Grounding of Receptacles, Overcurrent Protection for a Feeder and More

Article 110 Requirements for Electrical Installations

Article 250 Grounding

Article 310 Conductors for General Wiring

Article 312 Cabinets, Cutout Boxes and Meter Socket Enclosures

Article 430 Motors, Motor Circuits and Controllers

Article 518 Places of Assembly

Thermostat as motor disconnect

Q:May a thermostat located less than 5 feet away from a 1/6 hp attic exhaust fan be used as the disconnect to comply with 430.102? The branch-circuit overcurrent protection and disconnecting means are located on the floor below. They are out of sight of the motor and thermostat. The thermostat does not have marked “off” and “on” positions.

A:No, the disconnecting means is required to have a marked off and on position to qualify as a motor disconnect. Here is the NEC requirement: “430.104 To Be Indicating. The disconnecting means shall plainly indicate whether it is in the open (off) or closed (on) position.” For motors rated 2 hp or less and 300V or less, the disconnect is permitted to be a general-use switch with an ampere rating that is not less than twice the full load current rating of the motor or a general-use snap switch suitable only for use on AC circuits (not general use AC/DC snap switches) where the motor full-load current does not exceed 80 percent of the ampere rating of the switch. Permission for the use of these switches under the conditions mentioned appears in 430.109(C).

Overcurrent protection for a feeder

Q:A 3-wire, single-phase feeder is to be installed between a 200A service panelboard and a detached garage. The overcurrent protective device for this feeder is a spare 100A, two-pole circuit breaker. We intended to use three THWN copper conductors in nonmetallic conduit for this job. The load on this feeder is calculated at 38A. What is the minimum wire size that may be used with the existing 100A circuit breaker? Is it necessary to install an equipment-grounding conductor along with the circuit conductors?

A:Generally, conductors must be protected at their ampacities to comply with 240.4. However, there are seven subdivisions to 240.4 that allow overcurrent protection to exceed the ampacity of a conductor.

Without knowing the types of loads involved, and the temperature ratings of the terminations on the existing circuit breaker and panelboard in the detached garage, 2 AWG copper conductors should be used. If the wire terminations at both ends of the feeder are marked for 75 C, 3 AWG Type THWN copper conductors with an ampacity of 100 would be acceptable.

It is probably also necessary to insulate or isolate the grounded circuit conductor in the feeder from the panelboard in the garage. An equipment-grounding conductor must also be included in the feeder raceway. According to 250.32 and Table 250.122, the equipment-grounding conductor cannot be smaller than 8 AWG copper.

Working space around electrical equipment

Q: The electric service for an office building is being increased from 800A to 1,200A. The new service entrance switchboard is 96 inches wide and 30 inches deep. Clearance in front of the new switchboard complies with 110.26(A)(1). A single existing door into the room swings inward and has a lock and knob. Is it necessary to change the swing of the door and install panic hardware?

A:Because new service equipment is being installed, it must comply with the rules in effect, which I assume are in the 2002 edition of the National Electrical Code. The requirements in 110.26(C)(2) apply to electric equipment rated 1,200A or more that is more than 6 feet wide. Where the depth of the working space is 3 feet for a 208/120V distribution system, two doors are required at opposite ends of the switchboard. If the depth of the working space is 6 feet or more, only one door is required.

The door(s) must swing outward and be equipped with panic hardware. If the change in door swing obstructs a required exit passageway, walls and partitions will have to be relocated.

Nonmetallic sheathed cables installed in raceways

Q:Does the National Electrical Code permit the installation of nonmetallic sheathed cable in nonmetallic raceways? The cables would not be secured to the panelboard enclosure.

A:The NEC usually requires that cables be secured to enclosures, but there are exceptions. Here are some of them: “310.12 Mechanical Continuity—Raceways and Cables. Metal or nonmetallic raceways, cable armor and cable sheaths shall be continuous between cabinets, boxes, fittings or other enclosures or outlets. Exception: Short sections of raceways used to provide support or protection of cable assemblies from physical damage shall not be required to be mechanically continuous.” Part (C) of 310.15 reads like this: “Protection. A box or conduit body shall not be required where cables enter or exit from conduit or tubing that is used to provide cable support or protection against physical damage. A fitting shall be provided on the end(s) of the conduit or tubing to protect the cable from abrasion.”

