PG or Metric Thread: Cable Gland Compatibility Explained

PG and metric (M) cable glands are both common in industrial and electrical enclosures, both use a threaded compression design, and at a glance they can look close enough to swap. They aren't the same thread standard, and forcing one into a fitting designed for the other either won't thread properly or will cross-thread and damage the seal before the gland is even fully installed. For anyone sourcing replacement glands or specifying new enclosure hardware, knowing which standard is actually in use is worth confirming before ordering, not after a gland shows up that doesn't fit.

Where PG Thread Comes From

PG (Panzergewinde, German for "armored thread") is a legacy German industrial standard for cable gland and conduit fittings, historically widespread across European electrical equipment and still common today, particularly in older or European-designed enclosures. PG sizes are labeled by a number — PG7, PG9, PG13.5, PG16, PG21, and so on — that doesn't directly correspond to a simple metric measurement the way an M-series thread diameter does; the numbering follows its own historical convention rather than a straightforward millimeter measurement.

Where Metric (M) Thread Comes From

Metric cable glands use standard ISO metric threading, sized directly by thread diameter in millimeters — M12, M16, M20, M25, and so on. This is now the more common standard across new equipment design internationally, including most modern enclosures manufactured for global markets, in part because the metric sizing is more directly interpretable (the number is the actual thread diameter) than PG's legacy numbering system.

Why They Aren't Interchangeable Despite Looking Similar

PG and metric threads differ in thread pitch and diameter even at nominally similar sizes — a PG13.5 gland and an M20 gland are often mistaken for equivalent because they're used in similar-sized knockouts, but the actual thread geometry doesn't match. Attempting to thread a PG gland into a metric-tapped hole, or vice versa, typically either won't engage properly or will cross-thread and damage both the gland and the enclosure's threaded opening — and even where a mismatched thread appears to engage a few turns, it won't achieve the even compression the seal depends on for its rated IP performance.

A Rough Size Reference (Confirm Before Ordering, Don't Assume)

PG and metric sizes are sometimes listed side by side as "equivalents" for cable diameter range purposes, but this refers to similar cable-clamping capacity, not thread compatibility:

• PG7 is often paired near M12 in cable range, but the threads do not interchange
• PG9 is often paired near M16 in cable range, but the threads do not interchange
• PG13.5 is often paired near M20 in cable range, but the threads do not interchange
• PG16 is often paired near M25 in cable range, but the threads do not interchange

These pairings are useful for estimating cable capacity when comparing a PG spec against a metric spec in general terms, but the thread itself always needs to match the enclosure's actual tapped hole — never assume a "near-equivalent" size will physically thread in.

How to Confirm Which Standard You Actually Have

• Check the enclosure manufacturer's documentation or nameplate, which typically specifies the knockout or tapped thread standard directly
• If documentation isn't available, a thread pitch gauge can distinguish PG from metric threading on an existing installed gland or an open knockout
• For older European-manufactured enclosures specifically, default to checking for PG threading first, since it remains common on legacy equipment even as new designs shift toward metric
• When replacing a single gland on existing equipment, matching the exact original part number is more reliable than matching by nominal size alone

Where This Comes Up in Practice

• Retrofitting or servicing older European-designed control panels and enclosures still using PG-threaded knockouts
• Mixed equipment fleets where newer metric-standard enclosures and legacy PG-threaded units are both in service
• International sourcing, where a gland ordered by nominal size without thread standard confirmation arrives incompatible with the target enclosure
• New equipment builds, where specifying metric threading consistently across a design avoids introducing PG/metric mismatches down the line

Frequently Asked Questions

Can I use an adapter to convert between PG and metric thread?

Thread adapters exist for some size combinations, but they add an additional sealed joint to the assembly, which is another point that needs to maintain the IP rating. Where possible, sourcing a gland that matches the enclosure's native thread standard directly is more reliable than introducing an adapter.

How can I tell if my enclosure uses PG or metric threading without documentation?

A thread pitch gauge checked against an existing gland or an open tapped knockout will distinguish the two. Visually, PG and metric threads at similar nominal sizes can look close enough that confirming with a gauge or the enclosure manufacturer's spec is more reliable than a visual comparison alone.

Is PG or metric threading better for a new equipment design?

Neither is inherently better in performance — metric has become the more common default for new international equipment designs simply because the sizing is more directly interpretable and it's the more widely stocked standard globally today, which can simplify sourcing and replacement parts over the equipment's service life.