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Cable wiring methods combine insulated conductors with an outer sheath, armor, jacket, or other protective construction to form a complete wiring assembly. Unlike raceway systems, where conductors are installed into a separate conduit or tubing system, cable assemblies are manufactured as a unit and installed as the wiring method itself.
For the NASCLA Electrical Contractor Exam, you should be able to distinguish common cable types including NM, AC, MC, UF, and SE cable, locate their individual NEC articles, determine where each is permitted, recognize wet-location and physical-protection limitations, and apply the correct securing, supporting, grounding, and installation requirements.
Cable Assemblies Are Wiring Methods
A cable assembly contains the circuit conductors and the protective covering needed for the installation. Depending on the cable type, that covering may be a nonmetallic sheath, metallic armor, moisture-resistant jacket, or a combination of materials.
Because the cable itself is the wiring method, its permitted uses and limitations are determined by the NEC article covering that cable type. A cable suitable for one environment cannot automatically be substituted for another simply because the conductor sizes and number of conductors are the same.
NM Cable Is Not a Raceway
Type NM cable is a manufactured cable assembly. It is not a raceway and should not be treated as though its individual conductors were simply installed inside a removable sheath.
This distinction affects conductor installation, fill calculations, support requirements, replacement methods, and the Code articles used to answer exam questions.
Know the Cable Articles
As with raceways, quickly locating the correct Article is often more valuable than attempting to memorize every installation rule.
Article 320 — AC
Armored Cable, Type AC.
Article 330 — MC
Metal-Clad Cable, Type MC.
Article 334 — NM and NMC
Nonmetallic-Sheathed Cable, including the common Type NM wiring method.
Article 340 — UF
Underground Feeder and Branch-Circuit Cable, Type UF.
Article 338 — SE and USE
Service-Entrance Cable, Types SE and USE.
Type NM Cable
Type NM cable is one of the most familiar wiring methods in residential electrical construction. It typically contains insulated circuit conductors, an equipment grounding conductor, fillers where necessary, and an overall nonmetallic sheath.
Article 334 establishes where NM cable may be used and where it is prohibited. One of the most important limitations is that Type NM cable is intended for dry locations and cannot be used in wet or damp locations.
Residential Use
NM cable is widely used for branch circuits and feeders in qualifying dwelling and building construction.
Nonmetallic Sheath
The outer sheath provides a convenient cable assembly but does not provide the mechanical protection of rigid metallic raceway.
Dry Locations
NM cable is intended for dry-location installations and is prohibited in damp and wet locations.
Separate EGC
The cable assembly includes or otherwise provides the required equipment grounding conductor because the nonmetallic sheath cannot serve as the grounding path.
Putting NM Cable in Conduit Does Not Make It Wet-Location Cable
A raceway installed outdoors or underground can have a wet interior. Installing ordinary NM cable inside that raceway does not change the cable’s listing or make the cable suitable for a wet location.
Type NM cable in an underground raceway is therefore not permitted merely because the cable is enclosed by PVC or another raceway. The cable itself must be suitable for the environment. :contentReference[oaicite:1]{index=1}
Supporting NM Cable
NEC 334.30 contains the securing and supporting requirements for NM cable. Under the general rule, NM cable must be supported and secured at intervals not exceeding 4½ feet and generally secured within 12 inches of boxes, cabinets, or fittings, subject to the specific provisions and exceptions in the Code.
Cable routed horizontally through qualifying holes in framing members can be considered supported and secured when the installation complies with the requirements of 334.30. Fished cable in existing finished construction can also qualify for an exception where support is impracticable. :contentReference[oaicite:2]{index=2}
Protecting NM Cable From Damage
NM cable is relatively easy to install, but the outer sheath provides limited protection from nails, screws, impact, abrasion, and other physical damage. The installation must therefore comply with the physical-protection provisions of Article 300 and Article 334.
Framing Members
Cables passing through studs, joists, or similar framing must be positioned or protected to reduce the risk of penetration by nails or screws.
Exposed Locations
Where cable is exposed to physical damage, additional protection or another wiring method may be required.
Boxes and Entries
Cables must enter boxes and enclosures through approved openings and be secured where required without damaging the sheath.
