In residential and commercial wiring, Romex (NM-B) and THHN are two of the most common types of cables used. The confusion is that the NM-B wire is not that far from THHN in terms of construction. Whereas THHN is a cable with a single thermoplastic high heat-resistant nylon wire, NM-B wire is a sheathed cable with multiple conductors. The conductors inside the NM-B wire are either insulated or uninsulated, and the former is actually THHN, which commonly leads to confusion between the two types of cables. Let's settle the discussion on the choice between NM-B Romex and THHN.

What is Romex NM-B wire?

NM-B is a non-metallic sheathed cable with multiple conductors designed with residential applications in mind. It is primarily used for branch (B) wiring. The cable is usually available in gauges from 2 to 14 AWG, each color-coded in different colors, including yellow, white, and black. The voltage rating of the cable is 600 volts, and the temperature rating for dry conditions is 90 degrees Celsius. The cable has standard PVC insulation. The cable has an ampacity limit of 60 degrees Celcius.

NM-B wiring is never used outside. In the house, its primary applications are switches, lighting fixtures, and electronic appliances. It is also a common choice for interior walls. NM-B wiring can also be used in some of the rougher home applications, such as the garage, but in these instances, they should be protected by conduit. The cable is not practically durable and should be protected with connectors at both ends.

• Standards met by NM-B Romex include: 
• REACH
• RoHS
• NFPA 70
• ASTM B-3 and B-8
• UL 83 and 719

Common Sizes and Applications of NM-B Romex

1. 14/2 NM-B:
• Contains two 14-gauge conductors (one hot, one neutral) and a ground wire.
•  Ideal for general lighting and receptacle circuits in residential wiring.
• Rated for 15 amps.
2. 14/3 NM-B:
• Contains three 14-gauge conductors (two hot, one neutral) and a ground wire.
• Used for three-way switch wiring and split receptacle circuits.
• Rated for 15 amps.
3. 12/2 NM-B:
• Contains two 12-gauge conductors (one hot, one neutral) and a ground wire.
• Suitable for 20-amp circuits, including receptacles and kitchen appliances.
• Rated for 20 amps.
4. 12/3 NM-B:
• Contains three 12-gauge conductors (two hot, one neutral) and a ground wire.

• Used for three-way switch setups and split receptacles in kitchen and laundry areas.
• Rated for 20 amps.

1. 10/2 NM-B:
•  Contains two 10-gauge conductors (one hot, one neutral) and a ground wire.
• Appropriate for larger appliances such as water heaters and air conditioners.
• Rated for 30 amps.
2. 10/3 NM-B:
• Contains three 10-gauge conductors (two hot, one neutral) and a ground wire.
• Used for electric dryers and other 240-volt applications.
• Rated for 30 amps.
3. 8/2 NM-B:
• Contains two 8-gauge conductors (one hot, one neutral) and a ground wire.
• Suitable for larger electrical loads such as ovens and central air conditioning units.
• Rated for 40 amps.
4. 8/3 NM-B:
• Contains three 8-gauge conductors (two hot, one neutral) and a ground wire.
• Used for electric ranges and larger appliances requiring 240 volts.
• Rated for 40 amps.
5. 6/3 NM-B:
• Contains three 6-gauge conductors (two hot, one neutral) and a ground wire.
• Used for large electric ovens, subpanels, and other high-power applications.
• Rated for 55 amps.

NM-B vs. Romex

So, what is the difference between generic NM-B and Romex? 

"Romex" is a brand name for non-metallic sheathed cable, specifically manufactured by the American company Southwire. While not all nm-b wires are manufactured by this company, Romex has become a general term that points to this non-metallic wire, just like digital printing machines are often called xerox after the most famous brand that manufactures them.

While NM-B wires are mostly the same, quality may differ depending on the manufacturer. Actual Romex cables have a patented jacket that makes the installation easier. However, many high-quality NM-B wires are on the market, and purchasing the original brand is usually unnecessary. Simply make sure that the manufacturer is trustworthy.

Can NM-B Wire Be Run In A Conduit?

Despite the existing stereotypes, the nm-b wire can be run in conduit without any underlying issues. Article 334 of the National Electrical Code actively encourages NM-B wiring to be run in conduit in case there is a need for protection from mechanical damage. The conduit for the NM-B wire should be of a precise size so that the cable will not be damaged.

Why is Romex banned in Illinois?

The most popular type of home wiring in most of the United States, Romex, is not allowed by local legislation in many counties in Illinois. However, the ban is small, and some territories do allow the use of NM-B Romex. The reasons for NM-B wire not being used have a lot to do with bureaucracy. However, there is also an actual reason for the ban - the cable insulation is prone to be punctured by rodents. Fire safety regulations are also a consideration for the ban. Counties in Illinois, including Chicago, encourage using EMT insulation instead of NM-B Romex.

Check with your local county to ensure the exact rules regarding using NM-B Romex. Many counties outside of major urban areas do not have such ban.

What Is THHN Wire? 

THHN/ THWN is the most popular electrical cable on the market. Unlike Romex, THHN is an electrical wire with just one conductor. It is a thermoplastic high heat-resistant nylon coated wire that possesses superior resistance to heat and is water-resistant. While NM-B wire is exclusive to residential applications, THHN is also often used in industrial and office settings, thanks to the damage-resistant nylon being one of its key components. The cable is also rated at 600 volts and has a temperature rating of 90° C in dry and 75° C in moist conditions. It is worth noting that if you take two house cables with a similar rating, THHN will likely have superior performance with heat simply because of its nylon coverage.

