Article 110 Requirements for Electrical Installations
Article 210 Branch Circuits
Article 250 Grounding
Article 310 Conductors for General Wiring
Article 404 Switches
Article 408 Switchboards and Panelboards
Article 670 Industrial Machinery
Branch circuit for refrigerator in dwelling unit
Q: A 15-ampere, 125-volt branch circuit is installed for the refrigerator in the kitchen of a dwelling unit in an apartment building. Is this circuit allowed to serve any other load in the dwelling unit? Must the circuit terminate in a single receptacle or is a duplex receptacle acceptable?
A: According to exception No. 2 to 210.52(B)(1), an individual branch circuit with an ampere rating of 15 or more is permitted to supply refrigeration equipment in the kitchen of a dwelling unit. This is the way the exception reads: “Exception No. 2: The receptacle outlet for refrigeration equipment shall be permitted to be supplied from an individual branch circuit rated 15 amperes or greater.” Notice that the word “individual” is used to describe the branch circuit. And the definition of an individual branch circuit is: “A branch circuit that supplies only one utilization equipment.” These words lead to the conclusion that a single receptacle (not a duplex) is required. However, in response to a proposal to revise the 1999 National Electrical Code (NEC) language by substituting “A single receptacle...” for “The receptacle outlet...” the Code Making Panel rejected the proposal with this statement: “The panel has attempted to be reasonable with the exception and not require that a single receptacle be used.”
“The panel notes that the exception permits an additional circuit to supply the refrigerator. It is recognized that frequently the receptacle is indeed located behind the refrigerator, making the single-receptacle requirement overly burdensome. Should the outlet be close to or above the countertop, it would not count as the required countertop outlet by 210-52(c) and another outlet would be required to be installed and connected to the small appliance branch circuit.”
Although the words in the Exception and the definition for an individual branch circuit require a single-receptacle outlet, a duplex receptacle on a 15-ampere branch circuit that is installed on a wall behind a refrigerator will generally not supply more than one appliance or utilization equipment.
If the electrical inspector agrees with Code Making Panel No. 2’s statement and allows a duplex receptacle to be installed for the refrigerator in the kitchen, other loads can be supplied from this branch circuit. If he/she requires a single-receptacle outlet on this circuit, other loads cannot be served.
Q: Is sprinkler piping for fire suppression permitted to be run in or above the dedicated space for a 208Y/120-volt, 800-ampere service switchboard?
A: No foreign piping for any purpose is permitted in the 6 feet of dedicated space above an indoor switchboard. Part of 110.26(F)(1)(a) reads like this: “The space equal to the width and depth of the equipment and extending from the floor to a height of 1.8 meters (6 feet) above the equipment or to the structural ceiling, whichever is lower, shall be dedicated to the electrical installation. No piping, ducts, leak protection apparatus, or other equipment foreign to the electrical installation shall be located in this zone.”
The only foreign item permitted in this dedicated space is a suspended ceiling with removable panels. Sprinkler piping is permitted above the dedicated space provided that protection is installed to protect the electrical equipment from leaks, breaks or condensation.
Loads on a high-impedance grounded transformer
Q: One of the buildings in an industrial complex is supplied from a 500-kVA, three-phase, 480Y/277-volt, high-impedance grounded transformer. Is it permissible to use 480-volt, single-phase branch circuits from this transformer to supply HID luminaires (lighting fixtures) on poles for parking lot lighting?
A: There are restrictions on the use of a high-impedance grounded system. Among other things, line-to-neutral loads are not permitted on high-impedance grounded neutral systems. Where these systems are permitted and how they are to be installed are covered by 250.36.
HID luminaires must be suitable for pole mounting and have a voltage rating of 480. Circuit breakers must also be rated for 480 volts. Circuit breakers with a 277/480 rating are not acceptable. If the circuit breakers are used to switch the lighting on and off, they must be marked “HID.” The luminaires must be mounted at least 22 feet above the parking lot surface to comply with 210.6(d)(1)a. These are some of the items to be considered before attempting this installation.
Connecting grounded conductors in a panelboard
Q: Is there any rule in the NEC that prevents me from terminating two neutral conductors under the same screw on a neutral bus in a panelboard? How about a 12 AWG neutral and a 12 AWG equipmen-grounding conductor under the same screw on an equipment ground bus in a service panelboard?
