THHN Vs Romex: What’s The Difference? [Explained]

Choosing the right type of wiring for your electrical projects is a must. You have to factor in where it’s going, what it will be powering, and what size you need.

Two common types of wiring are THHN and Romex. But what is the difference between the two, and how do you know which you should use?

Both THHN and Romex are thermoplastic high-heat nylon wires. THHN is the generic acronym for this type of wire, while Romex is a specific brand. Romex consists of multiple THHN wires in a cable, which is surrounded by a non-metallic sheath. All Romex cables use THHN, but not all THHN products are Romex.

What Are THHN and Romex?

THHN stands for thermoplastic high-heat nylon. It’s the material that wraps around a single conductor electrical wire. You can use it in branch circuits to run power to lights, appliances, and other electrical systems.

Romex is actually a commercial brand of THHN wire rather than a different material altogether. The company Southwire produces Romex, and it comes in various sizes and styles.

Romex is available in cables, meaning each length contains multiple THHN wires inside another sheath. It’s typically used for residential branch wiring.

Differences Between THHN And Romex

Romex may contain THHN wire, but that doesn’t mean the two are interchangeable. As a brand, Romex utilizes THHN to offer its own unique product.

What Is THHN Wire?

THHN wire consists of two parts: the interior electrical wire and the outer sheathing. The interior wire is typically copper or aluminum

The acronym THHN refers to the materials and characteristics of the wire’s sheathing, also called its jacket. “Thermoplastic” simply means a material that fuses together under heat and hardens again after it cools.

The jacket of THHN wire is made of a thermoplastic nylon. This jacket surrounds the wire and protects it from cuts and damaging elements like water, oil, or chemicals.

“HH” is “high-heat.” The coating around THHN wire can withstand up to 194°F (90°C). This makes it suitable for many different types of buildings and electrical work.

THHN comes in a wide variety of colors. There’s no real difference between each one, but they can help you designate a different job to each wire you use.

This way, if you need to adjust a specific electrical current, you know which wire is the one you need to work on.

THHN also comes in different sizes, and these do make a difference. These include circuit sizes, which Romex cables often use. These are available in 10, 12, and 14 AWG (American wire gauge).

Then there are wires suitable for carrying more power. 1, 2, 4, 6, and 8 AWG are good for larger appliances like stoves or dryers.

There are even THHN wires which are useful for connecting a home’s electrical box to street power.

How Does Romex Use THHN?

Romex is a brand of electrical cables, which is different from a wire. A wire is a single electrical conductor, whereas a cable consists of a group of conductors.

In other words, a cable is a bundle of wires. Romex cables contain multiple THHN wires in one bundle.

The wires still have their own THHN sheath, but with Romex cables, there’s a non-metallic (NM) sheath around the whole group.

NM sheaths are not unique to Romex, but it is a popular brand. So, many contractors may use “Romex” to refer to any cable with an NM sheath rather than that specific brand.

A similar scenario is using the brand name “Scotch Tape” to refer to any brand of transparent sticky tape.

Romex cables in particular typically use polyvinyl chloride, or PVC, as their NM sheath. This is useful for indoor applications, but not outdoors.

The PVC does not offer enough protection from the elements and will degrade quickly outdoors.

Should You Use Romex, Single THHN, Or Something Else Altogether?

If you’re not sure which wire or cable to use, consider the type of job you’re doing.

Single THHN wires, which are available from many different brands and companies, can be used in residential wiring. It’s also good for both wet and dry conditions and has a higher heat rating than most Romex cables.

In general, THHN is cheaper per foot than Romex in big box home-improvement stores.

Romex cables are also great for home wiring or for small businesses. The PVC sheathing makes it useable in interior walls and above-ground installations such as over concrete flooring.

Romex also streamlines your wiring by grouping multiple wires together in one sheath. This makes it easier to run Romex through your workspace.

Usually, you can choose to run Romex through a conduit, but you should check your local building codes first. If you do use a conduit, Romex cables do require a specific size.

You have to match the conduit size to the size of the cable. Otherwise, the conduit could crush and damage the sheathing of the cable.

Both of these wires are not usable outside, however.

If you need your wiring to stand up to outdoor elements, consider underground feeder cables (UF-B). These are similar to the cables under the Romex brand except for the sheathing.

UF-B cables have a more durable sheath than standard Romex cables. This allows for installation both underground and outdoors.


The difference between THHN and Romex is like the difference between Kleenex and a tissue. A tissue is a generic paper product, while the name “Kleenex” refers to a specific brand.

Romex is simply a specific brand of THHN wire product. It consists of multiple THHN wires bundled together in a non-metallic (NM) sheath.

Many consumers may refer to any cable with an NM sheath as Romex, even if it’s a different brand.

Single THHN wires have similar applications to Romex cables, but they’re not interchangeable. THHN has higher heat resistance than Romex, and Romex is good for above ground and interior wall installations.

Consider your local building codes and your electrical needs to determine whether a single THHN, Romex, or even another brand of cable is right for your project.

Katherine Ann

Katherine is a freelance writer who enjoys DIY home décor and refurbishing tired furniture. In addition to writing for PlumbJoe, she blogs about books and movies and writes creatively in her spare time.

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