Outlet Boxes in Fire-Rated Construction, Derating Conductors and More

Article 110 Requirements for Electrical Installations; Article 230 Services; Article 300 Wiring Methods; Article 310 Conductors for General Wiring; Article 314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures; Article 330 Metal-Clad Cable: Type MC; Article 604 Manufactured Wiring Systems; Article 700 Emergency Systems; Article 701 Legally Required Standby Systems; Article 702 Optional Standby Systems; The Guide Information for Electrical Equipment (White Book), published by Underwriters Laboratories Inc., also is mentioned.

Conductors derating

Please give an example of the method used to calculate the conductor ampacity adjustment of raceways on rooftops. You should remind readers about the new requirement in Section 310.15(B)(2)(c) and how the terminals of equipment are based on 110.14(C)(1)(a) or (C)(1)(b), unless the equipment is listed and marked and has temperature terminal ratings of 75°F. Why is a 90°C column included in Table 310.16, since practically all terminations rated 600 volts and below are marked for 60°C or 75°C conductor insulation?

The Copper Development Association (CDA) developed a revision with a table, and the Code panel accepted it as a revision for the 2008 edition of the National Electrical Code (NEC).

I will go through the procedure assuming there are six Type THW 2/0 copper conductors supplying two three-phase, 3-wire cir-cuits in a metal raceway installed 4 inches above a roof exposed to the sun. Using the light cardboard or thick paper slide rule pro-vided by the CDA, the first step is to determine the ambient temperature in the city where the wiring is located. Assuming the city is Dallas, the ambient temperature shown on the chart is 99°F. This figure is taken from the ASHRAE Handbook. The fine print note that follows Section 310.19(B)(2)(c) permits this method of determining the average ambient temperature. For step 2, it is necessary to find the temperature in the metal conduit by placing 99 in the window and reading 129°F under the “Distance Above Roof up to 12? column. Now position the slide so that the black dot appears in the hole marked 123–131°F. Then read the corrected ampacity under the 75°C column in step 4, which is 117.

Since there are six current-carrying conductors in the raceway, they must be derated to 80 percent of their adjusted ampacity (0.8 × 117 = 93.6). For other than motor loads and other loads that permit overcurrent protection above conductor ampacity, the overcurrent protection for these circuits is 100 amperes.

If Type THHW insulation is used on the 2/0 AWG copper conductors and the same procedure as is used above with 2/0 AWG copper conductors with THW insulation, the adjusted ampacity is 118.4, a gain of about 25 amperes without changing the wire size. This is one use of 90°C insulation and is permitted by Section 110.13(C)(1)(b), which reads in part: “Conductors with higher temperature ratings, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equip-ment is listed and identified for use with such conductors.” Other parts of this same section allow the use of 90°C ampacities for derating provided that the 75°C ampacity is not exceeded.

Conductor insulation must be suitable for wet locations where installed in a metal raceway exposed to the weather. Section 300.9 requires that the interior of raceways installed in wet locations be considered a wet location. This was a new section in the 2008 NEC.

Fire-rated outlet box spacing

The electrical inspector says that I cannot install receptacles in fire-rated walls closer than 24 inches back-to-back. This is an apartment building with rooms on each side of the firewalls, and the spacing of receptacles is critical. Can you help?

Metallic and nonmetallic boxes are classified for fire resistance by Underwriters Laboratories Inc., and they are mentioned in the 2008 Guide Information for Electrical Equipment Directory (White Book) on pages 257–259. Generally, the spacing between boxes cannot be less than 24 inches, but closer spacing is permitted where Wall Opening Protective Materials are installed according to the requirements of their classification. These materials are found under CLIV in the Fire Resistance Directory published by UL.

Boxes are intended to be installed so that the surface area of individual boxes does not exceed 16 square inches, and the aggregate surface area of the boxes does not exceed 100 square inches per 100 square feet of surface area.

Nonmetallic boxes and metal raceways

May nonmetallic outlet boxes be used with metal raceways?

