Part 4 of our guide series "Converting units" deals with the conversion of metric, PG or NPT glands. The question often arises as to what the difference is between the individual standards for cable glands and how they can be converted, e.g. between the European standard IEC 60423 for connectors in accordance with DIN EN 62444 and cable glands for cables in accordance with DIN 89 280?
Cable glands conforming to DIN 89 280 are usually made of a corrosion-resistant material, such as brass cable glands, aluminium or chrome-plated stainless steel cable glands, as they are designed in particular for use in marine environments. As a rule, these connectors for electrical installations, which can be installed both inside and outside ships, have appropriate sealing rings to protect against dust and moisture ingress.
This is in contrast to the DIN EN 60423 standard, which specifies the outer diameter of conduits used for electrical and communication installations. In addition to the outer diameter, this standard also describes the exact requirements for the dimensions of the threads and their respective moulded parts.
PG gland table
Steel armoured conduit threads, often abbreviated to PG threads, are one of a series of historical threads that have a flank angle of 80 degrees. One of the characteristics of steel conduit is its low wall thickness. For this reason, the threads are not particularly deep.
The specification for PG threads can be found in the DIN 40430 standard, which has been superseded by DIN EN 60423. Typical areas of application for PG pipes and threads include electrical installations. They are still used there for cable glands, for example. Even though this historical type has been largely replaced by metric threads, there is still a need for their use.
Typical PG thread sizes at a glance
| Thread identifier | Outer diameter in mm | Diameter of the core hole in mm | Bore diameter | Thread pitch | Equivalent metric dimension in mm | Pipe diameter in mm |
|---|---|---|---|---|---|---|
| PG 7 | 12.5 mm | 11.3 mm | 12.5 + 0.2 mm | 1.27 | M 12 x 1.5 / M 16 x1.5 | 6 mm |
| PG 9 | 15.2 mm | 13.9 mm | 15.2 + 0.2 mm | 1.41 | M 16 x 1.5 | 8 mm |
| PG 11 | 18.6 mm | 17.3 mm | 18.6 + 0.2 mm | 1.41 | M 20 x 1.5 | 10 mm |
| PG 13.5 | 20.4 mm | 19.1 mm | 20.4 + 0.2 mm | 1.41 | M 20 x 1.5 / M 25 x 1.5 | 12 mm |
| PG 16 | 22.5 mm | 21.2 mm | 22.5 + 0.2 mm | 1.41 | M 20 x 1.5 / M 25 x 1.5 | 14 mm |
| PG 21 | 28.3 mm | 26.8 mm | 28.3 + 0.2 mm | 1.58 | M 25 x 1.5 / M 32 x 1.5 | 18 mm |
| PG 29 | 37 mm | 35.5 mm | 37.0 + 0.3 mm | 1.58 | M 25 x 1.5 / M 32 x 1.5 | 25 mm |
| PG 36 | 47 mm | 45.5 mm | 47.0 + 0.3 mm | 1.58 | M 50 x 1.5 | 32 mm |
| PG 42 | 54 mm | 52.5 mm | 54.0 + 0.3 mm | 1.58 | M 50 x 1.5 / M 63 x 1.5 | 38 mm |
| PG48 | 59.3 mm | 57.8 mm | 59.3 + 0.3 mm | 1.58 | M 63 x 1.5 |
Download table
Metric cable gland table
Typical metric thread sizes at a glance
| Threads | Outer diameter | Core hole diameter | Bore diameter | Thread pitch |
|---|---|---|---|---|
| M12 | 12.0 mm | 10.6 mm | 12.0 + 0.2 mm | 1.5 |
| M16 | 16.0 mm | 14.6 mm | 16.0 + 0.2 mm | 1.5 |
| M20 | 20.0 mm | 18.6 mm | 20.0 + 0.2 mm | 1.5 |
| M25 | 25.0 mm | 23.6 mm | 25.0 + 0.2 mm | 1.5 |
| M32 | 32.0 mm | 30.6 mm | 32.0 + 0.2 mm | 1.5 |
| M40 | 40.0 mm | 38.6 mm | 40.0 + 0.2 mm | 1.5 |
| M50 | 50.0 mm | 48.6 mm | 50.0 + 0.3 mm | 1.5 |
| M63 | 63.0 mm | 61.6 mm | 63.0 + 0.3 mm | 1.5 |
| M75 | 75.0 mm | 73.6 mm | 75.0 + 0.3 mm | 1.5 |
Download table
NPT thread sizes
NPT threads (National Pipe Thread) are US American threads in inch sizes. According to ANSI b 1.20.2, these threads are suitable for self-sealing connections.
