THHN vs. THWN: Which Is The Better Building Wire?


THHN vs. THWN

For contractors, one of the most commonly used types of cabling is building wire. This type of cable is most often used to transport electrical current to all external consumptions of power in a home or other structure.

As engineers and contractors are designing a facility, it’s important that they understand the specifications of each type of wire available for the job. THHN and THWN may seem like a lot of letters to understand but they are considered to be the most common types of building wire on the market.

THHN, or thermoplastic high-heat resistant nylon-coated wire is most often used for connecting appliances and branch circuits. THWN or THWN-2, as it’s sometimes referred to when dual-rated, functions essentially the same except for its water-resistance. Hence, the addition of the “W” in the acronym.

Let’s dive in deeper into both THHN and THWN to better understand their uses, characteristics and advantages.

THHN

Thermoplastic high-heat resistant nylon-coated, or THHN wire is arguably the most popular type of building wire used in construction. It was developed to endure heat up to 194 degrees Fahrenheit, dropping to 167 degrees if it’s exposed to oil. The wire is usually two-conductor and often found in ROMEX-style cabling that exists in most homes and other structures.

THHN is the original version of the now dual-rated cable. Previously it was only meant to be used for dry applications. Since both THHN and THWN have very few differences when used in minor applications, manufacturers have made them into one all-encompassing product (THHN/THWN or THWN-2).

THWN

The major reason for using THWN wire is because of its water resistance. The wire is essentially the same as THHN with the additional of its ability to assist in water resistance. These types of cables can be installed in areas where a conduit is not required. Stand-alone THWN cables are covered in PVC rather than vinyl, to provide resistance against water.

THHN/THWN and THWN-2 Dual-Rated Cable

As previously mentioned, THHN now comes with a dual rating (THHN/THWN or THWN-2) and can be used for both wet and dry applications. In the past, you could purchase THW, THHN, THHW, THWN and many other types of electrical wire.

However, as time passed, it became too tedious for manufacturers to build and inventory all the various types. Because of this, they began placing multiple approvals on a single type of electrical wire.

Wires rated as THHN/THWN have the characteristics of both THHN and THWN wires. Also, THHN/THWN-2 wires will not have the same limitations of the individual cables.

Where Are THHN/THWN Wires Used?

THHN wire is a superior wire, able to manage a variety of situations. A stand-alone THHN wire can only be used in dry or damp locations. For any sort of wet wiring, this is where THWN comes in. However, since most of the wires on the market today are dual rated as THHN/THWN, both types of wires can be used in dry, damp and wet settings.

Breaking Down The Acronyms

Nowadays, you can purchase THHN wire that has been rated both THW and THWN. The absence of the “N” in THW just means that it does not have a nylon outer coating. Instead, it is equipped with a thermoplastic insulation that is both heat and water resistant. For a more simplified breakdown of the various ratings, refer to the table below:

THHN THWN THWN-2
T = Thermoplastic
HH = High Heat Resistance
N = Nylon Coated
Temp Rating:
 90° C in dry locations
T = Thermoplastic
H = Heat and
W = Water Resistance
N = Nylon Coated
Temp Rating: 
90° C in dry locations and 75° C in wet locations
T = Thermoplastic
H = Heat and
W = Water Resistance
N = Nylon Coated
Temp Rating: 
90° C in both wet and dry locations

THHN wire can be found in a variety of forms:

  • Wire material: Aluminum or copper
  • Conductors: Can be solid or stranded, depending on the size
  • Jacket: Nylon outer jacket with a polyvinylchloride (PVC) insulation

The nylon jacket is essential for mechanical protection and protection from abrasion when the wire is fished through conduit. Also, the jacket helps prevent exposure to any hydrocarbons like gasoline, grease or other oils.

Precautions When Using THHN Wires

Like any sort of electrical wiring, there are some things you should be aware of and precautions you should take to prevent any dangers from occurring. The thermoplastic outer jacket of THHN wire has been known to release toxic gases when burned. It is strongly urged that these wires are positioned within conduits to prevent the release of any toxic materials.

THHN wires use a thinner PVC insulation and this is a key factor when it comes to the wire’s electrical characteristics. Because the insulation is thinner, it can often cause a current leakage or even a breakdown during environmental or chemical exposures.

You should consult with the National Electrical Code or any other relevant electrical codes in your area before beginning electrical work. As always, if you ever feel unsure or uncomfortable about what you are doing, consult a licensed electrician.

Advantages of THHN Wires

If you’re in need of a building wire that is strong and effortlessly handles heat, there’s no doubting that THHN wire is the route to go. However, using this type of cable comes with a number of other additional advantages:

  • Handles high voltages. THHN wire can manage up to 600 volts. Its copper conductor allows it to handle such high voltages without any issues.
  • Wet and dry application. With the existence of the dual-rating, you are not limited on the environment it can be installed in. Most THHN wires can now be used in settings that are both wet and dry.
  • Durability. The strong jacket makes it resistant to damage and abrasion.
  • Affordability. THHN is a very cost-effective choice for building wire. You can often find many deals by just simply searching the internet. It’s much more affordable than XHHW building wire.
  • Can save you energy and time. The flexible nylon coating makes THHN very easy to manipulate and install.
  • Simple application. THHN wire comes color-coded making it very easy to install.
  • Comes in a variety of different sizes. Possibly one of the greatest advantages of this type of building wire is that it comes in a wide variety of sizes. You are sure to find the perfect size to fit your project needs.

Relevant Questions

What is MWR?

MWR, or machine wire rated wire is another type building wire that is found in smaller gauges. It’s worth mentioning in this guide because it often comes with the same dual THHN/THWN rating and features either a nylon or PVC jacket. It’s typically found with thinned ends for easy soldering application.

However, the insulated version of MWR is not THHN/THWN rated. This type is used in the development of windings for motors and power supplies.

Does THHN have to be in conduit?

Yes, THHN wire must be put into some type of conduit. It is not rated to be installed in any building without a raceway.

Can THHN be used outdoors?

THHN wires are suitable for both outdoor and indoor use.

Can THHN wire be buried?

Provided that it is used in a conduit, it can be put underground. However, it cannot be used for direct burial alone.

Wrapping It Up

Aside from water resistance, there are very few differences between THHN and THWN wires. In fact, with the existence of the dual rating, you can now enjoy the characteristics and benefits of both types of building wire.

Whether you’re choosing THHN/THWN because of its affordability, durability, versatile application or ease of use, it will get the job done!

Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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