Guide to Electrical Cable Types: Romex, MC, THHN & More

Understanding the Essentials of Electrical Cable Types

As a licensed electrician, selecting the correct material is the foundation of every safe and compliant installation. This guide provides an expert overview of the primary electrical cable types you’ll encounter in residential, commercial, and industrial settings. Understanding the distinctions between romex wire, mc cable, and individual conductors like thhn wire is critical for code compliance and job-site efficiency. From the ubiquitous non metallic sheathed cable used in homes to robust armored cables for commercial build-outs, each has specific applications, limitations, and installation requirements dictated by the National Electrical Code (NEC). We will cover common sizes like 12/2 wire and 14/2 wire, explore their construction, and discuss how to choose the appropriate cable for your next project, ensuring your work is both safe and professional.

Commonly Used Electrical Cables in the Field

The vast majority of day-to-day electrical work involves a core group of cables. Let’s break down the construction and approved uses for these workhorses of the trade.

Non-Metallic Sheathed Cable (NM / Romex)

Often referred to by the popular brand name Romex®, nonmetallic-sheathed cable (NM-B) is the standard for most residential wiring. It consists of two or more insulated conductors (for example, a hot and a neutral wire) and an equipment grounding conductor (which may be bare, insulated, or covered) enclosed by an overall nonmetallic sheath. The familiar `NM-B` designation indicates the conductor insulation is listed for 90°C; however, NEC Article 334 limits the usable ampacity so that the final ampacity shall not exceed the values associated with the 60°C column. The 90°C rating can be used when applying adjustment and correction factors provided the final calculated ampacity does not exceed the 60°C rating.

Typical romex wiring involves common sizes like 14 2 wire for 15A circuits and 12 2 wire for 20A circuits. Larger appliances might require 6/3 wire or 8/3 wire, or specific feeder or range conductors sized per the appliance and load calculations. When terminating, you must use a proper romex connector to secure the cable to the box and protect it from sharp edges. It’s crucial to understand the latest NM-B cable installation requirements. For instance, recent NEC updates have clarified how to protect this wire romex from physical harm. For detailed guidance, electricians should review how 2023 NEC updates have changed NM and NMC cable installation rules and best practices for protecting NM and NMC cable from physical damage. If any wiring repaired is necessary, it must be done within an accessible junction box.

Metal Clad (MC) and Armored Cable (AC / BX)

For commercial applications or areas where physical protection is paramount, metal clad cable is the professional’s choice. MC cable features insulated conductors enclosed in a flexible metallic interlocking armor that may be aluminum or steel. Some MC assemblies include a separate equipment grounding conductor; others rely on the continuous metal sheath or armor as part of the grounding path depending on the cable construction and listing. This provides superior protection against impact. A key topic of discussion is often Romex vs MC cable; MC is generally permitted in many applications where NM cable is not appropriate. An older form of armored cable, often called bx cable colloquially, is more accurately known as Armored Cable (AC). Traditionally, AC provides the grounding path through its armor and bonding strip and may not include a separate insulated equipment grounding conductor. Installing mc wire in any MC cable electrical system requires the use of listed fittings and connectors identified for that use to properly terminate the cable and, where required, establish a grounding path. Common variants include 12/2 mc cable and 12/3 mc cable. You can find pre-fabricated assemblies, known as an electrical whip, for light fixtures. For work in attics, it’s essential to know how guard strip requirements for armored cable have changed. All regulations for this cable type are covered in NEC Article 330 (MC) and Article 320 (AC) depending on the cable type.

Individual Conductors: THHN, THWN, and XHHW Wire

When wiring is installed in conduit, electricians pull individual conductors rather than using a sheathed cable. The most common types are distinguished by their insulation.

• THHN Wire: Thermoplastic High Heat-resistant Nylon-coated. THHN is the common workhorse for dry locations; many conductors sold as THHN are also dual-rated THHN/THWN (or THHN/THWN-2), which makes them suitable for wet locations when so marked. Understanding the thhn wire meaning is key: its primary strength is its high temperature rating (90°C) and the nylon jacket which eases pulling.
• THWN Wire: Most THHN conductors are dual-rated as THHN/THWN. The “W” indicates suitability for wet locations when the marking is present. A conductor marked THWN-2 indicates wet-location suitability at 90°C.
• XHHW Wire: Cross-linked polyethylene High Heat-resistant Water-resistant. Often seen as a robust alternative, xhhw wire has thermoset-type insulation (cross-linked polyolefin) that offers good environmental resistance. The practical choice between THHN vs XHHW depends on project requirements, installation environment, and the conductor markings. All these conductors’ allowable ampacities depend on their wire insulation temperature ratings and the applicable ampacity tables and adjustment rules in the NEC.

Specialized and Service Entrance Cable Types

Beyond the basics, many jobs require specialized cables for unique environments or applications, from underground runs to low-voltage systems.

Underground, Service, and Specialty Cables

For outdoor and burial applications, you must use a cable rated for the purpose.

• UF-B Cable: Stands for Underground Feeder cable. This romex cable look-alike has a solid, water-resistant jacket that fully encases the conductors and is listed for direct burial without conduit where permitted by the NEC. Typical UF-B cable uses include runs to detached structures, outdoor receptacles, and landscape lighting, provided install practices and burial depths in the NEC are followed.
• Service Entrance Cable (SER/SE): As one of the main service entrance cable types, SER cable (service-entrance round) and SE cable (flat style) are used to bring power from the utility to the service equipment. SER/SE constructions and permitted uses are covered in NEC Article 338; these cables are built and sized specifically for service and feeder applications (utility-to-meter, meter-to-panel, or for large feeders) and must be selected per the required ampacity and installation method (service entrance, meter mounting, underground service lateral rules, etc.).
• URD Cable: Underground Residential Distribution cable is used by utilities for primary underground service laterals.
• Welding Cable: A highly flexible cable designed to handle the high-amperage, low-voltage demands of welding equipment; choose the proper conductor stranding and insulation for the environment.
• Circuit Integrity (CI) Cable: Essential for life-safety circuits, this family of cables (including mineral-insulated types) is designed to maintain circuit integrity for a specified period during a fire when listed for that purpose.
• Plenum Rated Cable: Cable intended for plenums (air-handling spaces) must be listed for that use (CMP) and have a low-smoke, limited-flame propagation construction; materials such as Low-Smoke Zero-Halogen (LSZH) are common in low-smoke cables used where required.

