Article 100-Definitions;
Article 250-Grounding;
Article 445-Generators;
Article 426-Fixed Outdoor Electric De-icing and Snow-melting Equipment;
Article 427-Fixed Electric Heating Equipment for Pipeline and Vessels; and
Article 702-Optional Standby Systems

Nonmetallic sheathed cables in dwellings with more than three floors

Q: I wanted to respond to your commentary on Article 336 in the December 1999 issue of Electrical Contractor magazine. The question was " ... are we allowed by the National Electrical Code (NEC) to wire this four-story building with nonmetallic sheathed cable?" Your reply was "... yes if the enforcing authority in your jurisdiction has adopted the 1999 NEC without any amendments to Sections 336-4 and 336-5."

I believe your answer to the question is incorrect based on a reading of the Code Sections you included in the column and verified by examination of those same sections in my copy of the 1999 NEC.

Regardless of revisions to 336-4, Section 336-5(a) in the 1999 NEC, it is clear that NM, NMC, and NMS are prohibited in "any multifamily dwelling or other structure exceeding three levels above grade." The structure in question is four levels above grade, is it not? So where is there anything in the 1999 NEC that overrules this section to allow NM cabling in a structure more than three levels above grade? Maybe I am missing something, but I am sure there are many others who are confused in the same way. Could you clarify these issues for us?

A: The permitted uses for nonmetallic sheathed cable were expanded in the 1999 NEC. Types NM, NMC, and NMS cables could not be installed in dwelling occupancies that exceeded three floors above grade. An explanation indicated how the first floor of a building was determined and pointed out that a first level utilized only for vehicle parking, storage, or similar use was not counted in arriving at the three-story limitation. In other words, a four-story building with three stories designed for human occupancy and the first level designed for off-street parking could be wired with nonmetallic sheathed cable. There was also an exception that allowed conversion of an attic space, vehicle parking space, or storage space to a habitable floor level in a one-family dwelling without having to change the existing wiring method.

Permission to use nonmetallic sheathed cable in one-and two-family dwellings appears in Section 336-4(1) of the 1999 NEC: "USES PERMITTED. Type NM, Type NMC, and Type NMS cables shall be permitted to be used in the following: (1) One-and two-family dwellings (2) Multifamily dwellings and other structures, except as prohibited in Section 336-5 (3) cable trays, where the cables are identified for the use." Notice that there is no restriction on the number of floors for one-and two-family dwellings. Item (2) allows nonmetallic sheathed cable to be used as the wiring method in multifamily dwellings and other structures if the limitation specified in Section 336-5 is satisfied. Item (a)(1) in Section 336-5 limits the height of multifamily dwellings and other structures to three floors where the wiring method is nonmetallic sheathed cable.

Definitions in Article 100 explain what a dwelling unit is and the read out defines one-family, two-family, and multifamily dwellings.

Portable generator connections

Q: What is the proper way to connect a 7,500 watt, 120/240 volt, single-phase portable generator to the interior wiring of a small flower shop? The generator will be stored in the back of the shop when it is not in use. Article 702 of the NEC covers Optional Standby Systems, but the second paragraph in the Scope (Section 702-1) indicates that portable generators are not covered by the Article. Will you give me your thoughts on how the generator wiring can be safely installed?

A: You are correct in your statement that Article 702 does not apply to portable units. Part of the Scope in Section 702-1 says: "The systems covered by this article consist only on those that are permanently installed in their entirety, including prime movers."

Section 250-20(b)(1) requires grounding of the generator neutral. This can be accomplished by using a two-pole, solid neutral transfer switch. With this arrangement, the generator neutral is grounded through the normal source neutral at the service equipment.

If flexible cord is used to connect the generator to the interior wiring, hard usage or extra hard usage multiconductor cord should be used. The minimum size should be based on 115 percent of full-load output current of the generator (7,500 divided by 240) multiplied by 1.15. This results in a current of about 36 amperes. Therefore, the minimum size flexible cord should be No. 8 copper. Section 445-5 is the reference that requires a minimum conductor ampacity of 115 percent between the generator terminals and the first overcurrent device.

