THHN and THWN differ in terms of their acronym but both approvals are placed on the same electrical wire nowdays. Years ago there was THW, THHN, THHW, THWN and even more types of electrical wire. As time went on it became tedious for manufacturers to make and inventory each different kind so they started putting multiple approvals on a single type of electrical wire.
That being said, you can now buy THHN wire that is also rated THWN and THW. To better understand take a look at what each letter of the acronym means below:
T – Thermoplastic
H – Heat resistant
H – High Heat resistant
W – Water resistant
N – Nylon Coating
As you can see above a single rated THW wire would be missing the “N”, or nylon coating. Therefore it would come with a thermoplastic insulation that's Heat and Water resistant. It's become much easier for suppliers to order from the manufacturers as well because they can sell one item that covers many approvals.
The initial comparison was THHN 6 AWG compared to THWN 6 AWG electrical wires . Do you understand the difference now based on the descriptions above? THHN is not “water resistant” and in the past was not approved for outdoors use. Many contractors, electricians and installers would have to specify THHN VS THWN because they may have been doing an installation indoors or outdoors which required a different type of wire.
Nowadays it's an all-in-one universal electrical wire for indoors, outdoor and conduit use. The only approval it does not have yet is for underground use. If you need an underground cable you'll need to reference UF-B cable which stands for “underground feeder”. If you need a single conductor 6 AWG wire that can be buried directly in the ground you can also use USE-2 solar panel wire.
You can probably imagine how many different types of insulation can be used for a simple 6 AWG copper wire . The options and applications are almost infinite due to the minor details, such as a “W” for water resistant that can change the entire application of that wire. There's also PVC insulation, rubber EPDM, PTFE high temperature, silicone, braided silicone and much more. Each type of insulation attacks requirements such as voltage, amperage, temperature, environmental conditions and more.
Have you ever been walking through a fair and seen black cables running along the ground? Those cables also need to be protected against people walking on them, being dragged across pavement and being run over by other trucks and cars before the fair starts. One puncture of a cable and one of the trucks will be closed for business that day.