Unlike Romex, THHN/THWN is a generic wire name without a connection to a particular brand.

THHN runs power to the building in lights, electrical outlets, branch circuits, control circuits, water heaters, cooling systems, garbage disposers, garages, and basements. THHN is rarely used outdoors for home applications, but it can be used if it has a UF-B jacket or is double-rated as THHN/ THWN and is used in conduit.

THHN is primarily available in sizes from 14 AWG to 600 MCM. It has solid and standed variations. Solid conductors are less flexible, but are cheaper and easier to terminate. Stranded conductors are more flexible.

• THHN/THWN in sizes 10 AWG, 12 AWG and 14 AWG are standard in circuits.
• Sizes 1 AWG through 8 AWG are for stoves, hot tubs, water heaters, and other high-power appliances.
• Any THHN/THWN larger than 1 AWG is used to bring the power from the utility pole to the electrical panel, similar to service entrance cables.

Common Use Cases (THHN vs. Romex)

Use Cases of Romex (NM-B) Wire

1. Residential interior wiring:

• Switches and outlets: Romex is commonly used for wiring switches and electrical outlets within residential buildings. 
• Lighting fixtures: Romex is ideal for wiring ceiling lights and other lighting fixtures. Lighting fixtures require flexible cables like NM-B.
• Appliances: Small household appliances indoors, such as dishwashers, refrigerators, and ovens, often use Romex.
• Interior walls: Romex is frequently used within interior walls, which are protected from physical damage by the structure of the building itself.

2. Dry and protected areas:

• Attics and crawl spaces: Romex can be safely used in attics and crawl spaces, provided it is not exposed to excessive moisture or potential physical damage.
• Interior room wiring: Suitable for wiring within rooms and behind drywall.

3. Limited commercial use:

• Office buildings: In certain situations, Romex can be used in office buildings for similar applications as in residential settings, provided it meets local building codes.

1. Industrial and commercial wiring:

• Control circuits: THHN wire is frequently used in control circuits due to its high heat resistance and durability. It’s ideal for industrial environments where temperatures can be higher.
• Lighting and outlets: In commercial buildings, THHN is used for wiring lighting systems and electrical outlets. Its single conductor design makes it easier to run through conduits.
• Power distribution: THHN is suitable for distributing power to various parts of commercial and industrial buildings, including connecting to electrical panels and breakers.

2. High-heat and moisture-prone areas:

• Garages and basements: THHN wire is used in garages and basements where it might be exposed to higher temperatures and moisture. Its nylon insulation provides additional protection against these elements.
• Outdoor use: When double-rated as THHN/THWN, this wire can be used outdoors in conduit, making it ideal for outdoor lighting, power to outbuildings, and other exterior applications.

3. Specialized residential use:

• HVAC Systems: THHN wire is used in heating, ventilation, and air conditioning (HVAC) systems because it can withstand higher temperatures.
• Water heaters and coolers: It’s suitable for wiring water heaters and cooling systems, where heat resistance is crucial for safety and performance.

4. Conduit installations:

• Conduit runs: THHN is often pulled through conduits in residential and commercial settings. 
• Control panels: Used extensively in wiring control panels and motor controls in industrial settings due to durability and heat resistance.

Choosing Between THHN/THWN and Romex 

Here is the list of tips on how to make a selection between NM-B Romex and THHN/THWN:

• Your regular branch wiring will likely do well with the Romex wire. It’s designed for typical residential applications and is straightforward to install.
• Running Romex inside the walls is steadier and easier. The non-metallic sheath provides protection and makes it easy to pull through wall cavities and ceilings.
• On average, the NM-B wire is more delicate to handle, and you need to take this into consideration when installing. Ensure you do not damage the sheath during installation.
• If you need a single wire, choose THHN. If your installation calls for a multi-conductor cable, go with the NM-B.
• Always compare the prices. In some harsher home applications, like basements and garages, you may run NM-B wire in conduit, but it might be cheaper to just go with THHN instead. Check with your professional electrician if you can run NM-B wire without the conduit by simply fastening it tightly to the wall.
• If you are in Illinois, choose THHN or another home wiring for your applications, or check with the local authorities for more information. The use of EMT conduit is encouraged in the state.
• If your application is exposed to high heat, always choose THHN. If it deals with water and moisture, use THHN double-rated as THWN.
• For outdoor home applications, use THHN with a UF-B jacket or THHN/THWN in conduit. Never use the NM-B wire outdoors, as it lacks the necessary protection against environmental elements.
• Use THHN/THWN in conduit when wiring a garage.
• If you run wires in an industrial kitchen where the cables might be exposed to grease, oils, and high temperatures, THHN/THWN will hold up better over time than NM-B cables. 
• Use NM-B for drywall and attic spaces where the risk of physical damage is minimal.
• For specialized applications like wiring hot tubs, pools, or exterior lighting, THHN/THWN is typically the better choice due to its enhanced protective qualities. 

To shop all types of residential wiring, visit nassaunationalcable.com. We sell all types of highest-quality residential wire at the best prices in the industry, including THHN/THWN-2 and Romex NM-B wire.