Trystan Casey
There are many different types of threads used for various applications. In the context of electrical fittings, some common thread styles include National Standard Pipe Tapered (NPT) and National Standard Pipe Straight (NPS). NPT threads are tapered or conical and are often found on conduit, nipples, elbows, and fittings. NPS threads, on the other hand, provide a secure and electrically continuous connection. Other thread types used in electrical applications include British Association (BA) threads for small electrical devices, steel armoured threads (Pg) used historically in electrical engineering, and knuckle threads found in electrical insulators and fittings. The choice of thread type depends on various factors such as design intent, industry standards, and specific application requirements.
| Characteristics | Values |
|---|---|
| Thread type | National Taper Pipe (NPT), National Standard Pipe Straight (NPS), Whitworth thread, British Association (BA) thread, Knuckle thread, Buttress thread, Worm threads, Fastening thread, Movement thread, Transport thread, Clockwise threads (right-hand threads), Left-handed threads (left-hand threads), Pipe threads, Tapered threads, Parallel threads, Metric threads |
| Angle | NPT, NPS, Whitworth thread, and the American Standard Institute's "V"-shaped thread: 60 degrees |
| British Standard Pipe thread angle | 55 degrees |
| Knuckle thread angle | 30 degrees |
| British Association thread nut bolts | 1/2" or 6 mm |
| Taper rate for NPT threads | 3/4-inch per foot or 1 inch per 16 inches of length |
| Tapered threads | Smaller in diameter as they approach the end of the fitting |
| Parallel threads | Maintain the same diameter throughout |
| Tapered threads sealing | Metal-to-metal wedging or minor thread distortion |
| Parallel threads sealing | Often require an o-ring or thread tape |
| Thread pitch size | Refers to the distance between threads |
| Compatibility | NPT and G threads are incompatible due to differing angles, shapes, and thread pitches |
| Fittings | NPT is found on conduit, nipples, elbows, and fittings |
| Electrical fittings with female threads | Generally not tapered |
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What You'll Learn
National Standard Pipe Tapered (NPT)
The NPT thread type features a 60-degree flank angle and a truncated peak and valley shape. When NPT threads are mated together, the tapered threads compress against each other to form a tight seal. This seal is created when the flanks of the threads compress, as opposed to straight thread fittings, where the threads do not provide a seal. This means that NPT fittings should be burr-free and lubricated with a lubricating paste or thread seal tape.
NPT threads are commonly used in North America and can be found on conduit, nipples, elbows, and fittings. Common sizes include 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1 1/4, 1 1/2, and 2 inches, although sizes smaller than 1/8 inch are occasionally used for compressed air applications. Sizes larger than 2 inches are uncommon due to the use of alternative joining methods for larger pipe sizes.
A variation of the NPT thread is the National Pipe Taper Fuel (NPTF) thread, also known as Dryseal American National Standard Taper Pipe Thread. NPTF threads are designed to provide a more leak-free seal without the use of sealants. This is achieved by adjusting the crest and root heights to create an interference fit, eliminating the spiral leakage path.
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National Standard Pipe Straight (NPS)
When selecting thread types for electrical fittings, manufacturers consider various factors, including design intent, industry convention, and UL regulations. The type of thread used can impact the sealing capabilities of the fitting. For example, tapered threads, such as those found in NPT, are pressure-tight and provide a sealing capability when torqued. On the other hand, straight threads like NPS do not form a seal and are designed to allow for the escape of unwanted substances.
It is important to note that the use of thread sealants can impact the sealing capabilities of different thread types. Thread sealants, such as thread seal tape or paste, can be used with both NPT and NPS threads depending on the specific application requirements. However, when using NPS threads, care must be taken to avoid electrically insulating a joint by using non-conductive thread sealants, as this would interrupt the path to ground. Therefore, the choice of thread sealant is crucial in ensuring the proper functioning of the electrical fitting.
In addition to NPT and NPS, other thread types used in electrical fittings include the steel armoured pipe thread (Pg) and the Whitworth thread. The steel armoured pipe thread, defined in DIN 40430, was commonly used in electrical engineering but is being replaced by metric threads (DIN EN 50262). The Whitworth thread, introduced by Sir Joseph Whitworth, is specified in inches and has a flank angle of 55 degrees. It is mainly used in the United Kingdom and is both a standard and screw thread.
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Whitworth thread
The Whitworth thread is a type of screw thread standard that uses imperial (inch-based) units. It was first introduced in 1841 by British engineer Sir Joseph Whitworth, making it the world's first standardised thread. Whitworth's standard specified a 55-degree thread angle, a thread depth of 0.640327p, and a radius of 0.137329p, where p is the pitch. The thread pitch increases with diameter, following a set of steps specified on a chart.
The Whitworth thread is mainly used in the United Kingdom, where it was widely adopted and influenced engineering practices. It also became the standard in the country's former colonies. The Whitworth thread laid the foundation for other thread standards, including British Standard Fine (BSF), British Standard Pipe (BSP), British Standard Conduit (BSCon), and British Standard Copper (BSCopper).
The Whitworth thread is a coarse thread that was originally designed to hold locomotives together. Due to its coarse pitch, it is more prone to vibrating loose, so it is not commonly used in motorcars. However, it is still used in certain legacy industries, restoration, and vintage machinery.
In terms of electrical fittings, the most common types of threads are National Standard Pipe Tapered (NPT) and National Standard Pipe Straight (NPS). These threads are used in rigid conduit systems and are selected based on factors such as design intent, industry convention, and UL regulation. While the Whitworth thread may not be as widely used in electrical fittings, it has played a significant role in standardisation and influenced engineering practices globally.
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Knuckle thread
The standard for knuckle threads is set by the German organisation Deutsches Institut für Normung (DIN), with DIN 405 specifying inch pitches and diameters ranging from 8 mm to 200 mm. The more recent standard, DIN 20400, employs a metric thread pitch and covers diameters from 10 mm to 300 mm. In the DIN thread charts, numbers are typically written with a comma as the decimal marker.
The rounded edges of knuckle threads also make them ideal for reducing stress on softer materials at connection points. This feature is advantageous in applications such as firefighting respirators, where metal and rubber parts need to be securely joined. Furthermore, the rounded crest and root design of knuckle threads resemble the Edison screw found on light bulbs, although bulbs typically have a shallower thread angle.
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Buttress thread
Buttress-thread screws are available in a wide range of sizes and diameters, typically ranging from 1 inch to 12 inches. The static load rating and operating load ratings vary accordingly, with larger screws capable of handling higher loads. Buttress threads are an important type of screw that has found applications in various industries, including construction, artillery, and oil field tubing.
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Frequently asked questions
The two most common types of threads used in electrical fittings are National Standard Pipe Tapered (NPT) and National Standard Pipe Straight (NPS).
NPT threads are tapered or conical, while NPS threads are straight. NPT threads form a tight seal when torqued, while NPS threads provide a secure, electrically continuous connection that is not sealed.
Other types of threads used in electrical applications include British Association (BA) threads, which are found in small electrical devices, and knuckle threads, which are used in electrical insulators and fittings.
It depends on the specific application and equipment. For example, if your equipment is manufactured in North America, you will likely need NPT threads, while equipment from China typically uses PT threads.
Yes, there are two variations of NPT threads: NPT and NPTF. NPTF threads are self-sealing and have overlapping internal and external threads, while standard NPT threads require a sealant.
