General Installation Requirements, Part XXXVIII

CIF March 2018 Figure 1

Article 110 in the National Electrical Code (NEC) is the second of only two articles in Chapter 1; the first is Article 100, Definitions. Article 100 contains only those definitions essential to the application of the NEC. It does not include commonly defined general or technical terms from related codes and standards.

Article 110, Requirements for Electrical Installations, is divided into five parts. Part I contains general requirements for approval, examination, identification, installation, use and listing of equipment. Part II contains provisions for electrical systems rated 1,000 volts (V), nominal, or less. Part III contains provisions for conductors and equipment rated over 1,000V, nominal. 

The provisions in Part IV shall apply to the installation and use of high-voltage power distribution and utilization equipment that is portable, mobile or both, such as substations, trailers, cars, mobile shovels, draglines, hoists, drills, dredges, compressors, pumps, conveyors, underground excavators and the like. Part V in Article 110 contains requirements for manholes and other electrical enclosures intended for personnel entry.

Working space requirements for electrical equipment rated over 1,000V, nominal, are in 110.34. Section 110.34(F) pertains to service equipment, switchgear and industrial control assemblies. Pipes or ducts foreign to the electrical installation and requiring periodic maintenance or whose malfunction would endanger the operation of the electrical system shall not be located in the vicinity of the service equipment, switchgear or industrial control assemblies.

Two different types of installations are prohibited in this one sentence. This first sentence mentions circumstances where pipes and ducts would not be permitted near the electrical equipment, but if the pipes or ducts are part of the electrical installation, they would be permitted. There are two types of installations where these pipes and ducts would not be permitted. The first type of installation is where the pipes or ducts would require some type of intermittent maintenance. The second type of installation is where the pipes or ducts would endanger the operation of the electrical system if they were to malfunction. In both installations, the prohibited area is not just the area directly over service equipment, switchgear and industrial control assemblies. Both of these types of installations are not only prohibited above the equipment, they cannot even be installed in the vicinity of this equipment.

While the area in the vicinity of the electrical equipment is not the working space, most, if not all of the working space area would be in the vicinity of the electrical equipment. For example, a clean-out for a drain pipe has been installed in the vicinity of a 4,160V switchgear. Because this drain pipe clean-out would require periodic maintenance and it is in the vicinity of switchgear rated over 1,000V, this installation is in violation of 110.34(F) (see Figure 1).

Figure 1

The second sentence in this section states that protection shall be provided where necessary to avoid damage from condensation leaks and breaks in such foreign systems. The foreign systems might be piping or ducts like in the first sentence, but these foreign systems can be installed in the vicinity if they do not require periodic maintenance and if they will not endanger the electrical system if they were to malfunction. If there is a possibility the piping or ducts would endanger the operation of the electrical system if they malfunction, they might be permitted in the vicinity if protection from condensation leaks and breaks is provided.

For example, a copper water pipe is in the vicinity of switchgear rated over 1,000V, nominal. The copper pipe is not a continuous unbroken pipe because there are couplings in it. In the case of a possible malfunction or condensation, a metal drip pan has been installed below the copper pipe to provide protection from condensation, leaks and breaks in this foreign system. The sides of the drip pan also cover the sides of the copper pipe to provide protection. Because protection is provided, this installation is permitted (see Figure 2).

Figure 2

The last sentence in 110.34(F) mentions piping and other facilities that are allowed in the vicinity of service equipment, switchgear and industrial control assemblies. Piping and other facilities shall not be considered foreign if provided for fire protection of the electrical installation. As long as the piping is installed to provide fire protection for the electrical installation, it is not considered foreign and shall be permitted in the vicinity of service equipment, switchgear or industrial control assemblies (see Figure 3).

Figure 3

The next section in Part III pertains to circuit conductors. A number of options are listed for conductors that will be supplying power to electrical systems rated over 1,000V, nominal. In accordance with 110.36, circuit conductors can be installed in raceways; in cable trays; as metal-clad cable Type MC; as bare wire, cable and busbars; or as Type MV cables or conductors as provided in 300.37, 300.39, 300.40 and 300.50.

Article 300 contains general requirements for wiring method and materials and all four of these sections are in Part II, requirements for over 1,000V, nominal. Requirements in these four sections pertain to above-ground wiring methods, exposed runs of braid-covered insulated conductors, insulating shielding and underground installations. As stated in 110.36, bare energized conductors shall be installed in accordance with the requirements in 490.24. Article 490 covers the general requirements for equipment operating at more than 1,000V, nominal. Section 490.24 pertains to the minimum space separation in field-fabricated installations. 

The second paragraph in 110.36 pertains to the insulators used to mount wires, single-conductor cables or busbars. Insulators, together with their mounting and conductor attachments, where used as supports for wires, single-conductor cables, or busbars, shall be capable of safely withstanding the maximum magnetic forces that would prevail if two or more conductors of a circuit were subjected to short-circuit current. A tremendous amount of magnetic force can move and twist conductors that are short circuited. If insulators or conductor attachments break during a short circuit and the conductors are no longer tied down, there could be additional damage from loose wires whipping around.

The last sentence in 110.36 also pertains to conductor supports. Exposed runs of insulated wires and cables that have a bare lead sheath or a braided outer covering shall be supported in a manner designed to prevent physical damage to the braid or sheath. Supports for lead-covered cables shall be designed to prevent electrolysis of the sheath.

Temperature limitation provisions for conductors are in 110.14(C) but for conductors used in systems over 1,000V, nominal, conductor temperature limitation provisions are in 110.40. Conductors shall be permitted to be terminated based on the 90°C (194°F) temperature rating and ampacity as given in Table 310.60(C)(67) through Table 310.60(C)(86), unless otherwise identified. Conductor ampacities shall be permitted to be based on the 90°C column because the terminations for these conductors are rated at least 90°C. Ampacities for conductors over 1,000V, nominal, are listed in Tables 310.60(C)(67) through (C)(86). Conductors in Table 310.15(B)(16) are based on an ambient temperature of 30°C (86°F) but conductors rated over 1,000V in air are based on an ambient temperature of 40°C (104°F). When the ambient air temperature is something other than 40°C (104°F), use the ambient temperature correction factors in Table 310.60(C)(4). Each of the 310.60 tables show ampacities for either copper conductors or aluminum conductors. To find the allowable ampacity, start by finding the correct table for the conductor material and the conductor installation.

For example, what size Type MV-90 aluminum conductors are required for a 100-ampere (A) load? The aluminum conductors will be triplexed in air and the ambient temperature will not exceed 40°C. The phase-to-phase voltage on these conductors will be 4,160V.

Start by finding the correct table; the correct table is Table 10.60(C)(68). The row containing the correct ampacities is the row under Type MV-90 and below 2,001–5,000V. The minimum size conductors for this installation is 2 AWG (see Figure 4).

Figure 4


About the Author

Charles R. Miller

Code Contributor
Charles R. Miller, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and NFPA’s “Electrical Reference.” He can be reached at 615....

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