THHN Wire: Why Is It So Common Around Us?


When building a new facility, you have to lay down the wiring to make connections. Since the whole functioning depends on your decisions, choosing the specific wiring type is not easy. The specifications for every wire are usually available, including all its strengths and weaknesses. Perhaps it might be a big challenge for the engineers to look into all characteristics, fully understand them and then choose, but it is critical to do so. Talking about wires, one type best suits heavy machines and building construction: THHN wire. Let’s get a deeper understanding of the topic.

Table of Contents

Understanding THHN Wire

Colourful THHN Cables

Caption: Colourful THHN Cables

THHN is not a brand but rather a type of wire dual-rated cable. 

THHN stands for Thermoplastic High Heat-resistance Nylon coated wire. These cables are typically used in control circuits, machine tools, and heavy appliances. It comes with a variety of characteristics, the conductor can be stranded or solid, of copper or aluminum, and jacketing can be of PVC insulation with Nylon. Also, it can withstand a 90-degree Centigrade in dry locations. However, if the system is under oil exposure, the wiring’s resistance may drop to 75 degrees Celcius.

Mostly, the contractors use the THHN wires to supply power to the outlets and lights. But, the uses do not end here, as it can work pretty well for every kind of application, either outdoor or indoor.

There are other alternatives to the wire, named THWN, THW, RHW, and RHH. Whereas the THW is without Nylon, RHW has a rubber coating. 


The THWN stands for Thermoplastic, Heat and Water resistance, and Nylon coated wire.

Then comes THWN-2, which is the same as the former but differs in the case of temperature tolerance since it has a resistance of 90 degrees in both dry and wet environments. Moreover, the THWN only has a maximum temperature resistance of 90 degrees Centigrade in wet areas, compared to THHN, so you can use them interchangeably. And, it is an inexpensive building wire, so best for less budget electrical current supplies.

Nylon Coating saves the wire from abrasion, and hence it is part of all of these types. Moreover, additional jacketing protects the cables from oil, grease, and gasoline so that they can last longer.


XHHW stands for Crossed-Linked Polyethylene (XLPE) High Heat and Water-resistant wire.

Moreover, similar to THHN, it can withstand up to 90 degrees Centigrade in dry areas and 75 degrees Celsius in wet areas.

On the other hand, the XHHW-2 can be called the next generation of the XHHW wire. Thus, the tolerance increases to 90 degrees in wet and dry environments. Also, common usage of XHHN-2 wires is in residential, industrial, or commercial buildings.

Moreover, manufacturers also use it for feeders, raceways, and circuit wirings.

In XHHW, the conductors can be copper wires or aluminum, and the inside wiring can be Solid or Stranded.

RHW and RHW-2

RHW stands for Rubber insulated Heat and Water Resistant wires. Although the name suggests its insulation to be only of Rubber, it also incorporates other materials such as Cross-Linked Polyethylene in it.

RHW can resist heat up to 75 degrees centigrade and can withstand moisture. On the other hand, the RHW-2 can resist heat up to 90 degrees, making it suitable for use in direct burial in wet and dry areas.


However, as the H in the name doubles up, the RHH is now a high heat-resistant cable that can easily resist 90 degrees centigrade heat. It is similar to RHW cable. The only thing it lacks is the ability to resist water and moisture.

Applications of THHN Wire

Bundle of Copper THHN Wires

Caption: Bundle of Copper THHN Wires

Here are some typical applications of Thermoplastic High Heat-resistant wires.

Indoor Applications

THHN wires are commonly rated for up to 600V. Hence the applications of THHN in indoor facilities are endless. So, you can find these wires in your water heaters, refrigerators, motors, or Garbage disposals. Also, as it is water, oil, and grease resistant, heavy industries find it safe to use these wires in their machinery and wet applications.

Moreover, THHN wires are inflexible, thus bundled together so that you can have an easy installation and planning with them.

Outdoor Applications

Since the THHN / THWN is also resistant to heat and other damaging factors, it is safe for you to use them outdoors, such as lighting a building or a house, etc. Thus, the producers might offer you the cable in a bundle along with UF-B jacketing. Hence, insert them in the conduit if you plan to connect with them, as they do not require additional insulation in industrial environments.

Specifications of THHN Wires

Electric THHN wire in double insulation

Caption: Electric THHN wire in double insulation

All the specifications of THHN wires are in their abbreviation.

  • Thermoplastic (T) is thermoplastic insulation with polyethylene PE or polyvinyl chloride PVC. Usually, these are easier to use and are available in different colors.
  • High-Heat Resistance (HH) has a dual rating for heats high as 90 degrees centigrade, making them suitable for outdoor use.
  • Nylon (N) refers to the Nylon jacket wrapped around the wire to protect it from cuts and abrasions.

Moreover, the THHN wires come in 10 colors that are

  1. Black
  2. Brown
  3. Orange
  4. Violet
  5. Gray
  6. Red
  7. Blue
  8. Yellow
  9. White
  10. Green with Yellow Stripes

Additionally, the THHN wires are available in many sizes, including 14 AWG to 1 AWG, for standard and 1/0, 4/0, and 250 thousand circular Mils (MCM) to 600 MCM for larger wires.

THHN Wire Ampacity Chart

Amperage Rating
Wire Size1412108643211/02/03/04/0
NM-B, UF, TW1520304055708595110125145165195
RHW, THEN, XHHW152030506585100115130150175200230
RHH, THHN in Copper152030557595110130150170195225260
RHH, THHN in Alum152545607585100115135150175205

Other Building Cables VS THHN

There are a few other options available for building cables in the market. These are

Romex Wire

The manufacturers make Romex copper electrical wires from multiple THHN/THWN wires. For instance, a Romex 12/2 consists of 2 THHN wires and uninsulated, bare ground wires insulated with PVC jacketing; it is easy to handle since you don’t need to run three separate cables in the entire facility. However, it has no resistance against water, gasoline, or other solvents. Hence, you can only use it in indoor applications.

UF-B Cable

UF or Underground Feeder electrical wire upgrades the Romex wiring. It is similar to the THHN conductors, except the insulation is more complex. Thus, it might be suitable for outdoor applications.

Service Entrance Type SE

Service cables distribute the power from the service drop to the meter base and distribution panel board. It is sunlight-resistant and can easily be used in wet and dry locations.


THHN Wires are best for indoor and outdoor applications since they are water, heat, and grease resistant.

Moreover, the Nylon jacketing can give it more protection than the bare one.

Here at Custom wiring loom, we offer a wiring harness, so your connection is made with attention to detail. To avail of our services, contact us now.