Feeder on the Roof, Metal Studs and More

Published On
Oct 15, 2022

Jim Dollard has an extensive background in codes and standards. Send questions about the National Electrical Code (NEC) to Jim at codefaqs@gmail.com. Answers are based on the 2020 NEC.

Feeder on the roof

The only viable path for a feeder we are installing is on a shopping mall’s roof. What are the minimum distances from the roof and the ampacity correction factors? I can’t find them.

The requirements for raceways or cables exposed to direct sunlight on or above rooftops were significantly revised in the 2017 NEC revision cycle. The proposal that revised this requirement included significant technical substantiation, proving that as long as there was air space between the raceway or cable assembly and the roof, no derating is required. This revision (310.15(B)(2)) now requires a minimum distance of 7/8 inch above the roof to the bottom of the raceway or cable. This dimension was chosen based upon an installation placing 7/8-inch unistrut directly on a roof to support a raceway.

Where a minimum distance of 7/8-inch above the roof to the bottom of the raceway or cable is not maintained, a 33°C (60°F) temperature adder must be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(1) or Table 310.15(B)(2). It is important to note that Type XHHW-2 insulated conductors are not subject to this ampacity adjustment. See the exception following 310.15(B)(2).

Metal studs

Where metal studs are used inside a building, are we required to bond or ground them? I heard of a situation where metal studs were energized by Type NM cable incorrectly installed by a homeowner.

The NEC requires normally noncurrent-carrying electrically conductive materials that are “likely to become energized” to be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path. See 250.4(A)(4). The key here is “likely to become energized.” Properly installed cable assemblies are not “likely to energize” metal studs. The same holds true for other metal on and in dwelling units. For example, are aluminum siding and metal roofs required to be grounded? No, they are not “likely to become energized.”

Metal water piping and ground rods

Where new services are installed or upgraded, we always supplement the metal water piping with two ground rods. The question is, when are two ground rods required? I have had a few inspectors tell me I only need one ground rod. Is that correct?

The metal underground water pipe must always be supplemented by another electrode. For example, if a concrete-encased electrode exists, it becomes the supplemental electrode and no ground rod is required. Section 250.52(A) lists all of the grounding electrodes permitted, including one of the most common electrodes in 250.52(A)(1), Metal Underground Water Pipe. See 250.53(D)(2), which requires that where a metal underground water pipe is used as an electrode, it must be supplemented by an additional electrode as specified in 250.52(A)(2) through (A)(8).

The most common supplemental electrode is a ground rod, and 250.52(D)(2) requires that where a ground rod is used to supplement the metal underground water pipe, it must also comply with 250.53(A). Section 250.53 provides the Code user with requirements for electrode installation, and 250.53(A)(2) requires that a ground rod be supplemented with an additional electrode as specified in 250.52(A)(2) through (A)(8).

The ground rod cannot be supplemented by the metal underground water pipe. A single ground rod is permitted when it is supplemented by 250.52(A)(2), a metal in-ground support structure; (A)(3), a concrete encased electrode; (A)(4), a ground ring; (A)(5), another rod or pipe; (A)(6), other listed electrodes (such as a chemical rod assembly); (A)(7), a plate electrode; (A)(8), other local metal underground system (typically a well casing); or where the single rod alone is measured at 25 ohms or less, as seen in the exception to 250.53(A)(2).

Is more than one service permitted?

A new project design has two service laterals to the building supplying two separate 800A fused service disconnects. Is that permitted? It is my understanding that only one service can supply a building unless different voltages or large capacity comes into play.

Yes, it is permitted. The general rule in Article 230 limits each building or structure to only one service. However, the parent text in Section 230.2 permits more than one service lateral under specific conditions. This permission is based on the provisions of 230.40 Exception No. 2 only. This exception allows a single service lateral to supply multiple service equipment enclosures. The permissive text in 230.2 recognizes underground sets of conductors, 1/0 AWG and larger, running to the same location and connected together at their supply end, but not connected together at their load end as supplying one service.

References extracted from other codes and standards

An inspector helped us out significantly on a recent fire pump installation. He mentioned that most fire pump requirements exist in another NFPA code for fire pumps. How do we get that right in the future, which rules apply and do I need to buy a fire pump code?

As the installer of the electrical installation, you must follow the rules in the NEC . The inspector was referring to NEC requirements extracted from another NFPA code or standard. For example, where another NFPA code or standard has purview over performance (such as with NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection), the NEC style manual permits an extracted requirement to be placed into the Code . It can be edited only to make the wording style consistent with the NEC Style Manual, and then only with the concurrence of the committee having primary jurisdiction.

Where this occurs, the extract reference is provided as a guide to indicate the source of the extracted text and is placed in brackets for informational purposes only immediately following the extracted text. For example, see the requirements for permitted individual fire pump power sources in 695.3(A). Each second-level subdivision for individual sources in 695.3(A)(1) through (A)(3) are followed by the NFPA 20 referenced section in brackets at the end of each subdivision. Each of these references provide the Code user with the specific performance requirement in Chapter 9 of NFPA 20.

GFPE protection requirements

Due to supply chain issues, we were directed by the engineer to substitute (3) 1,200A fused disconnects instead of 1,200A circuit breakers as service equipment at 480/277V. The inspector is asking how we will meet the arc energy reduction and GFPE protection requirements. Can we just use smaller fuses to be compliant?

The arc energy reduction requirements of Section 240.67 are specifically directed at fuses rated 1,200A or higher. This would allow the use of 1,000A fuses in a 1,200A fusible disconnect without the need for a means of arc energy reduction as required by 240.67. The GFPE requirement in Section 230.95 is very different where fuses are installed. It is important to note that the GFPE requirements in 230.95 apply only to solidly grounded wye electric services of more than 150V to ground, but not exceeding 1,000V phase to phase for each disconnect rated 1,000A or more. It is also important to understand how fuses are specifically addressed in this requirement. The application of GFPE in 230.95 is not based on the rating of the actual fuse installed; it is based on the “rating of the largest fuse that can be installed.” This means that 1,200A fused disconnects, as described in your question, must be provided with GFPE protection as required in 230.95.

Pneumatic airline in raceway?

We were asked by an owner and their engineering staff if it is permitted to install a pneumatic airline in some conduits and cable tray to supply industrial equipment. Is that permitted?

No, Section 300.8 prohibits any pipe, tube or equal for steam, water, air, gas, drainage or any service other than electrical in raceways or cable trays containing electrical conductors.

About the Author

Jim Dollard

Code Columnist

Jim Dollard is the safety coordinator for IBEW Local 98 in Philadelphia. He is a member of the NEC Correlating Committee, NEC CMP-10, NEC CMP-13, NFPA 70E, NFPA 90A/B and the UL Electrical Council. He can be reached at codefaqs@gmail.com.

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