THW Wire: A Free Advanced Selection Manual

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THW Wire


Do you intend to perform any form of general purpose wiring? There is a significant chance that a THW Wire is the most ideal option for your wiring needs.  THW is an acronym that stands for Thermoplastic Heat and Water-resistant Wire. Generally, this type of electrical wire is used in building wiring, feeders and branch circuits as well as internal secondary industrial distribution. Currently, the National Fire Protection Association is the governing standard for any type of THW Cable or wire.

Like most electrical cables and wires, it is often difficult to distinguish between this type of wire and electrical wires of the same nature. More specifically, most people find it difficult to differentiate between THW Wire, THWN, and THHN electrical wires. This confusion arises because the aforementioned electrical cables share a lot of similarities. Nonetheless, each wire is designed to suit certain specific wiring needs. It is important to understand the differences before selecting an appropriate electrical wire otherwise you may choose the wrong wire. Fortunately, by the time you finish going through this selection manual, you will have learned everything you need to know for free.


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1. What is THW Wire?

As I mentioned earlier, THW stands for, Thermoplastic Heat and Water-resistant wire. The THW electrical wire is specifically designed for use in dry locations up to 750 Celsius dry or wet. That being said, the National Fire Protection Association (NFPA) limits the wire’s ampacity to 750 Celsius conductor temperature. Like most electrical wires, this type of wire occurs in two forms namely, solid or stranded. Its conductor is made of soft copper and is insulated using Polyvinyl Chloride (PVC).


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As I indicated earlier, the THW wire is suitable for universal wiring, machine tool wiring and internal wiring of various electrical appliances. Still, for general ground wiring, it is always advisable to use a solid THW wire. What are some of those ground wiring situations? Well, some common general ground wiring situations that call for the use of solid THWs include CATV installations and swimming pool pumps.

Also, it is important to note that like most electrical wires and cables THW wires are not one-fits-all. For instance, the THW- 2 wire is specifically suited for general wiring applications associated with feeders, conduits or any other raceway services and branch circuits. The image below will give you a comprehensive overview of everything you need to know including THW wire sizes and the insulation thickness of any THW copper wire that you might want to buy;

thw wire data

2. What is the difference between THW and THWN Wire?

Most people often find it difficult to differentiate between a THW wire and a THWN-type Wire. The difficulty in differentiating the two electrical wires is quite understandable considering that they share a lot of similarities. Basically, THWN stands for, Thermoplastic Heat and Water-resistant Nylon-coated wire. As you can see, the difference between THW and THWN is the ‘N’ which stands for Nylon-coated.  So, logically, the only difference between THW and THWN is the nylon coating. To put it simply a THW Cable does not have a nylon coating whereas; a THWN is reinforced with a nylon coating. The difference between the two is pretty straightforward, right?

Regardless of the fact that a THWN wire is an additional jacket of extruded nylon, its uses are significantly similar to those of a THW. It is rated 750 Celsius wet or dry and is normally used in conduit installations, machine tools, regulated electrical circuits and any other general-purpose wiring needs. For new buildings and rewiring for 60-volt applications, the National Electrical Code (NEC) permits the use of THWN-2 electrical wires.

Do you intend to carry out some outdoor installations? If the answer is yes you should definitely go for a THWN-type wire. Why is it advisable to use this type of wire for outdoor wiring purposes? Well without the nylon coating, wire-type THW provides minimal protection against various environmental adversities. That being said, if your wiring needs are limited to an indoor environment, you should go for a THW building wire especially because it is less costly compared to THWN-type wires and suits your specific needs.

3. Is THW Wire the Same as THHN?

Well, the answer to the question posed above is definitely no. THW wires are not the same as THHN wires. However, it is quite understandable why the two electrical wires may be mistakenly construed to be the same considering they share certain similarities. What is the difference between THW vs. THHN?

Well, THHN stands for Thermoplastic High Heat-resistant Nylon-coated wire. Unlike the THW wire which is suited for both dry and wet locations, the THHN cable is suited for dry locations only and can withstand temperatures up to 900 Celsius. It can be used in machine tools wiring, control circuits, and certain specific appliances. Consequently, if you intend to use a THHN in electronic appliance wiring, you should make some inquiries and ensure that it is applicable to your electronic appliance.

thwn wire

4. How can I know the THW Size of a wire?

In most instances, the size of a particular THW wire is on its insulation. Thus, it is advisable to check for any written text on the insulation material when purchasing a wire. If you do not see any written text on the THW wire insulation, you should ask the electrical shop steward to tell you the size of the wire before buying it. Why is it important to confirm the size of the wire? Well, just like any other electrical wire, if you use a THW-type wire that is too small for your wiring needs, the wire will melt due to the excess current flowing through it.

There are several THW wire sizes that you could choose from. Each size is designed to withstand a specific amount of current passing through it. Assuming that you intend to use the wire for the internal wiring of an amplifier if you fail to choose the correct size of wire, the amplifier will not receive the necessary voltage that is needed for it to perform optimally. So, to put it in simple and clear terms, the size of the THW wire is one of the most important features that you must consider when shopping for an ideal electrical wire.



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About Me

Richard Zi

Hey, I am Richard Zi, General Manager of ZW Cable and expert in cable industry for over 15 years. I would like to share my experience in cable field. ZW Cable is leading Chinese cable manufacturer. We can choose right size cable and solve all your cable problem. If you have any question, please feel free to contact me. I will try my best to give your cable solutions.

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