Romex vs MC Cable vs THHN: An Electrician’s Identification Guide

In the world of electrical wiring, choosing the correct cable is critical for safety, code compliance, and efficiency. For any residential or commercial electrician, the debate of romex vs mc cable is a daily consideration, with THHN wire serving a different but equally vital role. Romex, technically Non-metallic sheathed cable (NM-B), is the go-to for most residential wiring due to its low cost and ease of use in protected, dry locations. In contrast, MC cable, or metal-clad cable, offers superior physical protection with its flexible cable armor, making it a staple in commercial applications and exposed areas. Meanwhile, THHN conductors are individual insulated wires that must be installed within a raceway like conduit, offering the most versatility for complex runs and high heat applications. Understanding the distinct construction, applications, and National Electrical Code (NEC) rules for each is fundamental to any professional electrician training program.

For a broader overview of different wiring options, ExpertCE offers a comprehensive electrical cable types guide to expand your knowledge base.

What is Romex® (Non-metallic Sheathed Cable)?

Romex® is a brand name for Type NM-B cable, a type of Non-metallic sheathed cable that is ubiquitous in modern residential wiring. As defined in NEC Article 334, it consists of two or more insulated conductors bundled together and wrapped in a flexible PVC (polyvinyl chloride) jacket. A typical configuration, such as a 12/2 wire, contains a black (hot) conductor, a white (neutral) conductor, and a bare copper grounding conductor. The sheathing is color-coded by gauge for easy identification: white for 14-gauge, yellow for 12-gauge, and orange for 10-gauge.

The primary advantage of romex wire is its affordability and speed of installation in standard wood-frame construction. However, its use is restricted by the NEC to dry, indoor locations and it cannot be installed in areas exposed to physical damage unless properly protected. For a deeper dive into its specific uses and limitations, you can explore this guide to NM cable types.

What is MC Cable (Metal-Clad Cable)?

MC cable, which stands for metal-clad cable, is a factory assembly of insulated conductors enclosed in a flexible armor of interlocking metal tape, typically aluminum or steel. Governed by NEC Article 330, MC cable provides its own physical protection, eliminating the need for conduit in many situations. Inside the cable armor, you’ll typically find individual THHN conductors, along with a green insulated equipment grounding conductor.

It’s important to distinguish modern MC cable from its predecessor, Armored Cable (AC), often called by the brand name BX cable. While they look similar, BX cable relies on the combination of its metal armor and a thin bonding wire for grounding, whereas MC cable contains a full-sized, dedicated grounding conductor, making it the modern standard for safety and code compliance.

Primarily used in commercial applications, MC cable is also gaining popularity in residential settings, especially for exposed runs in basements, garages, or for wiring appliances like water heaters. Its durability makes it a robust choice where wiring might be subject to physical impact.

What is THHN Wire?

Unlike Romex and MC cable, which are cable assemblies, THHN wire is a type of single conductor. The acronym stands for Thermoplastic High Heat-resistant Nylon-coated. This general-purpose building wire features a copper or aluminum conductor covered with PVC insulation and a tough nylon jacket. This construction gives it excellent resistance to heat, abrasion, oil, and gasoline.

Most THHN wire today carries a dual rating of THHN/THWN, where the “W” indicates a wet location rating. According to its UL listing, THHN is rated for 90°C in dry locations, while the THWN rating for this dual-rated wire is 75°C in wet locations. Because THHN conductors have no mechanical protection of their own, the National Electrical Code requires them to be installed inside a raceway, such as EMT, rigid conduit, or PVC conduit. It is the wire of choice for electricians when pulling conductors through conduit systems.

Core Differences: Romex vs MC Cable vs THHN

Choosing between these three wiring methods depends on the environment, budget, and code requirements. Each has distinct advantages and limitations that every electrician must know to pass an electrical inspection.

Construction and Physical Protection

• Romex (NM-B): Features a flexible PVC jacket that offers minimal protection against punctures or crushing. It is suitable only for installation within protected wall, ceiling, and floor cavities.
• MC Cable: The interlocking cable armor provides significant protection against physical damage, making it suitable for exposed indoor locations and fished runs. Standard MC cable is rated for dry locations only per NEC Article 330; however, specialized types with a listed corrosion-resistant jacket or covering are rated for wet or outdoor installations.
• THHN Wire: As an individual conductor, THHN has no inherent physical protection and is entirely reliant on the conduit or raceway it is pulled into. The strength and durability are determined by the raceway material (e.g., steel EMT vs. PVC).

Common Applications: Residential vs. Commercial

• Romex (NM-B): The dominant choice for residential wiring inside walls for outlets, switches, and light fixtures. Its use in commercial buildings is heavily restricted by the NEC.
• MC Cable: The standard for many commercial applications, including lighting, power circuits, and feeders. Its use is growing in high-end residential projects and in areas like basements and garages where wiring is exposed. For unique installations, like wiring Colorado mountain homes, the durability of MC cable can be a significant advantage.
• THHN Wire (in Conduit): Used universally in all construction types—residential, commercial, and industrial—wherever a conduit system is required. This includes underground runs, embedding in concrete, and areas requiring the highest level of physical or environmental protection.

