Derating Service Conductors, Appliance Garage Receptacles and More

Article 90—Introduction; Article 312—Cabinets, Cutout Boxes and Meter Socket Enclosures; Article 314—Outlet, Device, Pull and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures; Article 330—Metal Clad Cable: Type MC; Article 695—Fire Pumps; Article 700—Emergency Systems

Derating service conductors

Q: Is it necessary to derate service conductors on the line side of the utility meter? The service conductors are six 2/0 AWG copper conductors and a single 4/0 AWG copper neutral conductor, all with Type THWN-2 insulation in a single 2.5-inch rigid metal conduit. Also, what size overcurrent device is permitted for this combination of conductors? The system is 208Y/120V.

A: If these conductors are connected to the line side of the service point (see definition of service point in Article 100), the answer is no because these conductors are under the control of the serving utility. If the licensed electrical contractor installed these service conductors and they are customer-owned, the answer is yes, since there is no exemption from the derating of service conductors where more than three current-carrying conductors are installed in a raceway.

The parallel conductors must conform to the requirements in 310.4 and be derated according to 310.15(B)(2)(a) and Table 310.15(B)(2)(a). According to the table, the ampacities of the conductors as given in Table 310.16 must be derated to 70 or 80 percent. The percentage value depends on whether the major portion of the load on the neutral conductor is non-linear. If it is assumed that the neutral conductor carries only unbalanced linear load currents, the corrected ampacity for the phase conductors is 156A [0.80 × 195]. With two conductors per phase, the corrected ampacity is 312. The corrected ampacity of the neutral conductor is 208 [0.80 × 260].

To comply with 310.15(B)(4)(c) where a major portion of the neutral conductor load is nonlinear, a derating factor of 70 percent applies. This results in corrected ampacities of 136.5 [0.70 × 195] or 273A per phase. The corrected ampacity for the neutral conductor is 182 [0.70 × 260]. The corrected ampacities for the 70 percent derating factor are 273 for the phase conductors and 182A for the neutral conductor.

Overload protection for these service conductors can be 300A or above, depending on the motor load, or 350A and above for the conductors that have an 80 percent derating factor.

Where the service conductors are installed and maintained by the electric utility company, 90.2(B)(5) exempts them from the requirements of the National Electrical Code (NEC).

Receptacle in appliance garage

Q: Does the NEC require a receptacle in an appliance garage? Does an appliance garage on a kitchen countertop divide the countertop into two separate countertops?

A: A receptacle is not required in an appliance garage located on a kitchen countertop in a dwelling unit. Receptacles should be spaced along the countertop while ignoring the appliance garage. Space the receptacles no more than 2 feet apart without any regard to the appliance garage. Spacing of receptacles is covered by 210.52(C).

An appliance garage installed on a kitchen countertop does not create a separate space or divide the countertop into two spaces. This definition of separate spaces appears in 210.52(C)(4): “Countertop spaces separated by rangetops, refrigerators, or sinks shall be considered as separate countertop spaces in applying the requirements of 210.52(C)(1), (C)(2), and (C)(3).”

Equipment-grounding pigtailing

Q: Does the Code permit pigtailing two or three 12 AWG copper equipment-grounding conductors then connecting one to the grounded neutral bus in a service panelboard?

A: In a grounded system, the equipment-grounding conductor can be connected to the grounded service conductor (for a single-phase, 120/240V system, this is the neutral conductor).

A single equipment-grounding conductor is permitted where multiple circuits are run in the same raceway or cable, provided that the size of the equipment-grounding conductor is based on the largest overcurrent device protecting the conductors in the raceway or cable. Therefore, a single 12 AWG copper equipment-grounding conductor connected to the neutral bus and pigtailed to two or three 12 AWG copper conductors should not be considered a Code violation.

Panelboards containing splices or taps must have adequate wiring space. This is the requirement in part of 312.8: “Enclosures for Switches or Overcurrent Devices.... The conductors (in the enclosure) shall not fill the wiring space at any cross section to more than 40 percent of the cross-sectional area of that space, and the conductors, splices, and taps shall not fill the wiring space at any cross-section to more than 75 percent of the cross-
sectional area of that space.”