We have summarised common NPT sizes in the metric NPT table.
| Thread identification | Thread pitch | Outer diameter in mm | Bore |
|---|---|---|---|
| NPT 1/4" | 1.41 | 13.7 mm | 14.1 +0.2mm |
| NPT 3/8" | 1.41 | 17.1 mm | 17.4 +0.2mm |
| NPT 1/2" | 1.81 | 21.3 mm | 21.6 +0.2mm |
| NPT 3/4" | 1.81 | 26.7 mm | 27.0 +0.2mm |
| NPT 1" | 2.21 | 33.4 mm | 33.7 +0.2mm |
| NPT 1 1/4" | 2.21 | 42.2 mm | 42.5 +0.2mm |
| NPT 1 1/2" | 2.21 | 48.3 mm | 48.7 +0.2mm |
| NPT 2" | 2.21 | 60.3 mm | 60.7 +0.2mm |
| NPT 2 1/2" | 3.18 | 73.0 mm | 73.1 +0.4mm |
| NPT 3" | 3.18 | 88.9 mm | 89.0 +0.4mm |
| NPT 3 1/2" | 3.18 | 101.6 mm | 101.6 +0.4mm |
| NPT 4" | 3.18 | 114.4 mm | 114.4 +0.4mm |
Download table
| PG | Outer Ø PG | Pitch PG | Metric | Outer Ø metric | Pitch metric | NPT | Outer Ø NPT | Pitch NPT |
|---|---|---|---|---|---|---|---|---|
| M10 | 10.0 | 1 | ||||||
| PG 7 | 12.5 | 1.27 | M12 | 12.0 | 1.5 | |||
| 1/4" | 13,716 | 1,411 | ||||||
| PG 9 | 15.2 | 1.41 | ||||||
| M16 | 16.0 | 1.5 | ||||||
| 3/8" | 21,336 | 1,814 | ||||||
| PG 11 | 20.4 | 1.41 | ||||||
| PG 13.5 | 20.4 | 1.41 | M20 | 20.0 | 1.5 | |||
| 1/2" | 21,336 | 1,814 | ||||||
| PG 16 | 22.5 | 1.41 | ||||||
| M25 | 25.0 | 1.5 | ||||||
| 3/4" | 26,670 | 1,814 | ||||||
| PG 21 | 28.3 | 1,588 | ||||||
| M32 | 32.0 | 1.5 | ||||||
| 1" | 33,401 | 2,208 | ||||||
| PG 29 | 37.0 | 1,588 | ||||||
| M40 | 40.0 | 1.5 | ||||||
| 1 1/4" | 42,164 | 2,208 | ||||||
| PG 36 | 47.0 | 1,588 | M50 | 50.0 | 1.5 | |||
| 1 1/2" | 48,260 | 2,208 | ||||||
| PG 42 | 54.0 | 1,588 | ||||||
| PG 48 | 59.3 | 1,588 | 2" | 60,325 | 2,208 | |||
| M63 | 63.0 | 1.5 |
What is the difference between outer diameter, core hole diameter, bore diameter and bore?
The outer diameter of a cable gland is the outer dimension, so an M25 cable gland has an outer dimension of 25 mm.
The core hole diameter of a nut or connector indicates the dimension of the initial hole into which an internal thread is subsequently cut, e.g. for a blanking plugs for holes.
The bore of the cable gland indicates the diameter of the bushing through which the cables are fed, so the bore diameter of a cable gland is decisive for the maximum cable diameter.
What is the clamping range of a cable gland?
The clamping range of a cable gland indicates the minimum and maximum outer diameter of an inserted wire from which a cable gland maintains its tightness. This also ensures that the protection class specified by the manufacturer is achieved. The protection class is extremely important, especially in the industrial sector, depending on the area of application. This is the only way to ensure that neither moisture nor dust can penetrate the inside of an enclosure or control cabinet through the cable gland. The clamping range also provides information about the diameter from which and up to which the respective cables are strain-relieved. In this respect, the clamping range is one of the most important factors when selecting the ideal cable gland, regardless of whether it is a model with NPT, PG or metric cable glands.