Installation and NEC Considerations

The installation rules in the nec code book differ significantly for each type. Romex, under NEC 334.30, must be supported every 4.5 feet and within 12 inches of a box. MC cable, per NEC 330.30, generally requires support every 6 feet, offering a labor advantage on long runs. Both have specific bending radius limitations to prevent damage.

THHN installation is governed by the rules for the specific conduit type used, along with conduit fill calculations from NEC Chapter 9. Staying current with NEC rules is crucial for any electrician. To fully understand NEC rules for different cable types, particularly recent changes, consider reviewing material on how the 2023 NEC updates have changed NM and NMC cable installation rules.

Cost and Labor

On a material-cost basis, romex wire is generally the cheapest option, with some estimates suggesting it can be 25% cheaper than MC cable. However, the total installed cost can vary. While Romex is quick to staple in open wood framing, MC cable can be installed faster in certain scenarios, such as long, exposed runs where conduit would otherwise be required. THHN wire and conduit have the highest material and labor costs upfront but offer the most robust and future-proof installation, as wires can be replaced or added later without disturbing finished surfaces.

Step-by-Step: Calculating Conduit Fill for THHN Conductors

A critical skill in any electrician training is correctly calculating conduit fill to comply with the National Electrical Code. Overfilling a conduit can lead to excessive heat and damage to wires. Here’s a basic guide using NEC Chapter 9:

1. Identify Conductors and Conduit: Determine the size and insulation type of your THHN conductors (e.g., 12 AWG THHN) and the type and size of your conduit (e.g., 1/2″ EMT).
2. Find Conductor Area: Look up your conductor in NEC Chapter 9, Table 5 to find its cross-sectional area in square inches. A 12 AWG THHN conductor has an area of approximately 0.0133 in².
3. Calculate Total Conductor Area: Multiply the area of one conductor by the total number of conductors you plan to install. For example, six 12 AWG THHN wires would be 6 x 0.0133 in² = 0.0798 in².
4. Determine Allowable Fill Percentage: Use NEC Chapter 9, Table 1. For over two conductors in a conduit, the maximum fill is 40%.
5. Find Conduit Area: Look up your conduit in NEC Chapter 9, Table 4. A 1/2″ EMT has a total internal area of 0.304 in².
6. Calculate Allowable Fill Area: Multiply the conduit’s total area by the allowable fill percentage. For our example, 0.304 in² x 0.40 = 0.1216 in².
7. Verify Compliance: Compare your total conductor area to the allowable fill area. Since 0.0798 in² is less than 0.1216 in², this installation is code-compliant.

Key Takeaways for the Professional Electrician

• Romex (NM-B): Best for standard, protected residential wiring. It’s cost-effective and easy to work with but offers minimal physical protection.
• MC Cable: Ideal for exposed indoor runs, commercial applications, and areas where physical damage is a concern. It offers built-in protection and can reduce labor costs compared to installing conduit.
• THHN Conductors: Used exclusively within conduit or other raceways. This method provides the highest level of protection and is required for many commercial, industrial, and outdoor/underground applications.
• Code is Key: Always refer to the latest edition of the NEC code book and local amendments. Rules for support, protection, ampacity ratings (NEC Article 310), and permitted uses are non-negotiable for a safe and compliant electrical inspection.
• Consider the Entire Job: Evaluate material cost, labor, and long-term durability. The cheapest material (Romex) may not result in the lowest overall project cost if additional protection like conduit is needed.

Primary Sources

• NFPA 70, National Electrical Code (NEC), 2023 Edition

Frequently Asked Questions

What is the main difference in Romex vs MC Cable for a residential electrician?

For a residential electrician, the main difference is protection and permitted use. Romex is for use inside finished walls, while MC cable is used for exposed areas like unfinished basements, garages, or surface-mounted runs where the wiring needs physical protection.

Can you use THHN wire outside of conduit?

No. THHN wire is a single conductor with only a thin layer of insulation and a nylon jacket. The National Electrical Code requires it to be installed inside a conduit or other approved raceway to protect it from physical damage.

Is MC cable the same as BX cable?

No. While they are both types of armored cable, modern metal clad cable (MC) contains a dedicated green equipment grounding conductor. Older BX cable (Type AC) does not and instead relies on its metal armor and an internal bonding wire for grounding, which is an outdated method.

When does the NEC code book require metal-clad cable instead of Romex?

The NEC code book requires a wiring method with physical protection, such as metal-clad cable or conduit, in locations where non-metallic sheathed cable (Romex) would be subject to physical damage. This is common in commercial buildings, industrial locations, and any exposed areas where the cable isn’t protected by the building structure.