Low-Voltage and Data Cabling

Modern electrical work often overlaps with low-voltage systems. Familiarity with coaxial cable types (like RG-6 for video) and data cables is a must. When distributing a signal, a coaxial cable splitter or a more generic cable splitter is used. For networking, Cat 6 cable is a common choice, and knowing how to read a category 6 wiring diagram is a valuable skill. Even speaker wire, often in sizes like 14/2/2 wire, has its own installation rules. These cables should be secured neatly with cable ties and run in a dedicated cable raceway or wire raceway where separation from power conductors or other sources of interference is required.

Critical Considerations for Cable Installation

Properly selecting a cable is only half the battle. Correct installation, based on the NEC, is what ensures a safe system. This involves sizing, derating, and calculating for voltage drop.

Conductor Sizing, Derating, and Fill

Sizing wire is not just about matching the breaker. You must consult a conductor sizing chart or the NEC ampacity table. Several factors can reduce a wire’s safe current-carrying capacity, requiring you to use a larger gauge wire copper wire.

• Conductor Derating Factors: These must be applied when you have more than three current-carrying conductors in a raceway or cable, or when the ambient temperature is above 86°F (30°C), per NEC ampacity correction and adjustment rules.
• Conduit Fill Calculations: The NEC limits the percentage of a conduit that can be filled with wires to prevent heat buildup and allow for easier pulls.
• Large Conductors: For services and feeders, you’ll work with large conductors like 2/0 copper wire and 3/0 copper wire. Handling these heavier conductors requires specialized tools and techniques. You’ll also encounter sizes like 12 3 wire and 10/4 wire for specific multi-wire circuits.
• Wire Color Code: Adhering to the standard wire color code is essential for safety and troubleshooting. The color of a neutral wire is white or gray, the equipment ground is green, green with yellow stripe, or bare copper, and hot conductors are typically black, red, blue, etc. An open neutral wiring condition can be dangerous. While the US has its standards, be aware that the international colour wiring code can differ.

Step-by-Step Voltage Drop Calculation

A long wire run can result in a voltage drop that affects equipment performance. The NEC provides recommended limits (a common recommendation is a maximum 3% drop for a branch circuit alone). Here’s a basic voltage drop calculation for a single-phase circuit:

1. Formula: VD = (2 x K x I x L) / CM
2. Identify Variables:
   • K: Direct-current constant. A commonly used value for copper at typical operating temperatures is ~12.9 (use the value appropriate to the conductor temperature for precise calculations).
   • I: Current in amps.
   • L: One-way length of the circuit in feet.
   • CM: Circular Mills of the conductor (from NEC Chapter 9, Table 8).
3. Calculate: For a 100-foot run of 12 AWG wire (CM = 6530) carrying 16A (80% of 20A): VD = (2 x 12.9 x 16 x 100) / 6530.
4. Result: VD ≈ 6.32 Volts.
5. Verify: On a 120V circuit, this is about a 5.3% drop (6.32V / 120V), which exceeds the common 3% recommendation. You would typically upsize to 10 AWG wire to reduce the voltage drop. It’s critical to perform this check based on the actual cable length and load and to follow NEC guidance and any local amendments.

Mastering these electrical cable types and their associated NEC rules is a continuous process. Your ability to select and install the right 12/2 metal clad cable, run service entrance cable for a subpanel, or correctly diagnose a problem with a common wire is what defines you as a professional. Select the right wire for every job. Deepen your knowledge of the NEC.

Primary Sources for Electrical Codes

For the most accurate and up-to-date information, always refer to the official source documentation. E-E-A-T (Experience, Expertise, Authoritativeness, Trust) in the electrical trade is built on a foundation of code compliance.

• NFPA 70: National Electrical Code (NEC)
• Your state’s specific electrical licensing board and adopted code amendments.

Frequently Asked Questions (FAQ)

What is the main difference in Romex vs MC cable installation?

The primary difference is protection and grounding. Romex wire (NM-B) is used in wood-frame construction and must be protected from physical damage; it is a nonmetallic-sheathed assembly. MC cable has a metallic sheath for physical and mechanical protection and, depending on construction, may include a separate equipment grounding conductor or rely on the metal sheath for grounding. MC installation also requires listed fittings (mc connectors) to terminate the cable and maintain the grounding path as required, whereas Romex uses approved cable clamps or NM connectors into boxes.

When should I use UF-B cable instead of standard Romex wire?

You must use UF-B cable (Underground Feeder) for any application involving direct burial in the ground or in wet/damp locations where standard romex cable is not permitted. This includes outdoor runs to detached structures, landscape lighting, and certain outdoor receptacles where the listing and burial depth requirements are met. Its solid, water-resistant jacket and listed construction make it suitable for direct burial use where allowed by the NEC.

How does the wire color code identify the neutral wire?

In the United States, the standard wire color code identifies the neutral conductor (or wire neutral) with white or gray insulation. The equipment grounding conductor is green, green with a yellow stripe, or bare copper. Hot conductors are typically black, red, or blue. Correctly identifying the color of a neutral wire is fundamental to safe and correct wiring.