Article 210 Branch Circuits
Article 220 Branch Circuit, Feeder and Service Calculations
Article 250 Grounding
Article 334 Nonmetallic Sheathed Cable: Types NM, NMC, and NMS
Article 422 Appliances
The General Information for Electrical Equipment Directory published by Underwriters Laboratories Inc. is also mentioned.
Wiring an assisted living building
Q: I am wiring a 36-unit, 2-story assisted living facility with nonmetallic sheathed cable. The interior walls are constructed of 2 x 4-inch wood studs covered with gypsum wall board. Exterior walls are brick veneer. There is a combination dining room/recreation room and a small fitness room. The corridors have furred down ceilings with ceiling tiles in suspended T-bars to hide the air conditioning ducts. Nonmetallic sheathed cable is installed in this space to supply hallway lighting. The local electrical inspector has approved the wiring in the walls but has not approved the nonmetallic sheathed cable wiring in the suspended ceiling. He says that nonmetallic sheathed cable is not permitted in this space. What does the Code say? By the way, the building has a fire sprinkler system.
A: The answer depends on your Building Code. The NEC in 334.10 permits the use of nonmetallic sheathed cable in one-and two-family dwellings, and multifamily dwellings of Type III, IV and V construction. Also, nonmetallic sheathed cable is permitted in other structures of Types III, IV and V construction except in dropped or suspended ceilings.
If your Building Code classifies an assisted living facility as a multifamily dwelling, nonmetallic sheathed cable is permitted in the suspended ceiling provided that the suspended ceiling space is not used for return air for the air conditioning system. The wiring methods permitted where this ceiling space is used for environmental air-handling are listed in 300.22(C)(1), and do not include nonmetallic sheathed cable.
Because you mentioned a sprinkler system in the building, I suspect that your Building Code does not consider this occupancy as a multifamily dwelling unit; therefore, nonmetallic sheathed cable cannot be installed above or in the suspended ceiling. This is part of the requirement in 334.12(A). “Types NM, NMC, and NMS cables shall not be used as follows: (1) As open runs in dropped or suspended ceilings in other than one and two-family and multifamily dwellings.”
Grounding electrode conductor connections
Q: Am I permitted to connect two ground rods spaced eight feet apart to the service without running the grounding electrode conductor from one rod to the other? What parts of the NEC would allow or not permit such an installation?
A: Yes, you are permitted to run individual grounding electrode conductors from each ground rod to the service grounded conductor terminal in the service disconnecting means. The size of the grounding electrode conductor from each ground rod cannot be smaller than 6 AWG copper.
A single 6 AWG copper conductor is also permitted. It can be run from one rod to the other and then to the service or metal cold water pipe within 5 feet of where the pipe enters the building. The exception to this rule allows the connection to be made beyond 5 feet in commercial and industrial buildings under limited conditions [See 250.52(A)(1)].
If the ground rods are the only grounding electrodes for the service because there is no buried metal water pipe and the building structural steel is not effectively grounded, a single grounding electrode conductor is permitted to run from one rod to the other and then to the grounded service conductor, or two separate grounding electrode conductors (one from each rod) are permitted to be connected to the grounded (neutral) service conductor in the service disconnecting means. The part that allows this is 250.64(F).
Methods that are suitable for connecting grounding electrode conductors to grounding electrodes and grounded circuit conductors are outlined in 250.70.
Sizing conductors for electric ranges
Q: I am wiring a 120-unit apartment building with electric ranges. Each range is rated 18 kW at 240V, single-phase. My boss says that I can use 50A receptacles and range cords, but I say they are over 70A and must be hard-wired. Who is correct?
A: Your boss is right. Although the full load current for each range is 75A (18,000 divided by 240), Table 220.19 allows demand factors to be applied to household electric ranges. Note 1 to this Table requires an increase of 5 percent for each kW above 12. This increased percentage is then multiplied by the kW that is shown in Column C of the Table. The calculation shows a (18 – 12) x 5 percent = 30 percent increase above 8kW, or (8 x 0.3) = 2.4kW. This results in a total demand load of 10.4kW. Dividing this demand load by 240V results in a current a little over 43A.
