All-purpose metal clad cable
Q: At a recent chapter meeting of the International Association of Electrical Inspectors (IAEI), there were some samples of MC cable on a display table at the entrance to the meeting room. Some samples were colored green and some were not. There was a short discussion of these products, but I have two questions: Are these cables listed by a recognized independent testing laboratory? Does the green material interfere with the grounding continuity between the connectors and the armor?
A: This is a modification of interlocked armor Type MC cable in which the armor serves as an equipment-grounding conductor. In the assembly of the cable, the full-size equipment-grounding conductor is outside of the Mylar binding tape that encircles the circuit conductors. Therefore, the bare aluminum equipment-grounding conductor is in contact with the underside of the armor for its entire length.
Since the aluminum armor strip is wrapped tightly around the conductor assembly, the bare equipment-grounding conductor is in intimate contact with the armor. This results in a cable diameter that is somewhat smaller than conventional Type MC cable. Because of this smaller diameter, some cable connectors may not make up tight around the armor.
For this reason, a list of acceptable connectors is furnished with each coil or reel of cable. The list includes manufacturers’ names, catalog numbers and number and sizes of conductors in the cable assembly for each listed connector. Use of the proper size connectors ensures good grounding continuity between the connector and aluminum armor.
The green paint on the surface of the armor identifies cable with a green insulated grounding conductor in addition to the bare aluminum equipment-grounding/bonding conductor. It meets the requirements of 250.118(10)A, and may be used in patient care areas in healthcare facilities. The green paint on the armor is conductive and does not interface with grounding continuity between the armor and the connector.
These products are UL-listed and available in sizes 14 through 10 AWG copper. The insulation is Type THHN, and the maximum number of circuit conductors is four.
Nursing home on-site generator
Q: May a single transfer switch supply the emergency and the essential electrical systems in a limited-care nursing home? Is automatic connection to the on-site generator required? If yes, is there a time limit for power to be available from the generator? Are there any requirements for a critical branch in a limited-care nursing home? What loads, if any, must be supplied by the critical branch?
A: To decide whether a nursing home is judged under Article 517—Health Care Facilities, it is necessary to review the definition of a nursing home in 517.2. The definition is, “Nursing Home. A building or part thereof used for the lodging, boarding, and nursing care on a 24-hour basis, of four or more persons who, because of mental or physical incapacity, may be unable to provide for their own needs and safety without the assistance of another person. Nursing home, wherever used in this Code, shall include nursing and convalescent homes, skilled nursing facilities, intermediate care facilities, and infirmaries of homes for the aged.”
Assuming the limited-care nursing home mentioned in the question fits within the scope of this definition, 517.40 through 517.44 apply. The exception to 517.40 should be reviewed, because an on-site generator is not required where no surgical procedures requiring general anesthesia are performed or admitting and discharge policies that prevent accepting patients who require the use of electric life-support equipment are in force.
One automatic transfer switch may be used where the maximum demand on the essential electrical system is 150 kVA or less. Where the maximum demand is more than 150 kVA, at least two transfer switches are required: one for the critical branch and one for the life safety branch.
The load on the life safety branch must be transferred within 10 seconds of interruption of the normal power source. Loads that are permitted on the life safety branch are listed in 517.42(A) through (G).
The critical branch of the essential electrical system may have one or more transfer switches. They may be automatic delay operation or manual operation.
A list of loads that must be connected to the automatic delay operation transfer switch includes electrical equipment and locations in 517.43. Loads that are permitted to be supplied from a manual transfer switch are mentioned in 517.43(B).
Aluminum RMC in concrete
Q. Is there any restriction on the installation of rigid aluminum conduit (RMC) buried in the earth or encased in concrete in the earth? The words in 344.10(B) are not very clear on this subject.
A. Part (B) of 344.10, “Corrosion Environments” reads, “RMC, elbows, couplings, and fittings shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.” You are right, “… severe corrosive influences” are not defined, but there is some help in the General Information for Electrical Equipment Directory published by the UL. This one-sentence paragraph appears under “Rigid Nonferrous Metallic Conduit (DYWV). Aluminum conduit used in concrete or in contact with soil requires supplementary corrosion protection.”
To comply with 110.3(B), supplementary corrosion protection is required where aluminum conduit is installed underground.
Isolated ground receptacles
Q. Our electrical contracting company has a job in an office building to install isolated ground 15-ampere, 125-volt receptacles for computers and data-processing equipment. The requirement in 250.148(C) indicates that all equipment-grounding conductors associated with the circuit conductors must be connected together within the box or to the box. Does this include the isolated grounding conductor?
A. Yes it does, but the exception in 250.148 removes the isolated grounding conductor from the requirement. Receptacles with an insulated grounding terminal are recognized by 250.146(D), and the insulated equipment-grounding conductor is allowed to pass through panelboards without connection to a panelboard grounding bus or terminal. An exception to 250.148 indicates that this special insulated grounding conductor does not have to be connected to other equipment-grounding conductors or to the box.
Q. Are ground clamps that are marked “direct burial” also suitable for concrete encasement? This question is asked because a concrete-encased grounding electrode is required by 250.52(A)(3) of the 2005 edition of the National Electrical Code.
A. A revision to 250.50 requires the use of reinforcing steel in the foundation or concrete slab of a building or structure where one-half inch or larger diameter reinforcing steel at least 20 feet in length is present in the concrete slab that is in contact with the earth.
The connection of the grounding-electrode conductor to the reinforcing steel may be made in the concrete or above and outside of the concrete. If the ground clamp is connected to the rebar in the concrete, the clamp must be marked “direct burial.”
These words also mean that the clamp is suitable for concrete encasement. Under Grounding and Bonding Equipment (KDER) in the UL White Book this sentence also appears: “Clamps intended for use with rebar are marked with the size of rebar with which the clamp is intended to be used.”
Sunlight resistant cables
Q. The language in 310.8(D) allows listed cables to be sunlight resistant without being marked “sunlight resistant.” Are there any cables that are listed, but not marked “sunlight resistant”? If so, what type are they?
A. The requirement in 310.8(D) requires cables and conductors exposed to direct rays of the sun to be listed or listed and marked “sunlight resistant.”
Two types of cables are mentioned in the 2005 edition of the White Book published by the UL. They are Type SE with conductor insulations of the following Types RHW, RHW-2, XHHW, XHHW-2, THWN and THWN-2; Type USE cable with conductor insulation equivalent to RHW or XHHW; and Type USE-2 cable with conductor insulation equivalent to RHW-2 or XHHW-2.
Part of the remarks under the title Service Cable (TXKT) Service Entrance Cable (TYLZ) in the UL White Book are these sentences: “Type SE—Indicates cable for above ground installation. Both the individual insulated conductors are the outer jacket or finish of Type SE are suitable for use where exposed to the sun.”
For Types USE and USE-2 cables, this sentence appears: “Both the insulation and the outer covering, when used, on single and multiconductor Types USE and USE-2 are suitable for use where exposed to the sun.”
Clock-operated switch for swimming pool pump motor
Q. Does a time-clock switch controlling a swimming pool pump motor qualify as a disconnect switch for this load?
A. Yes, provided that the switch has the proper horsepower rating and has a marked “off” position. Listed clock-operated switches with horsepower ratings are tested at six times motor full load running current. Therefore, they are acceptable as motor disconnects. The clock-operated switch must be readily accessible and within sight of the pump motor.
For stationary motors two-horsepower or less, a general-use switch with an ampere rating that is not less than twice the full-load current of the motor is acceptable at the motor location where the clock-operated switch is not within sight of the motor. This permission is allowed by 430.109(C)(1). EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.