Staples and Fasteners
Fasteners must support the cable without crushing, cutting, or otherwise damaging the cable assembly.
Type AC Cable
Type AC cable is a factory cable assembly containing insulated conductors within flexible metallic armor. Article 320 governs its use and installation.
Although its metallic exterior resembles some types of MC cable, AC and MC are not the same wiring method. Their construction, grounding methods, wet-location suitability, fittings, and permitted uses differ.
Metal Armor
AC cable uses interlocked metallic armor surrounding the insulated conductors.
Bonding Strip
Traditional Type AC construction incorporates a bonding strip that works with the armor and fittings as part of the equipment grounding path.
Dry Locations
Type AC is permitted in dry locations and is not permitted in damp or wet locations. :contentReference[oaicite:3]{index=3}
Special Fittings
Fittings must be identified for Type AC cable and installed so the cable remains securely connected and properly bonded.
Do Not Call Every Armored Cable “BX”
Field terminology can create confusion. Older electricians may use the term BX generically for armored cable, but the NEC distinguishes Type AC cable from Type MC cable and other metallic cable systems.
On an exam, use the actual cable designation given in the question rather than relying on informal trade terminology.
Type MC Cable
Type MC cable is covered by Article 330 and is widely used in commercial and industrial installations. It contains insulated conductors enclosed in metallic armor or another metallic covering, and the construction can vary depending on the listed cable type.
MC cable has broader permitted uses than Type AC. Depending on the specific listed construction, it can be used for branch circuits, feeders, services, indoor and outdoor locations, exposed or concealed work, cable trays, wet locations, and direct burial when specifically identified for those applications. :contentReference[oaicite:4]{index=4}
Branch Circuits
MC cable is widely used for branch-circuit wiring in commercial and industrial buildings.
Feeders and Services
Article 330 permits MC cable in feeder and service applications where all applicable requirements are satisfied.
Wet Locations
Certain MC cable constructions are permitted in damp or wet locations when the cable has the required corrosion-resistant and moisture-protective construction.
Direct Burial
MC cable can be directly buried only when the cable is specifically identified for that purpose.
MC Does Not Automatically Mean Wet-Location Rated
Type MC cable can be manufactured for wet-location and direct-burial applications, but ordinary MC cable should not automatically be assumed suitable for those environments.
When the installation is wet, underground, corrosive, or direct buried, verify that the specific MC cable construction is identified for that application. :contentReference[oaicite:5]{index=5}
Similar Appearance, Different Wiring Methods
AC and MC cable are easy to confuse because both can have flexible metallic armor. The Code does not treat them as interchangeable.
Type AC
Article 320
Metal armor
Traditional bonding strip construction
Dry-location wiring method
Not permitted in damp or wet locations
Type MC
Article 330
Multiple metallic sheath or armor constructions
Typically includes an insulated or bare equipment grounding conductor depending on construction
Broader permitted uses
Some types identified for wet locations or direct burial
Securing and Supporting MC Cable
Article 330 contains the securing and supporting requirements for MC cable. MC cable must be supported and secured using methods such as suitable straps, hangers, listed cable ties, or other approved means that do not damage the cable.
Under the general rule, MC cable is supported at intervals not exceeding 6 feet and secured within the distance of terminations required by 330.30, subject to the exceptions and special conditions contained in that section. :contentReference[oaicite:6]{index=6}
Type UF Cable
Type UF cable is covered by Article 340. It is a cable assembly specifically intended for applications where moisture resistance and underground use are required.
Type UF is permitted underground in accordance with Article 300.5 and can be used for wiring in wet, dry, and corrosive locations where suitable. Where UF cable is installed in applications similar to NM cable, the applicable Article 334 installation requirements also apply. :contentReference[oaicite:7]{index=7}
Wet Locations
UF cable is suitable for wet-location applications within the conditions of its listing and Article 340.
Underground
UF cable can be installed underground when the installation complies with Article 300.5.
Direct Burial
Type UF is commonly used as a direct-burial cable where the installation meets applicable cover and physical-protection requirements.
