Q: Does the National Electrical Code permit Class 2, Class 3 and telephone conductors to be installed in the same bored hole in a wooden stud with Type NM or Type AC cable operating at 120V?
A: Class 2 and 3 conductors are permitted to occupy the same hole in a wood stud that contains Type NM or AC cables by 725.55(J). The basic requirement in Part J is two inches of separation between Class 2 and Class 3 conductors and conductors that supply electric light, power, or Class 1 nonpower-limited fire alarm, or medium-power network-powered broadband communications circuits.
However, Item 1 in Part J removes this separation requirement where all of the electric light or power conductors are installed in metal-sheathed or nonmetallic sheathed cables.
Similar permission is in 800.133(A)(2) for telephone conductors. Exception No. 1 does not require a two-inch separation for telephone and electric light or power conductors where the electric light or power conductors are contained in Type AC or Type NM cable.
Location of 50 kVA transformer
Q: May a 480V, single-phase, 50 kVA transformer that supplies 120- and 240V single-phase loads be mounted above a suspended ceiling where it will not be visible after the lay-in ceiling tiles are installed?
A: Yes, providing the various requirements in 450.13(B) are satisfied. Dry-type transformers with voltage ratings of 600 or less are permitted to be installed in hollow spaces provided that they are accessible and not permanently closed in by the structure.
The transformer must be separated from combustible material by 12 inches or be separated from combustible material by a fire-resistant or heat-insulating barrier. However, an exception does not require a separation or a barrier where the transformer is completely enclosed or has only ventilating openings.
Ventilation must be adequate to dispose of the heat caused by full-load losses of the transformer without exceeding a temperature rise that is in excess of the transformer rating.
Minimum clearances around ventilating openings as indicated on the transformer nameplate must be maintained to avoid overheating. It is important also to read Fine Print Note 2 to 450.9 and be aware of additional heating of the transformer that may occur where nonsinusoidal currents are present.
Derating parallel conductors
Q: Where three conductors per phase are run in a single raceway to supply a motor control center, are these conductors classified as three, current-carrying conductors because they are joined together at each end or is it necessary to derate because there are nine current-carrying wires in the conduit? Does the parenthetical phrase in 300.4 “electrically joined at both ends to form a single conductor” mean that derating because of the number of conductors in a raceway does not apply?
A: The conductors must be derated. According to Table 310.15(B)(2)(a), the ampacities listed in Table 310.16 must be derated to 70 percent. The phrase mentioned in the question appeared in the 2002 edition of the National Electrical Code. Changes made in the 2005 edition should clear up misunderstanding of the meaning of the words in the 2002 edition.
One change removed the words “to form a single conductor.” The other change involves 310.15(B)(2) in which this sentence was added: “Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.”
I believe the words “to form a single conductor” in the 2002 NEC were added to emphasize that taps to parallel conductors had to include all conductors connected to each phase.
A tap to a single conductor where conductors are parallel would cause unequal division of current. Also, the phrase allowed a single overcurrent device to protect all parallel conductors connected to the same phase.
Bathroom branch circuit
Q: I have a job to wire a new one-family dwelling unit that has two bathrooms on the first floor and one on the second floor. Does the NEC permit a single circuit for these three bathrooms?
A: Only one 20-ampere branch circuit must be provided to supply the receptacles. This branch circuit is required by 210.11(C)(3), and cannot supply any other outlets where more than one bathroom is supplied.
The receptacles must be protected by a ground fault circuit interrupter to comply with 210.8(A)(1), and be placed within three feet of the outside edge of each basin. However, the receptacles may be located on a wall or partition that is adjacent to the basin.
Separately derived systems
Q: The secondary conductors from a step-down transformer are installed in flexible metal conduit and terminate in a panelboard. The grounding-electrode conductor is connected to the XO terminal in the transformer. Is it necessary to isolate (insulate) the neutral bus in the panelboard? Is a bonding conductor required in the flexible metal conduit? If yes, how is it sized?
