For the National Electrical Code to be a complete regulation for the built environment, it must contain sufficient requirements to cover the overwhelming number of possible installations. Where it does not specifically address an installation or a method of doing something, it must provide a sufficient framework to judge whether an installation is safe. Many articles get reworked every cycle, and some high-profile sections get fine-tuned every cycle. Then there are those that we forget are there.

Article 326, Integrated Gas Spacer Cable (IGS), is a specialty cable that has been in the Code since 1984. These cables are insulated with dry kraft paper and pressurized sulfur hexafluoride. They cannot be used as inside wiring or be exposed in contact with buildings. In the NEC , IGS is limited to 600V.

Article 384, Strut-Type Channel Raceways, first appeared as part of the article on surface metal and nonmetallic raceways. At the time, it was Part C of Article 352. It first appeared as a separate article in the 2002 edition. Strut-type channel raceways are metal raceways that are intended to be mounted to the surface of or suspended from a structure, with associated accessories for electrical conductors and cable installation.

Article 394, Concealed Knob-and-Tube Wiring, is still in the Code because there are existing installations. Its only application today is for extensions to those existing installations. It is allowed elsewhere by special permission, but I don’t see that happening often. As this is an obsolete wiring system, there hasn’t been any Code activity in some time. A few cycles back there was a proposal for an article on historic wiring. The Code panel did not agree that it was a viable idea.

Article 398, Open Wiring on Insulators, has been in the Code for many cycles. It is an inexpensive wiring method that was used in mill construction and farm buildings. It is an exposed wiring method that was permitted in wet and dry locations. It is not permitted to be concealed by the structure of a building. It is required to be protected from physical damage. If it is within 7 feet of the floor, it is considered to be exposed to physical damage. Its construction is similar to knob-and-tube wiring, except for the prohibition on concealment. It is rarely used today.

Article 399, Outdoor Overhead Conductors Over 1,000 Volts, first appeared in the 2011 Code . Most overhead transmission and distribution systems are operated by public utilities, which are governed by the National Electrical Safety Code (NESC). However, some college campuses, military bases and industrial parks have privately owned systems not maintained by utilities. The NESC has many pages addressing hazards to overhead conductors, including wind loading, ice loading and sag. Such hazards also exist on private distribution systems. Article 399 is written in performance-based language to permit industry standards to be used for these installations, which would allow installations to comply with the NESC.

Article 411, Low-Voltage Lighting, covers lighting systems that operate at no more that 30V AC or 60V DC. It first appeared in the 1996 Code . Some of the early systems used fixtures suspended from exposed bare conductors.

Article 455, Phase Converters, was proposed as a new part to Article 430 in the 1990 Code , but did not become part of the NEC until the 1993 edition. Phase converters are used to convert single-phase power to three-phase power. Its primary application is areas where three-phase power is not commercially available, which tends to happen in rural areas. The most common method of obtaining three-phase power from a single-phase source is by using a rotary phase converter, which is a rotating machine-type generator. Some units also provide voltage regulation, which facilitates operation with voltage-sensitive electronic equipment. There are also some static power converters available, which use electronic means to convert a single-phase source to generate a three-phase output. These range from electronic circuits to simple arrangements that will switch capacitors during motor startups to provide an output that is out of phase with the existing line voltage.

Article 604, Manufactured Wiring Systems, discusses systems that use manufactured subassemblies that are assembled on-site. They are used for branch circuits, remote-control circuits, signaling and communications circuits. Manufactured wiring systems and all of their associated components are required to be listed.

Article 647, Sensitive Electronic Equipment, originated as Part G of Article 530, Motion Picture and Television Studios. It applies to separately derived 120V, three-wire, single-phase systems operating at 60V to ground. These circuits are primarily used where there is objectionable noise in sensitive electronic equipment. The primary application has been audio and video systems where there is a need to eliminate noise. These systems are permitted to be used in commercial and industrial establishments, but not residential. The location must be “under close supervision by qualified personnel.” This is one of very few places in the NEC where there is a limit on voltage drop. The limit on branch circuits is 1.5%. The combined voltage drop for feeders and branch circuits is 2.5%. The reason provided was to prevent impedances that might limit the ability of overcurrent devices to operate.

