Article 210—Branch Circuits; Article 240—Overcurrent Protection; Article 250—Grounding and Bonding; Article 300—Wiring Methods; Article 356—Liquidtight Flexible Nonmetallic Conduit: Type LFNC; Article 358—Electrical Metallic Tubing: Type EMT; Article 424—Fixed Electric Space Heating Equipment
Q: What methods are permitted in the National Electrical Code (NEC) for grounding 15- and 20-ampere, 125-volt receptacles?
A: Grounding type 15- and 20-ampere, 125-volt receptacles are permitted to be grounded by different methods, depending on the type of box (metal or nonmetallic), the location of the box in the structure, and the design of the receptacle.
Where the metal box is surface mounted, direct metal-to-metal contact between the receptacle yoke and the box is acceptable for grounding the device, provided that at least one insulating washer is removed from the receptacle. This method of grounding is found in 250.146(A).
For flush-mounted metal device boxes, receptacles with contact devices or yokes designed and listed as self-grounding along with supporting screws are acceptable as a means of grounding receptacles [see 250.146(B)].
Wiring methods that include equipment-grounding conductors must be spliced and connected to the receptacle and metal outlet box so that removal of the receptacle does not interrupt the grounding continuity. This requirement is in 250.148(B).
Where nonmetallic outlet boxes are involved, the equipment-grounding conductor(s) must be arranged so that a connection can be made to the grounding terminal on the receptacle. The Code reference is 250.148(D).
Detached building grounding
Q: Is a grounding electrode required at a detached garage for a dwelling unit that is supplied by a 20-ampere, multiwire branch circuit?
A: A grounding electrode at the garage is not required for the multiwire branch circuit, provided that an
equipment-grounding conductor is part of the branch circuit.
The exception to 250.32(A) removes the requirement for a grounding electrode at the garage. It states, “A grounding electrode shall not be required where only a single branch circuit supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the conductive non-current-carrying parts of equipment. For the purpose of this section, a multiwire branch circuit shall be considered as a single branch circuit.”
Rangehood cord-connected fans
Q: Our firm just received a contract to wire 16 single-family buildings in a subdivision. The plans show a duplex 15-ampere receptacle on a 15-ampere branch circuit to supply a kitchen exhaust fan above the range. The general contractor has asked for the duplex receptacle for a future built-in microwave oven. Does the NEC permit this?
A: Yes, it does, provided that the receptacle is installed more than 20 inches above the kitchen countertop. Because the 20-ampere, kitchen countertop appliance branch circuits are limited to a height of 20 inches above the countertop, any outlets above this height are not considered to be countertop receptacles by 210.52(B)(5) and are not required to be connected to 20-ampere branch circuits.
Rangehood exhaust fans may be cord-and-plug connected by the use of a cord-connection kit. Underwriters Laboratories Inc. classifies these kits. Rangehood cord-connection kits contain installation instructions along with the marking, “Rangehood Cord-Connection Kit for Use with Listed Rangehood Specified in Markings on the Packaging.”
To comply with 210.23(A), the full load current of the exhaust fan cannot exceed 7.5 amperes, and the total cord-and-plug connected load cannot exceed 15 amperes.
Tap conductor overcurrent protection
Q: Am I permitted to use a 400-ampere overcurrent device for protection of a tap that is 21 feet long with conductors that are 500 kcmil copper with Type THWN insulation? I rounded up to the next larger standard overcurrent device as permitted by 240.4(B).
A: According to Table 310.16, the ampacity of 500 kcmil copper conductors with Type THWN insulation is 380. If these conductors were not used for a tap, 240.4(B) allows 400-ampere overcurrent protection. However, a change in the 2005 edition of the NEC prevents increasing the size of overcurrent protection for conductors that are used for taps.
The change appears in 240.21(B) and reads, “Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). The provisions of 240.4(B) shall not be permitted for tap conductors.” The last sentence is new and places the restriction on increased overcurrent protection for tap conductors.
