Specifying a cable gland looks simple until the thread does not match the hole. A gland that seats on an M25 enclosure will not seal an NPT 3/4″ conduit, and forcing it cracks the body or leaves a path for water and dust. The four thread families you meet on real jobs — Metric (ISO), PG, NPT and G (BSPP) — are not interchangeable, and each seals in a different way. This guide explains what each standard is, where it is used, how to identify the one you already have, and how to pick the correct thread so the enclosure rating you paid for actually holds in service.
Why the Thread Standard Decides Whether the Seal Holds
A cable gland does three jobs at once: it grips the cable, it relieves strain, and it seals the entry point of the enclosure. The first two depend on the clamp and the seal inside the gland. The third depends entirely on the thread matching the threaded hole in the panel or junction box. If the thread is wrong, the gland either will not start, cocks at an angle, or appears to seat while leaving a helical leak path around the threads.
This is why thread choice is a sealing decision first and a mechanical-fit decision second. A gland with the right ingress rating is useless if its thread cannot form a proper joint with the enclosure. That makes the thread standard the first thing to confirm on any bill of materials, ahead of material or IP rating.
The Four Thread Standards Compared
Most industrial equipment uses one of four threaded standards. Knowing the family tells you the sealing method, the regional convention, and which replacement glands will fit without adapters.
Metric (ISO Metric Parallel)
Metric glands use a parallel ISO thread, almost always stated as M followed by the major diameter and pitch — for example M20×1.5 or M25×1.5. The pitch (the 1.5 mm figure) is as important as the diameter, because a gland and an enclosure with the same outer size but different pitch will not mate. Metric is the default on new European and Asian equipment and is increasingly the global standard for new installations. It seals with an O-ring or flat washer captured at the enclosure face, not with the thread itself.
PG (Panzergewinde)
PG, short for the German “Panzergewinde,” is a legacy parallel thread still common on older European panels, switchgear, and control cabinets — think Siemens and similar heritage equipment. Sizes read as PG13.5, PG16, PG21 and so on, and they do not convert cleanly to metric diameters, which is why a PG16 hole will not accept an M16 gland even though the numbers look close. PG glands seal with an O-ring at the panel, just like metric. For a deeper look at choosing between the two on real equipment, see our metric vs PG cable gland selection by application guide.
NPT (National Pipe Taper)
NPT is the North American standard and uses a tapered thread — the diameter shrinks along the length so the joint tightens as you screw it in. Common sizes are 1/2″, 3/4″ and 1″. Because the thread is tapered, an NPT gland seals through thread interference and requires a sealing compound such as PTFE tape or pipe dope; an O-ring alone will not close the helical gap. NPT dominates in the US, in oil and gas, and on legacy American machinery.
G / BSPP (British Standard Pipe Parallel)
G threads are parallel British Standard Pipe (BSPP). They look similar to NPT but are parallel, not tapered, and they seal with a bonded washer or O-ring trapped between the gland shoulder and the enclosure face. Sizes are given in inches (G1/2″, G3/4″, G1″) but the sealing principle matches metric and PG. G threads appear on UK equipment and on a great deal of continental European machinery built to British-influenced specs.
Parallel vs Tapered: The Sealing Mechanism Is the Real Difference
The single most important distinction is parallel versus tapered. Metric, PG and G are all parallel and rely on a face seal — an O-ring or washer squeezed between the gland and the flat surface of the enclosure. NPT is tapered and relies on the metal-to-metal wedge of the thread plus sealant. Mixing the logic is the classic failure: fitting an NPT gland into a parallel metric hole and hoping the O-ring saves it, or expecting an NPT taper to seal a PG opening without compound.
Practically, parallel threads are more forgiving during installation because the seal sits at a visible face you can inspect. Tapered threads demand correct sealant and the right number of turns; over-tightening can crack a thin enclosure, while under-tightening leaks. Choosing the right family up front removes this whole class of problems.
Matching the Thread to Your Enclosure and Region
Start from the enclosure, not the cable. Read the threaded holes on the panel: if they are marked M25×1.5, you need metric; if they are PG21, you need PG; if they are 3/4″ and the equipment is North American, assume NPT unless proven otherwise. When you are building new equipment for export, metric is the safest default because it mates with the widest range of international enclosures and avoids the tapered-seal complication.
