What is Cable Gland Thread
A Cable Gland Thread is the engineered spiral ridge system machined onto cable glands that creates the critical connection point between cables and electrical enclosures. According to the International Cable Protection Committee, improper thread selection accounts for 32% of all cable entry failures in industrial settings.
These precision-engineered threads serve three vital functions: creating watertight seals against environmental hazards, providing strain relief that prevents cable damage, and ensuring electrical continuity for grounding and EMI protection.
Types of Cable Gland Thread
Metric Thread (ISO)
Developed according to the International Organization for Standardization (ISO) standards, metric threads originate from the metric system established in Europe. These threads are currently the most commonly used type worldwide and represent the standard choice for countries following IEC (International Electrotechnical Commission) guidelines.
Metric threads used on cable glands are parallel (or straight), meaning their diameter remains constant along the thread length. They feature a symmetrical V-shaped profile with a 60° flank angle. Their naming convention consists of “M” followed by the nominal major diameter in millimeters, then the pitch (distance between adjacent thread crests) in millimeters, separated by an “x”. For example, “M20 x 1.5” indicates a nominal diameter of 20 millimeters with a pitch of 1.5 millimeters.
Common metric thread sizes for cable glands include M12, M16, M20, M25, M32, M40, M50, and M63. These standardized dimensions ensure interchangeability across manufacturers and simplify component selection for international projects.
National Pipe Taper Thread (NPT)
NPT is an American national standard, defined by ANSI/ASME B1.20.1. It serves as the primary thread type for fittings, conduit entries, and cable glands in North America (USA and Canada) and appears in other regions influenced by American engineering standards.
NPT features tapered threads with a taper ratio of 1 to 16 (1:16), meaning the diameter decreases by 1 unit for every 16 units of length along the thread axis. Like metric threads, NPT threads have a symmetrical V-shaped profile with a 60° flank angle. They are designated by the nominal pipe size in inches (e.g., 1/2″NPT, 3/4″NPT), which historically related to the internal diameter of the pipe rather than the actual thread diameter. The pitch is defined as the number of threads per inch (TPI).
The tapered design creates a self-sealing effect as the thread tightens, though additional sealant is typically required for liquid-tight applications. Common NPT sizes for cable glands include 1/2″ NPT (approximately 21mm), 3/4″ NPT (approximately 26.5mm), 1″ NPT (approximately 33mm), 1-1/4″ NPT (approximately 42mm), and 1-1/2″ NPT (approximately 48mm).
Panzer-Gewinde Thread (PG)
PG threads are a German standard (DIN 40430) named after their original design for “Panzerrohr” (armored conduit). They were historically prevalent in Germany and other parts of Europe before being largely superseded by metric threads
PG threads are parallel (straight) with a distinctive thread profile featuring an 80° flank angle—significantly wider than metric or NPT threads. This wider angle (think of it as a less pointed “V” shape) results in shallower thread depth compared to similar-sized alternatives. PG threads are designated by the prefix “PG” followed by a number (e.g., PG7, PG11, PG13.5, PG21, PG29) that relates to the approximate clearance diameter in millimeters.
While being phased out of new designs, PG threads remain critical in legacy industrial systems throughout Europe. According to the European Electrical Contractors Association, approximately €1.2 billion is spent annually on maintenance components with PG threads for existing installations, particularly in chemical processing plants, steel mills, and railway systems built between 1970-2000. Popular sizes include PG9 (15.2mm external diameter), PG11 (18.6mm), PG13.5 (20.4mm), PG16 (22.5mm), PG21 (28.3mm), and PG29 (37.0mm).
British Standard Pipe Thread (BSP)
BSP threads were developed from British Standards and are widely recognized and used in the UK, Europe, Asia, Australia, New Zealand, South Africa, and many other parts of the world, particularly in fluid power (hydraulic and pneumatic) systems.
BSPP (British Standard Pipe Parallel)
BSPP threads—identified by the designation “G” in ISO 228 standards—maintain a constant diameter throughout their length (parallel design). What distinguishes them from other parallel threads is their 55° flank angle profile, creating a slightly different thread depth and interference pattern.
The naming convention follows the pattern of “G” plus the nominal pipe size in inches (e.g., G1/2 equals approximately 20.9mm external diameter). For industrial applications requiring quick reference:
- G1/2 ≈ 20.9mm
- G3/4 ≈ 26.4mm
- G1 ≈ 33.2mm
- G1-1/4 ≈ 41.9mm
BSPT (British Standard Pipe Taper)
BSPT threads follow the ISO 7/1 standard with the same 55° flank angle as BSPP but incorporate a 1:16 taper ratio identical to NPT. This creates a hybrid thread system with clear designation markers to prevent confusion:
- “R” designates external (male) tapered threads
- “Rc” identifies internal (female) tapered threads
- “Rp” denotes internal parallel threads designed to mate with external tapered threads
Parallel vs. Tapered Forms
Parallel Threads
Metric (M), Panzer-Gewinde (PG), and British Standard Pipe Parallel (BSPP or G) threads maintain a constant diameter throughout their length. They typically pass through a clearance hole in an enclosure wall and are secured from the inside using a locknut, although they can also be screwed into internally threaded parallel holes.
Sealing is not achieved through the threads themselves but by compressing a separate sealing element (flat washer, O-ring, bonded seal) between a flange or shoulder on the gland body and the flat surface around the entry hole on the enclosure. This design allows for precise installation depth adjustment and simplified disassembly for maintenance purposes.
The International Protection Testing Laboratory reports that parallel thread systems maintain up to 93% of their initial torque values after thermal cycling tests, making them ideal for applications requiring frequent maintenance access. However, they typically achieve only IP65 protection (dust-tight and protected against water jets) without supplementary sealing systems.
Tapered Threads
National Pipe Taper (NPT) and British Standard Pipe Taper (BSPT or R) threads are characterized by a gradual reduction in diameter along the thread axis. NPT has a 1:16 taper. They are designed to be screwed into matching tapered internal threads (NPT or BSPT/Rc) or sometimes into parallel internal threads (NPS for NPT, Rp for BSPT), relying on the taper to form a mechanical lock and seal through interference.
Sealing primarily depends on the metal-to-metal interference fit created when the tapered external thread wedges into the matching internal thread. The thread flanks compress against each other. However, due to manufacturing tolerances and helical leak paths, achieving a perfect seal with thread interference alone is difficult. Therefore, it is almost always recommended (and often required) to use a thread sealant (PTFE tape or liquid/paste sealant) to fill any minute gaps.