/277-volt secondary. The transformer impedance is 5.5 percent. The primary full load current is (150,000) divided by (2,400 x 1.73) about 36 amperes. Table 450-3(a) allows a 100-ampere fuse to be used to protect the primary of the transformer. Also, this table and Note 2 to the table allows up to six circuit breakers to be used as the secondary overcurrent protection provided that the sum of the ampere ratings of the overcurrent devices does not exceed the value allowed for a single overcurrent device. The rated secondary current of the transformer is (150,000) divided by (480 x 1.73) 181 amperes. The Table (450-3(a)) allows the overcurrent protection to be increased to 125 percent of full load current; therefore, the sum of the ampere ratings of the overcurrent devices cannot be greater than 225 amperes. Four 30-ampere, three-pole circuit breakers and two 50-ampere, three-pole circuit breakers are one combination that does not exceed the ampere rating of the single overcurrent device permitted by Section 450-3(a) and the table. Since your job has been turned down by the electrical inspector, either the sum of the ampere ratings of the overcurrent devices exceeds the ampere rating allowed for a single secondary overcurrent device or all of the disconnects are not grouped at one location. Both of these requirements appear in Note 2 to Table 450-3(a). Although this size transformer is probably not adequate to carry the total load in a six-story building, it was used to make the calculations easier to handle. Replacing a standard motor with a Design E motor Q: Is it necessary to increase the horsepower rating of a 10 hp motor starter and disconnect switch when replacing a 10 hp, three-phase, 460 motor with a Design E motor of the same horsepower and voltage? A: Yes it is. The motor starter and disconnect switch must be replaced because of the different electrical characteristics of a Design E motor. Section 430-83(a)(1) covers the requirements for motor controllers and part of this paragraph reads as follows: “A controller for a Design E motor rated more than 2-hp shall (1) be marked as rated for use with a Design E motor, or (2) have a horsepower rating not less than 1.4 times the rating of a motor rated 3 through 100 hp, or not less than 1.3 times the rating of a motor rated over 100 hp.” Therefore, the controller will have to be replaced with one that is suitable for use with a Design E motor or one with a 15 hp rating. If the existing disconnecting means is a motor-circuit switch rated in horsepower, it will also have to be replaced. The switch requirements are similar to those for the motor controller, and are found in Section 430-109(a)(1). The switch can be replaced by one that is marked for Design E motors or by a 15 horsepower rated switch. Sports arena lighting Q: Am I permitted to use 70-ampere branch circuits with No. 4 Type THWN copper conductors to supply 1000-watt lighting fixtures in a sports arena? The full-load current for each fixture is four amperes and the voltage is 277. I plan to install 14 lighting fixtures on each circuit, which will limit the load to less than 80 percent of the branch circuit rating. A: The maximum ampere rating of a branch circuit that supplies lighting fixtures is 50 amperes. This restriction is located in Section 210-23(c) and (d). Part (c) recognizes 50-ampere branch circuits for supplying fixed lighting units with heavy-duty lampholders in other than dwelling units. Part (d) prohibits branch circuits larger than 50 amperes from supplying lighting equipment. Therefore, the maximum branch circuit rating cannot exceed 50 amperes, and the maximum number of lamps permitted on this circuit is 10. (The number of lamps permitted on a circuit is based on 80 percent of the circuit capacity, because lighting in the arena is considered a continuous load.) A heavy-duty lampholder is defined as one having a rating of not less than 660 watts if of the admedium type, and not less than 750 watts if of any other type. This information is found in Section 210-21(a). Bonding gas piping Q: What size of bonding conductor is required to bond the gas piping in a building? This requirement appears in part (b) of Section 250-104 and is new in the 1999 edition of NEC. A Notice that an “x” appears along with this change. Actually, this text is extracted from paragraph 3.14(a) of NFPA 54-1966 National Fuel Gas Code. Where it can be determined which circuit could energize the gas piping, I would use the ampere rating of that circuit to select the size of the bonding conductor from Table 250-122. Where there are a number of gas-fired appliances, or it is difficult to determine what wiring may energize the gas piping, the bonding jumper may be sized from Table 250-66. In the 1996 and previous editions of the NEC, all interior metal piping with the potential to become energized was required to be bonded by an equipment bonding conductor whose size was based on the ampere rating of the circuit that may energize the piping. In the 1999 Code this wire size is shown in Table 250-122, which was formerly Table 250-95. Since underground metal gas piping cannot be used as a grounding electrode, I see no reason why the bonding conductor cannot be sized according to Table 250-122 where the ampere rating of the circuit that may energize the piping can be determined. Wiring fluorescent fixtures Q: May I use 3/8-inch flexible metal conduit containing two No. 18 type TFN fixture wires from an outlet box to a fluorescent fixture? The branch circuit rating is 20 amperes. A: Yes, provided that the length of flexible metal conduit is not longer than six feet to comply with Section 350-10(a)(2)(c) and the tap conductors do not exceed six feet in length. This restriction on the length of the tap conductors appears in Section 410-67(c) and limits the length of 3/8-inch flexible metal conduit to about 5 feet for this application. Finally, Section 240-4(b)(2) permits No. 18 fixture wire to be tapped to a 20-ampere branch circuit, provided that the length of the run of No. 18 is not greater than 50 feet. 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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