Above Ground
UF cable can also be used in qualifying above-ground applications when installed according to the applicable Article 340 and Article 334 provisions.
Moisture Suitability Is the Major Difference
NM and UF cable can appear similar in some installations, but their environmental ratings are very different. NM is a dry-location wiring method, while UF is designed for wet and underground environments.
If an exam question involves an underground feeder to a detached structure, exterior equipment, or another wet-location installation, ordinary NM cable is not made acceptable merely by placing it inside conduit.
Type SE Cable
Article 338 covers service-entrance cable, including Types SE and USE. Type SE cable is commonly associated with service conductors and feeders, particularly in dwelling installations.
SE cable may also be used for branch circuits and feeders when the requirements of 338.10 are satisfied. Interior branch-circuit and feeder installations generally follow applicable Type NM installation provisions, subject to the additional limitations of Article 338. :contentReference[oaicite:8]{index=8}
SE
Service-entrance cable intended primarily for above-ground service applications and other permitted feeders and branch circuits.
SER
A common multiconductor Type SE configuration containing insulated circuit conductors and an equipment grounding conductor suitable for qualifying feeder applications.
USE
Underground Service-Entrance cable is intended for underground service applications but has different restrictions from Type SE, including important limitations on interior wiring.
SE and USE Are Not Interchangeable
Article 338 separates Type SE and Type USE because their permitted applications differ. Type USE is intended primarily for underground use and is generally prohibited for interior wiring unless the cable also carries another listing that permits the interior application.
Likewise, ordinary Type SE cable is not simply treated as underground cable. When an exam question involves service-entrance cable, identify the exact marking before choosing the applicable rule. :contentReference[oaicite:9]{index=9}
How Cable Assemblies Provide an Equipment Grounding Path
Different cable wiring methods provide equipment grounding in different ways. The outer covering should never be assumed to be the grounding path unless the NEC specifically recognizes the cable construction for that purpose.
Wire-Type EGC
NM cable normally contains a bare or insulated equipment grounding conductor because its nonmetallic sheath cannot serve as the grounding path.
Armor and Bonding Path
Qualifying Type AC construction uses the armor together with its bonding strip and fittings to provide the equipment grounding path.
Depends on Construction
Many MC cable assemblies contain a separate equipment grounding conductor, while certain listed constructions can use other recognized grounding methods.
Wire-Type EGC
The nonmetallic cable assembly includes the equipment grounding conductor needed for the circuit where grounding is required.
Boxes, Connectors, and Fittings
Cable wiring methods must enter boxes, cabinets, panels, and equipment through fittings or openings appropriate for the cable type and enclosure. The fitting must secure the cable without damaging the armor, sheath, or conductor insulation.
NM Connectors
Secure nonmetallic-sheathed cable where the box construction or installation requires a connector or clamp.
AC Fittings
Must be identified for armored cable and maintain the required grounding continuity.
MC Fittings
Must be listed for the particular MC cable construction and for the installation environment.
Wet-Location Entries
Where a cable enters equipment in a wet location, the cable, fitting, box, and enclosure entry must all be suitable for the environment.
Minimum Bend Radius and Handling
Cable assemblies must not be bent, crushed, twisted, or installed in a manner that damages their conductors, armor, sheath, or jacket. Each cable article includes bending provisions applicable to the particular construction.
A bend that looks physically possible may still be too tight for Code compliance or may damage the conductors inside. Larger cable assemblies generally require larger bend radii.
Cable Construction Can Affect Conductor Ampacity
The ampacity of conductors contained in a cable assembly is not determined solely by reading the conductor size from Table 310.16. Cable-specific rules can limit the temperature column or require adjustment under particular installation conditions.
For example, NM cable has specific ampacity provisions in 334.80, while AC and MC cable have their own ampacity requirements in Articles 320 and 330. Bundling multiple cables or installing them in thermal insulation can also require conductor ampacity adjustment.
Cable Can Be Installed in Raceway for Protection
A cable wiring method can sometimes be installed through a raceway where additional physical protection is needed. A common example is protecting a cable where it passes through a vulnerable area or emerges from grade.