A: The neutral bus or terminal must be insulated from the panelboard enclosure. The grounding-electrode conductor size is determined by 250.66 and Table 250.66. A system-bonding conductor must be provided in the flexible steel conduit that contains the secondary conductors.
This bonding conductor is connected to the XO terminal in the transformer and the equipment-grounding conductor bus in the panelboard. Its size is determined from Table 250.66 or 12.5 percent of the largest secondary conductor where secondary conductors are larger than 1,100 Kcmil copper or 1,750 Kcmil aluminum.
Flexible metal conduit is not acceptable as an equipment-grounding conductor where the conductors in the conduit are protected by overcurrent devices greater than 20 amperes. It is also not recognized as an equipment-grounding conductor where the length exceeds 6 feet. These and other restrictions on the use of flexible metal conduit as an equipment-grounding conductor appear as Item 5 under 250.118.
Swimming pool pump motor
Q: Is there any requirement in Article 680 of the NEC that prohibits a 240V single-phase swimming pool pump motor from being cord-and-plug connected?
A: Wiring for swimming pool pump motors used with permanently installed swimming pools is covered in Part II of Article 680. Generally, wiring methods that are suitable for pool-associated motors include rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, and Type MC cable listed for the location. However, other wiring methods are acceptable in specific locations or applications where mentioned in 680.21.
Item 5 in 680.21 deals with cord-and-plug connected pool associated pump motors. Two of the requirements in Part 5 limit the cord length to 3 feet and require an equipment-grounding conductor in the cord that complies with Table 250.122. For a 20-ampere branch circuit to the pump motor, the minimum size equipment-grounding conductor in the flexible cord is 12 AWG copper.
Where a double-insulated pump motor is installed and a means for grounding internal inaccessible, noncurrent-carrying metal parts is provided, the equipment-grounding conductor in the flexible cord must be connected to this terminal, and the equipotential bonding grid is not connected, but a sufficient length of 8 AWG solid copper wire connected to the bonding grid must be provided to bond a replacement motor that is not double-insulated.
The receptacle that supplies power to the pump must be located at least 10 feet from the inside walls of the pool. This distance can be reduced to five feet where these conditions are satisfied: it must be a single receptacle, a locking type, a grounding type, and it must be GFCI protected.
These requirements appear in 680.22(A). Even though the pump receptacle is located 10 feet from the inside walls of the pool, GFCI protection is required for 15 and 20 ampere, 125V through 250V receptacles that supply this motor.
Continuity of grounding electrode conductors
Q: Where the only grounding electrode for a service is two ground rods, am I permitted to run the grounding-electrode conductor to the first ground rod and then splice the grounding-electrode conductor from the first rod to the second?
A: Where it takes two ground rods to satisfy the 25 ohms or less requirement for a service grounding electrode as required by 250.56, the answer is no unless an irreversible compression-type connector or exothermic welding is used.
Although two rods are used, this is a single electrode. Where it is necessary to use two rods because the resistance to earth exceeds 25 ohms, Part C of 250.64 requires that the grounding-electrode conductor be installed in one continuous length from the service-grounded conductor to the grounding electrode. However, exothermic welding or irreversible compression-type connectors are permitted where splices are made.
Q: Article 695 limits voltage drop on a fire pump branch circuit. Are there any other parts of the NEC that limit voltage drop?
A: Yes, there are. As stated in the question, 695.7 limits voltage drop to 15 percent below normal controller voltage during fire pump motor starting, and not more than 5 percent below the motor voltage rating when the motor is operating at 115 percent of full-load current.
In Article 700, there is a voltage drop requirement for batteries that supply emergency equipment. The battery voltage cannot drop below 87.5 percent for a period of 1.5 hours while supplying emergency loads. This same requirement applies to batteries that provide Legally Required Standby Power.
For Sensitive Electronic Equipment 647.4(D) has this requirement: “The voltage drop on any branch circuit shall not exceed 1.5 percent. The combined voltage drop on feeder and branch circuit conductors shall not exceed 2.5 percent.”
Fine Print Notes following 210.19(A), 215.2(A)(3) and 310.15(A)(1) also suggest limiting voltage drop on branch circuits and feeders. EC
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.