Twist-on wire connectors must be suitable for the application and must conform with the requirements in 110.14.

Fire-pump service and feeder

Q: Where fire-pump service conductors are run through a building to a room for a fire pump, emergency and standby equipment, and feeder conductors from an outdoor generator enter this same room, do the service or feeder conductors have to be protected according to 230.6(1) or (2) or by 700.9(D)(1)?

A: Service conductors must be protected to comply with 230.6(1), (2), (3) or (4) where they enter the building. After entrance into the building, these conductors must be installed in accordance with 230.6(1) or 230.6(2).

Fire-pump supply conductors on the load side of a service disconnecting means and overcurrent devices must be kept entirely independent of all other wiring and encased in 2 inches of concrete or be within enclosed construction with a minimum one-hour fire-resistive rating dedicated to the fire-pump circuit. A listed electrical circuit-protective system with a minimum one-hour fire rating also is acceptable. All of these protection methods are recognized by 695.6(B). Feeder circuit wiring from the emergency generator must meet the fire protection requirements of 700.9(D) in assembly buildings that can accommodate 1,000 or more people or in buildings that are more than 75 feet in height with the following occupancy classes: assembly, educational, residential, detention and correctional, business, and mercantile. Types of fire protection that are acceptable include a listed electrical circuit protective system with a minimum 1-hour fire rating, a listed thermal barrier system for electrical systems components, a fire-rated assembly listed to achieve a minimum fire rating of one hour, embedded in not less than 2 inches of concrete, or a cable listed to maintain circuit integrity for not less than one hour. Another fire protection method allows the feeder conductors to be protected by an approved fire-suppression system.

Supporting Type MC cable

Q: Am I permitted to run MC cable from luminaire to luminaire in an accessible ceiling without securing the cable within 12 inches of each luminaire?

A: Yes, you are permitted to have 6 feet of unsupported cable between luminaires (lighting fixtures). 330.30(D) states, “Unsupported Cables. Type MC cable shall be permitted to be unsupported where the cable: (2) Is not more than 1.8 m (6 ft) in length from the last point of cable support to the point of connection to a luminaire (lighting fixture) or other piece of electrical equipment and the cable and point of connection are within an accessible ceiling. For the purpose of this section, Type MC cable fittings shall be permitted as a means of cable support.” Similar language appears in 320.30(D)(3) for Type AC cable.

Outlet box dimensions

Q: Small boxes, such as the 3.5-by-0.5-inch box, have less than 2 cubic inches of volume, yet they often are used under roof overhangs on dwelling units for mounting lampholders with flat covers. For what use are these boxes listed?

A: They are listed for use with luminaires (lighting fixtures) with dome covers where the dome cover is marked with its cubic inch capacity. The volume of the box is then added to the volume of the dome cover to obtain the total volume that can be used to determine the number of conductors that may be installed in the box/canopy combination. Information on box volume and conductor fill is in 314.16.

Sizing fire-pump overload protection

Q: An electrical drawing shows a 100-horsepower, 460V, three-phase fire pump motor being supplied by two sets of 500 kcmil Type THWN copper conductors per phase. Is it necessary to size the service conductors for locked rotor current? If not, what size conductors should be used? What is the minimum size overload protection for this feeder?

A: Conductor ampacity for the fire pump motor cannot be less than 125 percent of the fire pump motor full-load current. Therefore, the minimum size service conductors cannot be smaller than 150A [1.25 × 120]. According to Table 310.16, three 1/0 Type THWN copper conductors is the minimum size service and feeder conductors that may supply this motor. Because the voltage drop cannot exceed 15 percent of fire-pump controller voltage rating marked on the nameplate during motor starting, this size conductor may have to be increased (see 695.7).

The overcurrent protective devices must be sized to carry fire pump motor locked-rotor current. According to Table in the 2007 edition of NFPA-20, Installation of Stationary Pumps for Fire Protection, the locked-rotor current for a 100-horsepower, 460V, three-phase motor is 725A. Therefore, the fuses or circuit breaker cannot have a rating that is less than 800A. This is the next-larger standard size fuse or circuit breaker above 725A that is listed in 240.6 of the NEC. EC

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.

About the Author

George W. Flach

Code Q&A Columnist

George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical...

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