Branch Circuits, Overcurrent Protection, Grounding, and More

CODE CITATIONS: Article 100—Definitions; Article 210—Branch Circuits; Article 230—Services; Article 240—Overcurrent Protection; Article 250—Grounding; Article 347—Rigid Nonmetallic Conduit; Article 430—Motors, Motor Circuits, and Controllers; and Article 702—Optional Standby Systems Grounding electrode conductor connection point Q: Does the National Electrical Code (NEC) permit me to connect the supplementary ground rods for residential and small commercial services to the interior metal water pipe? There is more than 10 feet of buried metal water pipe outside of the buildings. A: The grounding electrode system is covered by Section 250-50 in the NEC. Where the buried metal water pipe outside of the building is at least 10 feet long, it must be used as a grounding electrode. Part (a)(2) of Section 250-50 requires that the water pipe be supplemented with an additional grounding electrode and states that a supplemental grounding electrode that is a pipe, rod, or plate must have a resistance to earth of not more than 25 ohms. If a single grounding electrode’s resistance exceeds 25 ohms, an additional pipe, rod, or plate, or any of the grounding electrodes mentioned in Section 250-50(b)(c) or (d) must be provided. If two 8-foot ground rods are necessary because the resistance of one exceeds 25 ohms, the rods must be driven at least 6 feet apart. The grounding electrode conductor from the rod(s) does not have to be larger than No. 6 copper, and may be connected to the grounding electrode conductor that is connected to the water pipe, the grounded service-entrance conductor, and the nonflexible grounded service raceway, or any grounded service enclosure. Although it appears that the wording in Section 250-50 permits the grounding electrode conductor for the rod(s) to be connected to the water pipe, part (a)(2) of the same Section does not mention a water pipe as an acceptable connection point for the grounding electrode conductor that connects to the rod(s). The first sentence of the second paragraph in Section 250-50 reads as follows: “An unspliced grounding electrode conductor shall be permitted to be run to any convenient grounding electrode available in the grounding electrode system or to one or more grounding electrode(s) individually.” However, part (a)(2) of this Section has the title: “Supplemental Electrode Required,” and contains this sentence: “The supplemental electrode shall be permitted to be bonded to the grounding electrode conductor, the grounded service-entrance conductor, the nonflexible grounded service raceway, or any grounded service enclosure.” Notice that water pipe is not mentioned. Although not required by the Code, to provide a more reliable ground for the neutral from the supplementary electrode(s), the grounding electrode conductor should be connected to the neutral bus in the meter socket or the neutral or grounding bus in the service disconnecting means. Optional standby systems Q: Is it right for an electrical inspector to use the requirements in Article 702—Optional Standby Systems for a generator set that is portable and not permanently installed? A: It is not proper to use Article 702 for standby systems that are not permanently installed. The scope of Article 702 reads like this: “The provisions of this article apply to the installation and operation of optional standby systems. “The systems covered by this article consist only of those that are permanently installed in their entirety, including prime movers.” The scope of Article 702 was written this way because Section 250-34 has requirements for portable and vehicle-mounted generators and Code Making Panel No. 15 did not want to create a conflict. The 1996 edition of the NEC contained a requirement for transfer equipment in Section 230-83 which was deleted in the 1999 edition. This left a void in the Code, since Section 230-83 required a transfer switch that would disconnect all ungrounded conductors of one supply source before the ungrounded conductors of the second source were connected. There were seven proposals to change the wording in Section 702-1. All of the proposals suggested deleting the words “permanently installed in their entirety, including prime movers.” The Code Making Panel unanimously accepted this revised second paragraph to Section 702-1: “The systems covered by this article consist only of those that are permanently installed in their entirety, including prime movers, and those that are arranged for a connection to a premises wiring system from a portable alternate power supply.” Overcurrent devices must be accessible Q: In a flea market open on weekends, the owner provides a 20-ampere, 120-volt receptacle at each stall. An individual branch circuit is used for this purpose. These branch circuits are supplied from panelboards at various locations. The doors on these panelboards are kept locked