Disconnect for baseboard heaters
Q: May a single-pole, line-voltage thermostat that controls two baseboard heaters in a bedroom of a multifamily dwelling serve as the disconnecting means for the heaters? The branch circuit for these electric baseboard heaters is 20-amperes, 240-volts single-phase.
A: A single-pole, line-voltage thermostat is not recognized as a disconnecting means for the 240-volt baseboard heaters. To be considered an acceptable disconnecting means, the thermostat must open all ungrounded conductors of the branch circuit, it must be provided with a marked “off” position, and it must be designed so the circuit cannot be energized after the thermostat is placed in the “off” position. These requirements are in 424.20(A).
If a unit switch is provided on each baseboard heater by the manufacturer and the switch has a marked “off” position, it may serve as a disconnecting means provided that there is a disconnecting means within the dwelling unit or on the same floor that will disconnect the baseboard heaters.
The disconnect for baseboard heaters without motors, or motors rated not over one-eighth horsepower, may be the branch-circuit switch or circuit breaker when within sight of the heaters, or it must be capable of being locked in the “off” position when out of sight. For additional information, see 424.19.
Securing and supporting EMT
Q: I have to install electrical metallic tubing (EMT) above a lay-in ceiling in a commercial building. I plan to install support wires from the building structure. May I secure these support wires to the lay-in ceiling grid?
A: I assume you will secure the EMT to the wires that you install from the building structure. The other end of these wires will be secured to the ceiling grid. It will stabilize the EMT and prevent it from moving because the support wires are secured at both ends. Article 300.11 permits this installation method. According to 300.11, “Support wires and associated fittings that provide secure support and that are installed in addition to the ceiling grid support wires shall be permitted as the sole support. Where independent support wires are used, they shall be secured at both ends. An independent means of secure support shall be provided and shall be permitted to be attached to the assembly. Where independent support wires are used, they shall be distinguishable by color, tagging or other effective means from those that are part of the fire-rated design.”
Electrical metallic tubing must be securely fastened every 10 feet and within 3 feet of each termination unless modified by the exceptions to 358.30.
Q: Does a length of liquid-tight flexible nonmetallic conduit (LFNC) Type B require any intermediate support when less than 6 feet long? It is used as the wiring method between an outdoor disconnect and air conditioner.
A: Yes, the conduit must be secured every 3 feet and within 12 inches of terminations. However, liquid-tight flexible nonmetallic conduit cannot be used where it is subject to physical damage, outdoors unless marked “outdoor,” for ambient temperature, or conductor insulation exceeding 60°C unless marked with a maximum temperature rating.
Disconnection of branch circuits
Q: Multiwire branch circuits in dwellings are required to have simultaneous disconnection of all ungrounded conductors. Why is this requirement limited to dwellings?
A: I suspect you are using the 2002 edition of the NEC because the 2005 edition requires simultaneous disconnection of the ungrounded conductors in all occupancies. This is a change that was made in the 2005 NEC and is in 210.4(B). Also, 210.7(B) requires simultaneous disconnection of the ungrounded conductors where two or more branch circuits supply devices or equipment on the same yoke.
Fluorescent luminaires supplied by flexible cords
Q: We plan to install single-tube fluorescent luminaires 10 feet above the aisles of a warehouse building where the ceiling is 18 feet above the floor. The luminaires will be suspended from the ceiling with stranded steel cable, and 18 AWG fixture wire will be secured to the cable with tie-wraps. The outer end of the fixture wire will terminate in a twist-lock attachment cap that will plug into a twist-lock receptacle on the ceiling. The branch circuits are 20-ampere, 120-volts. Does this proposed installation comply with the NEC?
A: Cord-connected fluorescent luminaires are permitted by 410.30(C) where all of the following restrictions are met: The luminaire is located directly below the outlet, the flexible cord is visible for its entire length, the flexible cord is not subject to strain or physical damage, and the cord is terminated in a grounding-type attachment plug.
The minimum flexible cord conductor size is 18 AWG. Article 240.5(B)(2) permits this size for lengths less than 50 feet. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.