The branch circuit conductors for these ranges can be 6 AWG copper with 60 C or 75 C insulation or 8 AWG copper conductors with 75 C insulation if terminals on both ends are suitable for 75 C. Fifty-amp overcurrent protection is permitted for the branch circuit, and a 4-pole, 50A receptacle can be used to connect the 4-wire 6 AWG copper range cord.
Suitable only for use as service equipment
Q: Does the Code permit the installation of a 200-ampswitch with 200-ampfuses marked “Suitable Only for Use as Service Equipment” as the overcurrent protection and disconnecting means for the secondary conductors of a transformer that is a separately derived system?
A: Grounding of separately derived systems is located in 250.30 and part (A)(2)(a) requires that the grounding and bonding connections to the grounded circuit conductor of the transformer be made at the same point. In other words, the secondary conductor cannot be grounded at the transformer terminal because the neutral bus in the switch marked “Suitable Only for Use as Service Equipment” is bonded to the switch enclosure at the factory. Therefore, the grounding electrode conductor must terminate on the neutral bus in the switch and the neutral terminal in the transformer must not be grounded or bonded to the transformer enclosure.
The General Information for Electrical Equipment Directory published by Underwriters Laboratories Inc. has this information under the title “Switches, Enclosed (WIAX). Some enclosed switches are marked, ‘Suitable Only for Use as Service Equipment.’”
“Enclosed switches marked for use at services may also be used to provide the main control and means of cutoff for a separately derived system or for a second building.”
Storage tank water heaters
Q: When wiring 30-gallon water heaters rated 240V, single-phase, 4600W the branch circuit conductors have to be sized for 125 percent of the full load current of the heater to comply with 422.13, but the overcurrent protection must satisfy the requirements in 422.11(E). Do these two different requirements mean that I must use 10 AWG copper conductors protected by a 20A overcurrent device?
A: No, they do not. Branch circuit conductor ampacity must be 125 percent of full load current and overcurrent protection must also be at least 125 percent of full load current.
The full load current for the water heater is (4600 divided by 240) 19.2A. One hundred and twenty-five percent of this figure is 24A. Therefore, the branch circuit conductor size must be 10 AWG copper, and the overcurrent protection should be 25A. This size overcurrent protection should not be in conflict with 422.11(E)(1) or (3). If the water heater nameplate indicates overcurrent protection of 30A, this size overcurrent device may be provided for protection. If there is no overcurrent protection rating marked on the heater, Item (3) of 422.11(E) would allow overcurrent protection of 30A.
Trip current for GFCI devices
Q: What level of ground-fault current is permitted for ground-fault circuit-interrupters installed in residential occupancies? Is a GFCI with a 20 mA trip acceptable? Are different trip levels permitted for indoors and outdoors?
A: For dwelling units, all 125V, single-phase, 15A and 20A receptacles installed at the various locations mentioned in 210.8 must have ground-fault protection for personnel. This means that the GFCI must trip when the ground-fault leakage current exceeds 4 to 6mA. The same level of ground-fault trip current applies whether the GFCI protected receptacles are installed indoors or outdoors.
Underwriters Laboratories Inc. classifies the 4-6mA GFCIs as Class A. This information appears in the 2002 edition of the General Information for Electrical Equipment Directory published by UL: “A Class A ground-fault circuit-interrupter trips when the current to ground has a value in the range of 4 through 6 milliamperes. A Class A ground-fault circuit-interrupter is suitable for use in branch and feeder circuits, including swimming pool circuits. However, swimming pool circuits installed before local adoption of the 1965 National Electrical Code may include sufficient leakage current to cause a Class A ground-fault circuit-interrupter to trip.
“A Class B ground-fault circuit interrupter trips when the current to ground exceeds 20 mA. This product is suitable for use with underwater swimming pool lighting fixtures installed before the adoption of the 1965 National Electrical Code.”
Underwater swimming pool lighting fixtures operating at 120V that were installed prior to the 1965 NEC often had leakage current to ground that exceeded 10mA. This is the reason for the 20mA trip level for Class B GFCIs. Notice that the discussion on the Class B GFCI indicates that it is for use with swimming pool underwater lighting fixtures installed before enforcement of the 1965 edition of the NEC. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.254.2132.