A four-wire, 50-ampere male inlet (motor attachment plug) should be secured to the building at the location where the generator will be placed. If outdoors, the male inlet should be mounted in a box that will protect it from the weather while in use. A four-wire cord is recommended to provide an equipment-grounding conductor from the generator frame to the equipment grounding bus in the transfer switch. It is very important to ground the generator frame. One end of the flexible cord should be directly connected to the generator output terminals or generator disconnecting means and the other end should terminate in a female cord connector that matches the male inlet on the building. This cord should not be longer than necessary to do the job.

The transfer switch should be located close to the panelboard to reduce the amount of wire needed to connect the transfer switch into the electrical system and to keep the cost down.

Because you have a small genset, it would probably be more economical to use the requirements in Section 250-34. This Section contains special rules for portable generators, and allows the generator frame to serve as the grounding electrode where the generator supplies cord- and plug-connected equipment through receptacles mounted on the generator. In addition, noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles must be bonded to the generator frame.

If the generator can be operated in the storage room, there is adequate ventilation, and the engine exhaust can be piped to the outside, consideration should be given to mounting receptacles on the generator and plugging the electric appliances in the flower shop directly into these receptacles or through properly sized and listed extension cords.

Ground-fault protection for electric heating systems

Q: Articles 426-Fixed Outdoor Electric Deicing and Snow Melting Equipment and 427- Fixed Electric Heating Equipment for Pipelines and Vessels require ground-fault protection. Does this mean that a GFCI, or a GFP as mentioned in Section 230-95 for a 480-volt service is required or are standard fuses or circuit breakers that provide branch circuit overcurrent protection all that is needed?

A: For resistance heating elements used for outdoor snow-melting and de-icing, ground-fault protection of equipment is required by Section 426-28. The ground-fault sensor is typically set at 30 to 50 milli-amperes (ma) and may be part of the branch circuit overcurrent device or a separate unit supplied by the manufacturer of the heating system.

GFCI protection for personnel with a trip setting of 5 ma plus or minus 1 ma is required for impedance heating systems where the voltage of the secondary winding of an isolation transformer is more than 30 but not more than 80. This requirement appears in Section 426-32.

Similar rules for ground-fault protection of equipment appear in Section 427-22 for electric heat tracing and heating panels for pipelines and vessels. However, the ground-fault protection sensor does not have to deenergize the circuit where qualified persons maintain the heating system and continued circuit operation is necessary for safe operation of equipment or processes. Under these conditions, a visual or audible alarm or both is required to indicate a ground-fault.

A revision in the 1999 NEC changed the requirement for ground-fault protection of the branch-circuit supplying the heating equipment to ground-fault protection of the heating equipment. This change removes the requirement for ground-fault protection at the branch circuit overcurrent device to any location between the branch circuit overcurrent device and the input terminals of the heating system.

GFP for these heating systems typically has trip settings of 30 ma to 50 ma.

Fittings for electrical metallic tubing

Q: Is electrical metallic conduit (EMT) recognized as an equipment grounding conductor for circuits operating at 600 volts or less where 'sock-on' or indentor-type fittings are used?

A: Listed drive-on (sock-on) and indentor fittings provide adequate grounding continuity when they are installed in accordance with instructions supplied with the fittings. Section 250-118(4) recognizes electrical metallic tubing as an equipment-grounding conductor. 'Sock-on' connectors and fittings must be driven tight and indentor fittings must be crimped with a proper tool supplied by the manufacturer.

Tools with two indent bumps placed 180 degrees apart require that the tool be rotated about 90 degrees after the first set of indentations are made so that another set of indentations can be made. In other words, four indentations approximately 90 degrees apart are required with this tool. Triple indent tools require only one set of indentations.

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.