Understanding NEC Article 320: The Blueprint
In the electrical trade, choosing the right wiring method for a specific environment is the difference between a long-lasting installation and a hazardous failure. One of the oldest and most reliable wiring methods used in both residential and commercial construction is Armored Cable, commonly referred to as Type AC.
For professionals and students alike, Understanding NEC Article 320 is a fundamental requirement. This section of the National Electrical Code provides the definitive rules for the construction, installation, and usage of Type AC cable. By mastering the guidelines within this article, electricians ensure that their projects are physically protected and electrically sound.
What is Type AC Armored Cable?
Before diving into the installation rules, you must identify exactly what qualifies as Type AC. It is a fabricated assembly of insulated conductors enclosed in a flexible metallic enclosure. This enclosure is typically made of galvanized steel or aluminum strip, which is interlocked to provide significant mechanical protection.
A key feature to remember when Understanding NEC Article 320 is the internal bonding strip. Type AC cable contains an internal bonding strip of copper or aluminum in intimate contact with the armor. This combination of the armor and the strip provides a reliable path for equipment grounding.
Permitted Uses in Construction
Knowing where you are legally allowed to install Type AC cable is essential for passing any inspection. Section 320.10 clearly outlines the permitted applications for this armored wiring method.
It is widely permitted for both exposed and concealed work in dry locations. You will frequently find it used in the wall cavities of residential homes and in the overhead spaces of commercial office buildings. Additionally, it is allowed to be embedded in plaster finishes or brick, provided the environment remains dry.
When Understanding NEC Article 320, you should also note its use in cable trays and as a feeder or branch circuit. Its robust metal exterior makes it an excellent choice for areas where conductors need more protection than a standard nonmetallic (NM) cable can provide.
Strict Code Prohibitions
Just as vital as knowing where to use it is knowing where Type AC is strictly forbidden. Section 320.12 establishes the “no-go” zones for this material.
You are explicitly prohibited from using Type AC cable in wet or damp locations. Since the armor is not liquid-tight, moisture can penetrate the assembly and cause the conductors to fail. Furthermore, it cannot be used in locations subject to physical damage that could crush the metal armor.
Thoroughly Understanding NEC Article 320 also reveals that it cannot be used in most hazardous (classified) locations, except as permitted elsewhere in the code. It is also generally prohibited for direct burial in the earth or encasement in concrete.
Installation and Support Mandates
A metal cable left hanging without support is a violation and a safety risk. Section 320.30 establishes the strict requirements for securing and supporting Type AC runs.
Generally, the cable must be securely fastened in place by an approved staple, strap, or cable tie at intervals not exceeding 4.5 feet (1.4 meters). Additionally, a support must be placed within 12 inches (300 mm) of every outlet box, junction box, or fitting.
When Understanding NEC Article 320, you will find practical exceptions for “fished” work. If you are pulling cable through finished walls where support is impossible, the requirement for securing the cable is waived for those hidden sections.
Bending Radius and Protection
Bending a metal-clad cable too sharply can kink the armor and damage the internal insulation. The code mandates that the radius of the curve of the inner edge of any bend shall not be less than five times the diameter of the cable.
Furthermore, if the cable is run through bored holes in wood members, it must be protected. If the hole is less than 1.25 inches from the edge of the wood, a steel nail plate must be installed to prevent screws or nails from piercing the armor. This is a critical detail in Understanding NEC Article 320 that prevents future electrical fires.
Fittings and Anti-Short Bushings
One of the most unique requirements of this article concerns the termination of the cable. Because the cut edge of the metal armor is sharp, it can easily slice through wire insulation.
Section 320.40 requires that an insulating bushing (often called a “redheaded” or “red devil”) be installed between the conductors and the armor at every termination point. This bushing provides a smooth, rounded edge that protects the wires. In the context of Understanding NEC Article 320, failing to install these bushings is one of the most common reasons for a failed electrical inspection.
Conclusion
Ultimately, Understanding NEC Article 320 provides the technical blueprint for utilizing one of the industry’s most durable wiring methods. By following the rules for support, bending, and termination, contractors can execute projects that withstand the test of time.
Mastering Type AC cable requirements ensures that you are providing a high-quality, grounded system that protects both the conductors and the structure. As you continue your journey in the electrical trade, keep these Article 320 principles at the forefront of every armored cable installation.