Although not permitted by the basic requirements in Section 314.3, two exceptions allow their use where bonding can be provided be-tween all metal raceways and metal armored cable entries. Here is what the exceptions say: “Exception No. 1: Where internal bonding means are provided between all entries, nonmetallic boxes shall be permitted to be used with metal raceways or metal-armored cables.” Exception No. 2 also allows nonmetallic boxes: “Where internal bonding means with a provision for attaching an equipment bonding jumper inside the box are provided between all threaded entries in nonmetallic boxes listed for the purpose, nonmetallic boxes shall be permitted to be used with metal raceways or metal-armored cables.”

Securing Type MC cable

A manufactured wiring system uses Type MC Cable. Does this wiring method need to be secured in accordance with the requirements in Article 330, or is it exempt from these requirements because it is part of a manufactured wiring system?

Securing and supporting of Type MC cable must comply with the rules in Article 330. Section 604.7 reads: “Installation. Manufactured wiring systems shall be secured and supported in accordance with the applicable cable or conduit article for the cable or conduit type involved.”

Type MC cable must be secured every 6 feet unless the cable contains four or fewer conductors not larger than 10 AWG; then, the cable must be secured within 12 inches of every box, cabinet, fitting or other cable termination. Unsupported cables are permitted where fished or run between luminaires in accessible ceilings.

Service-disconnects grouping

The utility provides a 300-ampere service-drop to a one-family dwelling unit. Does the NEC permit a 100-ampere service-disconnect on the outside of the residence to supply a large garage and a 200-ampere service disconnect inside the building?

Exceptions to Section 230.40 allow more than one set of service-entrance conductors to be supplied from one service drop; therefore, Section 230.72 applies. Part (A) says: “Two to six disconnects as permitted in 230.71 shall be grouped. Each disconnect shall be marked to indicate the load served.”

The two disconnects must be marked and grouped at one location to comply with Section 230.72.

Retirement home emergency system

I currently have a retirement home with a 2,500-amp, 480-volt, three-phase service. It has an emergency generator for some of its emergency lighting, and other parts of the facility have battery-backup exit signs and bug eyes. The facility wants to install a new 500-kW backup generator for the whole facility. They want to interrupt the line between the main disconnect switch and the main distribution panel to install a transfer switch, which would provide power to the whole facility if they lose their permanent power. My concern with this is that all of the power, both emergency and normal building power, would transfer at the same time. Is this allowed by Code? If it is allowed, I would think the automatic transfer switch would need to be 2,500 amps to match the main disconnect switch. I look forward to your comments on this.

A single transfer switch cannot be used to transfer the total load to the emergency system. The 2008 edition of the NEC in Section 700.6(D) requires a transfer switch that supplies emergency loads only. This transfer switch must be listed for emergency system use by part (C) of Section 700.6.

Where a legally required standby system also is necessary, a separate transfer switch listed for legally required standby systems or emergency systems must be provided to comply with Section 701.7(C). The legally required standby system may also supply the optional standby system with load shedding as permitted by Section 701.6. If preferred, a transfer switch (manual or automatic) may be provided for the optional standby loads (see Sections 702.5 and 702.6).

Since the 500-kW generator probably cannot provide power to all of the remaining load that is not part of the emergency or legally required standby loads, a manual transfer switch should be provided for the optional loads so that these loads can be selected to prevent overloading of the generator while it is delivering power to the emergency and legally required loads.

Use of Type SJO cord

Is SJO cord (300 volt rated) acceptable for use on a 277-volt luminaire? The system voltage is 480Y/277. In an “open neutral” condition, the voltage between conductors in the cord could exceed 300 volts.

A Type SJO cord may supply 277-volt luminaires provided that the cord contains only one ungrounded conductor. This means that the cord can contain only one ungrounded conductor and one circuit--grounded conductor. An additional equipment-grounding conductor also is permitted. There can be no more than one ungrounded phase conductor in the cord assembly (see definitions of “voltage of a circuit,” “voltage, nominal,” and “voltage to ground.” Also, see 110.4 “voltages”).

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. Questions can be sent to ecmag@necanet.org.

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 in...

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.