Installing cable inside raceway does not convert the cable into individual conductors and does not eliminate the cable’s own environmental limitations. Raceway fill and other applicable requirements must still be evaluated.
The Raceway Does Not Change the Cable Listing
If NM cable is prohibited because the location is wet, installing the NM cable in PVC does not correct the problem. If a cable is not identified for direct burial, placing soil around it does not make it direct-burial cable.
Always evaluate both the cable type and the installation environment independently.
Choosing the Correct Cable Wiring Method
Interior Dwelling Branch Circuit
Type NM is commonly used where the building construction and dry-location requirements permit it.
Commercial Interior Wiring
Type MC is commonly selected for branch circuits and feeders because of its armored construction and broad permitted uses.
Underground Branch Circuit
Type UF is a common cable method where direct burial or other wet-location underground wiring is permitted.
Service or Feeder
Type SE or another permitted service/feeder wiring method may be appropriate depending on whether the installation is interior, exterior, overhead, or underground.
Identify the Cable Before Answering
Cable questions frequently include subtle differences in the wording. A question that changes one designation from NM to UF or AC to MC may completely change whether the installation is permitted.
Identify the Cable Type
Look for NM, NMC, AC, MC, UF, SE, USE, or another specific cable designation.
Locate the Cable Article
Use the exact Chapter 3 article covering the wiring method.
Identify the Environment
Determine whether the location is dry, damp, wet, underground, exposed, concealed, corrosive, or subject to physical damage.
Check Uses Permitted and Not Permitted
Verify that the cable type is allowed in the environment before working on dimensions, ampacity, or support requirements.
Check Support and Protection
Apply the securing, supporting, physical-protection, and fitting requirements of the cable article.
Evaluate Grounding and Ampacity
Determine how the cable provides an equipment grounding path and whether cable-specific ampacity rules apply.
Use the Same Pattern for Every Cable Article
When you locate the cable article, look first for the sections covering uses permitted, uses not permitted, bending, securing and supporting, fittings, and ampacity. The recurring structure of Chapter 3 articles can make cable questions much faster to solve during an open-book exam.
Code Locations to Know
Article 320
Armored Cable, Type AC.
Article 330
Metal-Clad Cable, Type MC.
Article 334
Nonmetallic-Sheathed Cable, Types NM and NMC.
334.30
Securing and supporting requirements for Type NM cable.
Article 340
Underground Feeder and Branch-Circuit Cable, Type UF.
Article 338
Service-Entrance Cable, Types SE and USE.
300.5
Underground installation requirements affecting UF and other underground wiring methods.
Article 250
Grounding and bonding requirements affecting cable assemblies and equipment grounding paths.
Key Points to Remember
- Cable assemblies are complete wiring methods and are not raceways.
- Type AC is covered by Article 320, MC by Article 330, NM by Article 334, UF by Article 340, and SE/USE by Article 338.
- Type NM cable is widely used in qualifying dry-location residential and similar installations.
- NM cable is not permitted in damp or wet locations.
- Putting NM cable inside conduit does not make it suitable for a wet or underground location.
- NM cable is generally supported and secured at intervals not exceeding 4½ feet and secured near terminations as required by 334.30.
- Type AC and Type MC cable are separate wiring methods even though both may have metallic armor.
- Type AC is generally limited to dry-location applications and is not permitted in damp or wet locations.
- Type MC has broader permitted uses than Type AC.
- Certain MC cable constructions can be used in wet locations or direct buried when specifically identified for those applications.
- MC cable does not automatically qualify for wet-location or direct-burial use merely because it has metal armor.
- Type UF cable is permitted in wet and underground applications and is commonly used for direct-burial circuits.
- Type SE and Type USE have different permitted applications and must not be treated as interchangeable.
- Type USE is primarily an underground wiring method and has important interior-wiring limitations.
- Different cable types provide the equipment grounding path in different ways.
- Cable fittings must be appropriate for both the cable type and the environment.
- Cable-specific ampacity requirements can modify the ampacity that might otherwise be expected from general conductor tables.
- Installing cable in raceway for protection does not change the cable’s listing or environmental limitations.
- On the exam, identify the exact cable type before applying the wiring-method requirements.
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