and the owner is the only one on the site with keys. Sometimes the owner does not arrive on the site until late in the afternoon. He is usually met by renters with no electricity in their booths. The owner then goes to the panelboards and resets the circuit breakers. Does this installation comply with Sections 230-72 and 240-24? A: No; it does not. Part (c) of Section 230-72 requires that the occupant’s service disconnect means be accessible to the occupant. An exception allows the service equipment to be inaccessible to a tenant where building management personnel are continuously on the premises. This is not the situation described here. Section 240-24(c) requires that each occupant have ready access to all overcurrent devices supplying that occupancy. However, ready access is not required where electrical maintenance is continuously provided under building management supervision. To comply with these two rules, the electrical equipment should not be locked while the flea market is open, or the owner must make arrangements to have electric maintenance personnel on the premises at all times that tenants (renters) occupy the flea market. Another section that deals with locked service overcurrent devices is Section 230-92. It reads as follows: “Where the service overcurrent devices are locked or sealed, or not readily accessible to the occupant, branch-circuit overcurrent devices shall be installed on the load side, shall be mounted in a readily accessible location, and shall be of lower ampere rating than the service overcurrent device.” Expansion joints in PVC conduit Q: Is an expansion joint required in a 55-foot length of rigid PVC conduit installed on an outside wall where the temperature varies between 25 degrees Fahrenheit in the winter to 95 degrees Fahrenheit in the summer? A: If this is a straight run between outlet boxes or other securely mounted enclosures, the answer is yes. Section 347-9 and Table 347-9(A) cover the requirements for expansion joints in rigid PVC conduit. For a temperature change of 70 degrees, such as a reduction from 95 to 25 degrees Fahrenheit, the expansion is 2.8 inches per 100 feet. For a 55-foot length, the change in length is (0.55 x 2.8) or 1.54 inches. Section 347-9 requires expansion fitting(s) where the calculated change in length exceeds 1/4 inch. Kitchen refrigerator branch circuit Q: If I installed a 15-ampere branch circuit in the kitchen of a dwelling unit to supply the refrigerator, am I allowed to extend this circuit to other parts of the house? A: Exception No. 2 to Section 210-52(b)(1) allows an individual branch circuit rated at 15 amperes to supply refrigeration equipment but nothing else. The key word in the exception is “individual.” An individual branch circuit is defined in Article 100 as one that supplies only one piece of utilization equipment. Section 210-21(b) mentions a single receptacle installed on an individual branch circuit and points out that the ampere rating of the receptacle cannot be lower than the ampere rating of the branch circuit. If you install a 20-ampere branch circuit rather than a 15, other receptacle outlets in the kitchen, pantry breakfast room, dining room, etc., can be supplied from this 20-ampere branch circuit. Sizing flexible cord for a motor load Q: I wired a 10 horsepower, 208-volt, three-phase motor with three No. 8 copper Type THWN conductors and a No. 10 equipment grounding conductor in 3/4-inch rigid nonmetallic conduit. For the last 3 feet of the branch circuit, I used No. 8 Type SO cord to prevent vibrations from the motor from being transferred to the conduit system. The electrical inspector has not objected to the use of flexible cord for this installation, but he says that the No. 8 type SO cord is too small. According to Table 310-16, No. 8 copper wire with 60 degrees Celsius insulation has an ampacity of 40 amperes. And according to Table 430-150, the full load current of a 10-horsepower, 208-volt, three-phase motor is 30.8 amperes. The branch circuit is protected with 50-ampere dual-element-time delay fuses. Am I missing something? A: Since the inspector agrees that Type SO cord can be used to prevent the transmission of vibration from the motor to the conduit system, this use of flexible cord is permitted under Item (7) of Section 400-7. However, the four-conductor cord (three current-carrying and an equipment grounding conductor) has an ampacity of only 35 amperes. This ampacity is shown in Column A of Table 400-5(A) and is based on the fact that there are not more than three current-carrying conductors in the multiwire cord or cable. According to Table 430-150, Section 430-6, and Section 430-22(a) the minimum motor branch circuit conductor ampacity is (30.8 +1.25) 38.5 amperes. This means that the No. 8 Type SO cord has to be increased in size to No. 6. 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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