Article 210 Branch Circuits; Article 250 Grounding and Bonding; Article 300 Wiring Methods; Article 310 Conductors for General Wiring; Article 408 Switchboards and Panelboards; Article 430 Motors, Motor Circuits, and Controllers
Grounding isolated EGCs
Is it necessary to use the isolated grounding conductor that supplies isolated ground receptacles to ground metal boxes and other metal enclosures? This question is asked because only panelboards are mentioned in 250.146(D).
A grounding receptacle with an insulated equipment-grounding conductor (EGC) terminal is permitted by 250.146(D) for the reduction of electrical noise (electromagnetic interference). With these devices, an equipment-grounding conductor must be run with the branch-circuit conductors, must be insulated and may pass through panelboards without connection to the equipment-grounding bus in the panelboards.
A new sentence has been added to 250.146(D) that clears up the status of connections to other metal enclosures. It appears at the end of part (D) and reads, “Where installed in accordance with the provisions of this section, the equipment grounding conductor shall also be permitted to pass through boxes, wireways, or other enclosures without being connected to such enclosures.”
A fine print note following this part (D) advises that an isolated equipment-grounding conductor does not relieve the requirement for grounding the raceway system and other metal enclosures.
Rooftop conductor derating
Will you give an example of derating conductors in raceways installed on or above roofs?
This is an addition to the derating requirements for conductors installed in raceways on and above roofs. The Copper Development Association sponsored tests and results, in the form of proposals, were submitted to Code-Making Panel 6 for inclusion in the 2008 edition of the National Electrical Code (NEC).
Since the question originates from someone in North Carolina, an outdoor design temperature for Greensboro or Wilmington, N.C., will be used in the calculations. This combination table and slide rule published by the Copper Development Association shows an outdoor design temperature of 92°F for the two cities and is recommended for use by the fine print note following 310.15(B)(2)(c).
In the table and slide rule, there are about 170 cities listed from the 50 states. The temperature ranges from 58°F in Valdez, Alaska, to 111°F in Palm Springs, Calif. If the city of interest is not listed, one nearby can be used.
The requirement in part (C) reads, “Conduits Exposed to Sunlight on Rooftops. Where conductors or cables are installed in conduits exposed to direct sunlight on or above rooftops, the adjustments shown in Table 310.15(B)(2)(c) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.16 and Table 310.18.”
Now for the calculation: Assume the bottom of the conduit is 2 inches above the roof. According to Table 310.15(B)(2)(c), 40°F must be added to the ambient temperature to obtain the temperature in the raceway. This results in an internal temperature of 132°F. Conductors with 60°C insulation cannot be used because the ampacity at this temperature is zero. For 75°C insulated copper conductors, 14 AWG has an ampacity of 12, 12 AWG has an ampacity of 15, 10 AWG has an ampacity of 20, and 1/0 AWG has an ampacity of 87. Where 90°C insulated copper conductors are used, the corrected ampacities are 14 AWG—18 amperes; 12 AWG—21 amperes; 10 AWG—28 amperes; and 1/0 AWG—121 amperes.
Where more than three current-carrying conductors are installed in the raceway, additional derating is required to comply with 310.15(B)(2)(a) and the table.
Panelboard overcurrent protection
Will you explain the requirements for overcurrent protection of panelboards now that lighting and appliance branch-circuit panelboards and power panels are no longer segregated into two different categories?
The 42-circuit limitation and the words lighting and appliance branch circuit panelboard and power panelboard no longer appear in Article 408, except for the exception in 408.55 that allows the wire bending space in panelboards with not more than 42 overcurrent devices for panelboards rated 225 amperes or less. A maximum of 42 overcurrent protective devices is mentioned in Exception No. 2 that allows two main overcurrent devices as the disconnecting means where the maximum number of overcurrent devices does not exceed 42.
Generally, a single overcurrent device or set of fuses with ratings not greater than the ampere rating of the panelboard are required to comply with section 408.36. The first exception to this rule does not require a main where the panelboard is used as service equipment.