Of course, there is also a standardised basis for cable glands, which is formed by DIN EN 62444. Among other things, this standard sets out all the essential requirements regarding the design, construction and testing of a cable gland.
Why is the correct calculation of the clamping range important?
Based on the cable cross-section of a wire, the diameter of a cable can also be calculated using a formula. However, it is important to take into account the thickness of the sheath in order to be able to calculate the total outer diameter of the wires. To determine the correct sizes for a cable gland that conforms to protection class IP67, for example, the clamping range of the respective cable gland is specified in a table. Depending on the sizes of the cable gland, this also determines the minimum and maximum clamping range.
As already mentioned, the calculation of the clamping range is always important if the cable gland not only provides strain relief, but also has to be dustproof and waterproof under all circumstances. This is the case, for example, with switch cabinets for industrial use or distribution boxes on a construction site. In outdoor areas, these systems must remain functional even under adverse weather conditions. Short circuits and the resulting failure of the devices, construction machinery or production systems supplied with electricity should be avoided at all costs. In ATEX zones in particular, for example, it is essential that the cable glands fulfil the requirements of DIN EN IEC 60079-0 and DIN EN IEC 60079-7. This is the only way to ensure that no explosive gases enter an enclosure and are ignited by a spark.
Incidentally, it is difficult to define a generally applicable formula for calculating the clamping range. One of the reasons for this is that what appears to be the same cable type differs depending on the manufacturer - in terms of the number of wires and their outer diameter.
What information do you need for the calculation?
To calculate the clamping range, the outer diameter of a cable is required in all circumstances. This also results in the clamping range to be selected for strain-relieved and tight cable entries for the respective wires. In order to determine the appropriate clamping range based on the cable diameter, it is necessary to take the manufacturer's specifications into account. It should also be noted that some manufacturers extend their clamping range cable gland table to include the typical sealing range. For metric cable glands - M12 to M63 - this is around 0.5 to 3 mm below the minimum clamping range of a connector. This discrepancy is caused by a deformable or compressible sheath material of the respective cable.
Table of clamping ranges
Metric, imperial or in the PG sizes that are still commonly used: Cable glands for tight cable entries that are protected from tensile loads are available in all conceivable designs with different sealing materials. We have summarised the most common sizes, including the respective clamping ranges in mm, in a clamping range cable gland table. Please note that these values are for guidance only. To determine the exact clamping range, it is essential to observe the manufacturer's specifications!
| Thread | Unfeasible clamping range |
|---|---|
| PG 7 | 2 - 6.5 mm |
| PG 9 | 4 - 8 mm |
| PG 11 | 4 - 10 mm |
| PG 13.5 | 5 - 12 mm |
| PG 16 | 8 - 14 mm |
| PG 21 | 11 - 18 mm |
| PG 29 | 16 - 25 mm |
| PG 36 | 19 - 32 mm |
| PG 42 | 28 - 38 mm |
| PG 48 | 34 - 44 mm |
| Thread | Unfeasible clamping range |
|---|---|
| M 12 | 2.5 - 6.5 mm |
| M 16 | 2.5 - 8.0 mm |
| M 20 | 5.0 - 12.0 mm |
| M 25 | 9.0 - 18.0 mm |
| M 32 | 14.0 - 25.0 mm |
| M 40 | 18.0 - 32.0 mm |
| M 50 | 24.0 - 38.0 mm |
| M 63 | 30.0 - 44.0 mm |
| Threaded Skintop MSR | Unfeasible clamping range |
|---|---|
| 1/4 inch | 3.5 - 7 mm |
| 3/8 inch | 4.5 - 9 mm |
| 1/2 inch | 7 - 12.5 mm |
| 3/4 inch | 9 - 16.5 mm |
| 1 inch | 11 - 21 mm |
| 1 1/4 inch | 19 - 28 mm |
| 1 1/2 inch | 27 - 35 mm |
| 2 inches | 34 - 45 mm |
Despite careful editing and checking of the content, Stecker Express assumes no liability for the topicality, correctness, completeness and quality of the information provided.
Further advice topics
Tube dimensions
Converting hose dimensions into inches or millimetres for (cable protection) hoses and pipes.
Inches in mm
Conversion from inches to millimetres - How do you convert inches to mm or cm?
AWG in mm²
Conversion of AWG cable cross-sections to mm² - Strand construction and AWG tables
Strain relief
Strain relief - What needs to be considered to fix and protect cables!