A: It is my understanding that there are some panelboards that are marked to allow two equipment-grounding conductors to be terminated under a single screw, but there is not any marking on a panelboard that would allow two grounded circuit conductors to be terminated in a single hole in a neutral bus.
Markings on panelboards, load centers, etc., indicate that grounded circuit conductors must be individually terminated on the neutral bus. Doubled-up neutral conductors are not permitted.
Although the UL Standard for panelboards requires an individual terminal for each branch circuit neutral conductor, it is not uncommon to see two neutral conductors connected to the same neutral terminal. This is a violation of 110.3(B) and 408.21. This last reference is new in the 2002 edition of the NEC and reads: “Grounded Conductor Connections. Each grounded conductor shall terminate within the panelboard in an individual terminal that is not also used for another conductor.” An exception allows parallel grounded conductors to terminate in a single terminal if the terminal is identified for connection of multiple conductors.
Installing grounding-type receptacles
Q: Is it necessary to remove the fiber or thin cardboard screw retainers from receptacles and switches before installing them in surface-mounted outlet boxes? I believe that an adequate ground or bond is established with the device strap by the underside of the head of the mounting screw and the outlet box, since the screw makes metal-to-metal contact with the device strap and the screw threads are in contact with the box. The local electrical inspector does not agree with this theory and wants the screw retainers removed before installation. What is your opinion?
A: I agree that metal-to-metal contact exists between the underside of the metal screw head and the device strap where the device screws are fully tightened, but the cardboard or plastic retainers can collapse over a period of time because of the pressure on them that is caused by the tight device screws. This could result in loose device mounting screws and inadequate grounding of the receptacles grounding terminal.
Metal-to-metal contact is recognized as an acceptable method for grounding receptacles in 250.146(A). This is the way part of this rule reads: “Where the box is mounted on the surface, direct metal-to-metal contact between the device yoke and the box shall be permitted to ground the receptacle to the box.” I think these words mean that the receptacle yoke must be in direct contact with the box; therefore, the nonmetallic retainers should be removed from receptacles before installation.
The requirements for grounding snap switches are found in 404.9(B) and differ from those for receptacles, since there is no requirement for a surface-mounted box, and metal-to-metal contact is not mentioned. Here is the part that is of interest: “Snap switches shall be considered to be effectively grounded if...the switch is mounted with metal screws to a metal box or to a nonmetallic box with integral means for grounding devices.” This allows the screw retainers on switches to remain in place during installation.
Machine tool wiring
Q: Where can I get some information on Machine Tool Wiring?
A: Machine Tool Wiring is listed as Type MTW in Table 310.13. This Type has a moisture-, heat- and oil-resistant thermoplastic insulation. It has an insulation temperature rating of 60 degrees Celsius in wet locations, and 90 degrees Celsius in dry ones. The “Application Provisions” column in Table 310.13 indicates that this wire is suitable for wiring of industrial machines as covered by Article 670 of the NEC and NFPA 79-Electrical Standard for Industrial Machinery. There are two types of construction: single conductors may have PVC insulation only, or PVC insulation with a nylon jacket.
Working space around a switchboard
Q: How many doors are required for an electrical equipment room that contains a main 2,000 ampere, 480-volt, three-phase circuit breaker, and four 600-ampere, three-phase, 480-volt feeder circuit breakers? The electrical room is 8 feet by 10 feet, and the switchboard will be installed along and against the 8-foot wall. There is over 6 feet of clear space in front of the switchboard. A single 32-inch by 6.5-foot door is provided on the opposite 8-foot-long wall at about the center of this wall. Is only one door required for this installation? Must the door swing outward and is panic hardware required?
A: With the switchboard on one side of the working space and no grounded objects or live parts on the opposite side of the working space, a minimum distance of 3 feet is required. This minimum working space is mentioned in Condition 1 in Table 110.26(A)(1). Assuming that this situation exists, one exit door is required to satisfy the rule in 110.26(C)(2)(b). There is no requirement for the door to swing outward or that a panic bar or pressure plate be provided on the door. A simple twist knob or lever is all that is needed. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.254.2132.