Article 650, Pipe Organs, covers large organs used in houses of worship. Modern pipe organs may produce sound electronically or through pressurized air. They are often very large and custom-built systems that are assembled in the field.

Article 665, Induction and Dielectric Heating Equipment, discusses industrial plants with processes that used induction and dielectric heating systems to provide process heating. Induction heating is used to heat metal parts, and dielectric heating is used to heat nonmetallic parts. Induction heating uses alternating current operating in a range of 50 hertz to 500 kilohertz and 5 kilowatts to 42 megawatts.

Dielectric heating operates on assigned frequencies of 13.56 megahertz (MHz), 27.12 MHz and 40.68 MHz. Some operate on microwave frequencies.

Article 670 covers industrial machinery. The NEC does not cover the internal wiring of equipment, so other requirements were needed. In 1961, NFPA 79, Industrial Machinery, was adopted as a tentative standard, subject to comments, under “Standard for Machine Tools.” It was created to address requirements for industrial machinery, which can consist of a number of different motors and controllers that work together in a manufacturing process. The standard has evolved over time so that it now recognizes equipment built to IEC standards with U.S. deviations. This allows some equipment to be imported if it complies with the U.S. deviations contained in NFPA 79.

Article 675 covers electrically driven irrigation machines, and these are occasionally found in rural areas where three-phase power is not available. They may be supplied by a phase converter, which is governed by the requirements of Article 455.

Article 685, Integrated Electrical Systems, discusses some facilities with processes that could become a serious hazard if the process lost power. The hazard could endanger personnel or damage equipment. Examples include chemical processes, steel production and large electromagnets used for lifting scrap iron. In the case of the scrap iron, dropping it could seriously injure workers. Removal of power for chemical processes can create a hazard for personnel, including the potential for an explosion. Removal of power for melting steel can result in damage to processes and equipment.

The purpose of the NEC is the protection of personnel and property from hazards arising from the use of electricity. The use of Article 685 is very limited. As noted in the scope, it can be used where “an orderly shutdown is required to minimize personnel hazard and equipment damage.”

There is a requirement for qualified personnel to supervise and maintain the installation. The responsible person must have documented safety training on the hazards involved, which must be maintained in the office responsible for the installation. Finally, the safeguards used for the installation must be approved by the authority having jurisdiction. A circuit fault will usually be indicated by an alarm. This gives the trained personnel the opportunity to initiate an orderly shutdown. There is a table in 685.3 that refers to a number of sections in other parts of the Code that correlate the requirements for shutdown. The overcurrent protection rules in 240.12 and 240.13(1) govern installation where an orderly shutdown is essential.

There isn’t a lot in Article 720, Circuits and Equipment Operating at Less Than 50 Volts. Several articles have their own requirements for installations operating at less than 50V.

Article 727, Instrument Tray Cable, is located in an odd place. These cables operate at 150V or less and 5A or less. For the 2023 Code , relocation to Chapter 3 has been proposed so that it appears with other wiring articles.

Article 810, Radio and Television Equipment, covers radio and television receiving equipment installations, amateur radio transmission and receiving equipment and citizens band (CB) radio equipment. Amateur radio is licensed by the Federal Communications Commission (FCC). CB radio no longer requires a license. Most people using either of these services are unaware that they are also covered by the NEC because such installations are never inspected by an AHJ. The FCC reserves the right to inspect an installation; however, it is not inspecting for Code compliance, but is primarily concerned with transmissions that interfere with other radio services.

Some of these articles may be obscure. However, having rules in the NEC provides the framework for a safe installation.