Finally, there is a long and detailed exception to 312.5(C) which requires that each nonmetallic sheathed cable be secured to the cabinet, cut-out box or meter socket enclosure. However, nonmetallic sheathed cable is not required to be secured to the enclosure where all of these restrictions are satisfied: 1) The cables enter a raceway that is not less than 18 inches and not more than 10 feet long; 2) The enclosure is surface mounted and the raceway is directly above the enclosure; 3) A fitting is provided at each end of the raceway and they are accessible after installation; 4) The raceway is sealed at its outer end; 5) The cable sheath(s) is continuous and extends at least 0.25 inch into the enclosure; 6) The raceway is fastened at its outer end and at other points as required by the applicable article for the raceway; 7) Cable fill does not exceed that permitted for the conduit system as shown in Table 1 of Chapter 9. This is a condensed version of a long exception. Read it carefully to make sure you comply with all of the requirements.

Branch circuit conductor size for motor-operated appliances

Q:A motor-operated appliance has the following nameplate data: 1 hp, 120V, 11.9A, Frequency 60. Is it necessary to use the horsepower rating shown on the nameplate to size the branch-circuit conductors and overcurrent protection? The full-load current marked on the nameplate indicates the actual rating of the motor is less than 3/4 hp.

A:Under exception 3 to 430.6(A) there is permission to use the full-load current marked on the appliance to determine the ampacity of the branch-circuit conductors, the rating of the disconnecting means, and the branch-circuit short-circuit and ground-fault protection. This exception applies to listed appliances with inflated horsepower ratings. Underwriters Laboratories Inc. personnel do not check the horsepower output of the motor but do verify the full-load current marked on the data plate. Therefore, the full-load current marked on the nameplate is accurate but the horsepower rating may not be.

NEC 430.6(A)(1) requires the use of Tables 430.147 through 430.150 to determine branch-circuit conductor ampacity, ampere rating of the disconnecting means, etc. However, they are not used for the motor-operated appliance mentioned in the question. For comparison, Table 430.148 indicates a 1 hp, 120V, single-phase motor has a full-load current of 16A. This means that branch-circuit conductor ampacity must be not less than 20 (1.25 x 16). By using the full load current marked on the nameplate, the minimum branch-circuit conductor ampacity is approximately 14.9 (1.25 x 11.9).

Maximum short-circuit and ground-fault protection for this appliance is a 30A breaker or 35A nontime delay fuse. For a 1 hp motor, a 40A circuit breaker or 50A nontime delay fuse would satisfy 430.52(C).

Grounding of receptacles

Q:Is a reliable, low-impedance ground connection made between the metal yoke of a 15A, 120V duplex receptacle and a surface-mounted metal outlet box if the fiber or cardboard device screw retainers are not removed before installation?

A:It is doubtful that good metal-to-metal contact will be obtained with the cardboard or fiber-screw retainers in place. With the receptacle fastened securely to the box, there will be metal-to-metal contact with the underside of the mounting screw heads in contact with the receptacle yoke, but this contact surface may not provide a reliable low-impedance path for phase-to-ground fault current.

The grounding terminal on a receptacle is permitted to be grounded by securing the metal mounting screws on the receptacle to a surface-mounted metal box by 250.146(A). A change has been accepted for the 2005 edition of the NEC that will require the removal of at least one insulating retaining washer to assure grounding continuity. This is the way part of the revision to 250.146(A) will read: “At least one of the insulating washers shall be removed from receptacles that do not have a contact yoke or device that complies with 250.146(B) to ensure metal-to-metal contact.”

Temporary wiring in exhibition halls

Q:In many convention centers there are exhibit halls for temporary use by exhibitors that are part of the convention. Some of these exhibit spaces have cable trays that are used for flexible cords and cable. Flexible cords and cables usually include SPT-3, STO, SJT and G. Does the NEC permit the use of these flexible cords and cables in convention centers?

A:I assume the exhibition areas and convention building can accommodate more than 100 people, and the construction (walls, floors and ceilings) is required to be fire-rated by the local building code. With this assumption, the building wiring must conform to the rules in Article 518—Places of Assembly.

Temporary wiring in exhibition halls must comply with 518.3(B) and Article 527. Flexible cords and cables are permitted for temporary wiring if they are types that are suitable for hard usage or extra-hard usage. In the list of cords and cables included in the question all are acceptable except SPT-3. This is not a hard usage or extra-hard usage cord or cable according to Table 400.4

This temporary wiring may be installed in cable trays where the cables and cords are installed and maintained by qualified persons, and the cable tray is marked: CABLE TRAY FOR TEMPORARY WIRING ONLY at intervals not exceeding 25 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. 


About the Author

George W. Flach

Code Q&A Columnist
George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical in...

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