Region is a strong hint but not a guarantee. Plenty of Asian-built machines use NPT for the US market, and plenty of European panels still carry PG holes. Always verify the actual thread rather than trust the nameplate country of origin.
How to Identify the Thread You Already Have
When a panel is in front of you and the marking is gone, measure instead of guessing. Work through these four checks.
Step 1: Measure the Major Outside Diameter
Caliper the widest part of the thread on the existing gland or the enclosure hole. Note the measurement in both millimetres and inches so you can compare against standard tables. A 25 mm major diameter points toward M25 or PG, while roughly 26.4 mm is the major diameter of a 3/4″ NPT.
Step 2: Count Threads per Inch (or Measure Pitch)
Lay a ruler across the threads and count how many peaks fall in one inch, or measure the distance between peaks in millimetres for the pitch. Metric and PG use fine pitches (around 1.5 mm), NPT 3/4″ runs about 14 threads per inch, and G/BSPP 3/4″ is 14 TPI as well — which is exactly why diameter plus pitch together are needed to tell them apart.
Step 3: Check for Taper
Measure the major diameter at the mouth and again a few millimetres deeper. If it shrinks, the thread is tapered (NPT). If it stays constant, it is parallel (metric, PG or G). This one test separates NPT from everything else and tells you immediately whether you need thread sealant.
Step 4: Compare Against a Standard Table
Match your diameter, pitch and taper result to a published thread table to land the exact size. Once the family and size are confirmed, order the gland in that standard rather than forcing an adapter, which only adds a second potential leak path.
Common Mistakes That Break the Seal
The recurring errors are predictable. Forcing an NPT gland into a metric or PG hole cross-threads and cracks the enclosure. Assuming PG and metric are the same because the numbers look alike leaves a loose, leaking joint. Skipping sealant on a tapered NPT thread defeats its only sealing mechanism. And re-using a gland whose O-ring has flattened or perished quietly removes the face seal on a parallel thread. Each of these turns a correctly specified IP66 or IP68 installation into a failure during the first heavy rain or washdown.
Quick Selection Guide
| If your enclosure is… | Choose this thread |
|---|---|
| New European or Asian panel, marked M×1.5 | Metric (ISO) parallel, O-ring seal |
| Older German control cabinet, PG sizes | PG parallel, O-ring seal |
| North American or oil & gas, tapered feel | NPT tapered, with thread sealant |
| UK or British-spec machinery | G / BSPP parallel, washer or O-ring |
| New export equipment, no legacy constraint | Metric — widest compatibility |
Frequently Asked Questions
Can I use an NPT gland in a metric hole if it looks close?
No. NPT is tapered and metric is parallel, so the threads will not engage properly; the gland will cock, cross-thread, or appear seated while leaving a leak path. Use an NPT-to-metric adapter only as a deliberate, sealed transition, never as a forced fit.
Do metric, PG and G glands all seal the same way?
Yes — all three are parallel threads and rely on an O-ring or bonded washer at the enclosure face rather than on the thread. The difference is purely the thread geometry and pitch, which is why they need the correct matching hole even though the sealing principle is identical.
Why does my NPT gland need tape if it has an O-ring?
An O-ring on an NPT gland seals the face, but the tapered thread itself still has a helical gap that only closes with sealant. Without PTFE tape or pipe dope, water tracks along the thread grooves. Face O-rings and thread sealant do different jobs and both are needed on a tapered joint.
Is PG still worth specifying on new projects?
Only when you are matching existing PG panels. For new builds, metric is the better default because it is globally recognised, avoids the PG size confusion, and uses the same simple O-ring sealing as PG. Reserve PG for legacy interchange.
How do I confirm a gland is the right thread before installation?
Measure the major diameter, count the threads per inch (or pitch), and check whether the thread tapers. Match those three readings to a standard table. A five-minute check prevents a cracked enclosure and a failed ingress rating on commissioning day.
Need glands in the exact thread your enclosure requires?
Niuli Electric supplies cable glands in Metric, PG, NPT and G threads. Send us your enclosure thread size and cable diameter and we will match the gland, seal and approvals.