Part of the supporting material included with the proposal reads, “As documented by the Task Group, an original (circa 1933) intent of the circuit limitation was to prevent overheating by installing too many rubber-insulated wires in the panelboard. This preceded the present UL 67 Panelboard Standard constructional requirements for Wiring Space, Wiring Gutters, and Wire Bending Space. UL 67’s thermal test requirements that incorporate ‘worst-case’ loading conditions, coupled with the industry advances in conductor insulation, entirely eliminate this as a concern.”
EMT under roof deck
Does the 2008 edition of the NEC prohibit running electrical metallic tubing (EMT) under the roof deck of a small commercial building?
In a section titled “Protection Against Physical Damage,” a new Part (E) appears and reads, “Cables and Raceways Installed Under Roof Decks. A cable- or raceway-type wiring method, installed in exposed or concealed locations under metal-corrugated sheet roof decking, shall be installed and supported so the nearest outside surface of the cable or raceway is not less than 18 mm (1.5 inches) from the nearest surface of the roof or decking.” The exception removes this requirement for rigid metal conduit and intermediate metal conduit. Notice that this rule applies only to metal corrugated sheet roof decking.
Are all 250-volt, 15- and 20-ampere receptacles required to be tamper resistant in a residence? Which 15- and 20-ampere, 125-volt receptacles are required to be tamper resistant?
There is no requirement for 250-volt tamper-resistant receptacles in dwelling units. However, tamper-resistant receptacles are required for all 125-volt, 15- and 20-ampere receptacles installed in locations mentioned in 210.52. According to 408.11, all 15- and 20-ampere, 125-volt receptacles installed in the following locations in dwelling occupancies are required to be tamper resistant: the kitchen, family room, dining room, living room, parlor, library, den, sunroom, bedroom, recreation room, ballroom, balconies, porches, decks, laundry rooms, basement, garage, hallways and bathrooms. These all are rooms and spaces covered by 210.52.
Are color codes of MC and AC cables available to satisfy the requirements in 210.5(C) for branch circuits?
Yes, a change in this part (C) requires identification of conductors by system and phase by color code, marking tape, tagging or other approved means. The method used for conductors originating within each branch-circuit panelboard must be documented and readily available or posted at each branch-circuit panelboard.
Types MC and AC cables are produced with conductors of various colors that meet this rule. Here are some examples: MC and AC cable with brown, yellow, orange and gray insulation; MC and AC cable with black, white, red and blue insulation. These cables also are available with two wires in various color combinations. They also may have an insulated, green-colored conductor where insulated ground receptacles are required for clean ground branch circuits or for redundant ground requirements in 517.13 for AC cable or Type MC AP cable installed in patient care areas of healthcare facilities.
The change in the 2008 edition of the NEC added the requirement for phase identification of conductors in addition to the requirement for system voltage identification.
Several motors on a single branch circuit
I ran a 12-AWG copper, 240-volt, single-phase branch circuit to connect four 240-volt, single-phase, ?-horsepower pump motors. The branch circuit overcurrent protection is a 20-ampere two pole circuit breaker. A 15-ampere switch controls each individual motor. The inspector turned the job down, saying the 20-ampere overcurrent device was not Code-compliant. What am I missing?
Table 430.248 says the full-load current for a ?-hp, 240-volt, single-phase, motor is 3.6 amperes. Therefore, minimum branch-circuit conductor ampacity is 15.3 amperes (1.25 x 3.6 + 3.6 + 3.6 + 3.6) as required by 430.24. The 12 AWG copper conductors are adequate for the load served. However, 430.52 has limits on branch circuit ratings that supply more than one motor. Where the motors are not over 1 horsepower, 120 volts, they are allowed to be connected to a 20-ampere branch circuit. Where the branch circuit exceeds 120 volts but is not greater than 600 volts, the maximum overcurrent protection cannot exceed 15 amperes. To qualify for this provision, each motor cannot exceed 6 amperes, the rating of the branch-circuit, short-circuit and ground-fault protective device marked on any of the controllers is not exceeded, and individual overload protection conforms to 430.32.
FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.