Incorrect sizing is the single most common cause of cable gland seal failure in industrial installations. Many procurement teams and field technicians select glands based only on thread size, overlooking the critical cable diameter clamping range. A gland that is too large will fail to form a watertight seal, while one that is too small can damage the cable jacket, strip threads, or prevent proper compression. This guide breaks down the core dimensions, measurement methods, and sizing logic for unarmored, armored, and special cable types, aligned with IEC 62444 performance requirements.
Why Cable Gland Sizing Matters
A properly sized cable gland achieves three simultaneous outcomes: a uniform environmental seal that meets the rated IP class, sufficient strain relief to resist cable pull and vibration, and no damage to the cable outer sheath or internal conductors.
When sizing is off, failures follow quickly. An oversized gland leaves gaps between the sealing insert and the cable jacket, allowing dust, moisture, and contaminants to enter the enclosure — even if the gland carries an IP68 rating. An undersized gland compresses the seal beyond its designed range, crushing the cable insulation, cracking the gland body, or stripping the compression nut threads during installation. For hazardous-area installations, incorrect sizing can also invalidate explosion-proof certifications and create ignition risks. Sizing is not a minor detail; it is the foundation of reliable gland performance.
Key Dimensions That Define Cable Gland Size
Cable gland size is defined by two independent dimensions that serve very different purposes. Confusing the two is the number one sizing mistake made by first-time buyers.
2.1 Thread Size (Entry Size)
Thread size describes the external thread on the gland body that passes through the enclosure panel and secures with a locknut. It is matched to the pre-drilled entry hole on the enclosure, not to the cable itself. Common thread standards include Metric (ISO 965), PG (DIN 40430), and NPT (ANSI B1.20.1), each with standard nominal sizes such as M20, PG11, or 1/2″ NPT.
Thread size determines mechanical fit to the enclosure and sets the maximum physical bore of the gland, but it does not by itself tell you what cable diameter will fit inside. Two glands with identical thread size from different manufacturers can have different cable clamping ranges.
2.2 Cable Diameter Range (Clamping Range)
The clamping range is the working window of cable outer diameters that the sealing insert can properly compress. It is always stated as a minimum and maximum value, for example 6–12 mm. This is the dimension that must match your cable.
The range is determined by the bore of the gland body and the elasticity of the sealing material. NBR rubber and neoprene seals have a predictable compression range; if the cable falls below the minimum, the seal will not make contact; if it exceeds the maximum, the seal cannot compress evenly and will either leak or be damaged during tightening. For consistent performance, select a gland where the measured cable diameter falls in the middle 60% of the stated clamping range.
How to Measure Your Cable for Correct Sizing
Nominal cable cross-section (such as 2.5 mm² or 4 mm²) is not a reliable sizing reference. Actual outer diameter varies by jacket material, number of conductors, shield construction, and manufacturer. Always measure physically before specifying.
Start with the complete cable as it will enter the gland. For unarmored cables, measure the outermost diameter of the outer sheath using calipers at three points around the circumference and take the largest reading. Do not measure just the conductor or inner insulation. Account for manufacturing tolerances — most industrial cables have a diameter tolerance of ±0.5 mm, and jacket thickness can increase slightly at higher temperatures.
For armored cables (SWA/AWA), take two separate measurements: the outer diameter of the inner sheath (after removing the armor wires) and the full outer diameter including the armor layer. Double-compression glands seal against the inner sheath and clamp onto the armor, so both dimensions must fall within their respective ranges. Finally, for shielded or EMC cables, confirm that the shield braid diameter will fit within the gland bore without bunching.
Sizing for Different Cable Types
Sizing logic changes with cable construction. What works for a simple unarmored PVC cable will not work for a steel-wire armored cable or a flexible conduit assembly.
4.1 Unarmored Cables (Single Compression)
For standard unarmored cables with a single outer jacket — the most common type in indoor control panels and distribution boards — sizing follows one simple rule: the measured outer diameter must fall within the gland’s stated clamping range.
Single-compression brass and nylon glands are sized to cover typical industrial cable diameters. For example, an M16 metric gland commonly covers 4–8 mm cables, while an M20 covers 6–12 mm. PG sizes follow a similar pattern, with PG7 covering 3–6.5 mm and PG13.5 covering 6–12 mm. When in doubt between two adjacent sizes, choose the larger one only if the cable diameter is closer to its minimum; never force a cable into a size that is too small. For a full range of single-compression sizes in nickel-plated brass and stainless steel, explore the XingWei standard cable glands collection.
4.2 Armored Cables (SWA/AWA, Double Compression)
Armored cables require double-compression glands that grip both the inner sheath and the steel or aluminum armor layer. Sizing these glands means verifying two separate ranges, not just one.
First, match the inner sealing range to the diameter of the cable’s inner sheath after armor removal. Second, match the armor clamping range to the full outer diameter of the armored cable. The armor wires must sit fully within the clamping cone or basket to provide proper pull-out resistance. A common error is sizing based only on total cable diameter, which can leave the inner seal loose and allow water migration along the armor. For certified hazardous-area armored glands, sizing must also align with the ATEX or IECEx certificate scope. For verified Ex d armored gland sizes across common SWA cable sections, view the XingWei explosion-proof armored cable gland range.
4.3 Flexible Conduit & Special Cables
Flexible conduit entries require glands sized to match the conduit outer diameter, not the internal wires. Conduit glands use a different clamping mechanism that grips the conduit wall, so standard cable sizing charts do not apply. For shielded and EMC cables, allow extra bore space for the shield braid and 360° contact spring; an undersized bore can fold the shield and break electrical continuity.
Common Cable Gland Size Standards at a Glance
Cable gland sizes follow regional thread conventions, but clamping ranges are not universally standardized across manufacturers. This quick reference outlines typical size pairings for general guidance — always confirm exact ranges with the product datasheet.
PG sizes, a legacy European format still widely used on control panels, run from PG7 up to PG48, with clamping ranges scaling with thread size. Metric sizes, the global ISO standard, follow nominal thread diameters from M12 to M63 and are dominant in new EU and Asian installations. NPT sizes, the North American tapered thread standard, are specified by nominal pipe size from 1/4″ up to 2″. Within each thread standard, smaller sizes serve instrumentation and signal cables, while larger sizes serve power and multi-core feeder cables.
Because clamping ranges vary by manufacturer and seal material, there is no universal conversion table between thread size and cable capacity. Two M20 glands from different suppliers can differ by 1–2 mm in usable cable range.
Common Sizing Mistakes to Avoid
Most sizing errors stem from rushed specification and assumption-based ordering. Watch for these five recurring pitfalls.
First, selecting by thread size alone without checking the clamping range is the most frequent mistake. Second, using conductor cross-section to estimate diameter instead of measuring the actual jacket leads to consistent undersizing or oversizing. Third, sizing right at the edge of the clamping range leaves no margin for cable tolerance, temperature expansion, or seal compression set over time. Fourth, for armored cables, measuring only the overall diameter and ignoring the inner sheath dimension causes inner seal failure. Fifth, swapping seal materials from the original specification — for example, replacing NBR with a thicker silicone seal — effectively reduces the usable clamping range.
Step-by-Step Cable Gland Size Selection
Follow this structured workflow to get sizing right on the first try.
Start by identifying the cable type: unarmored, armored, shielded, or conduit. Next, measure the relevant outer diameter or diameters with calipers, recording the maximum reading. Then confirm the enclosure entry hole thread standard and nominal size. With those three values, filter product options by thread size first, then verify that the cable diameter falls within the published clamping range, ideally in the middle portion of the range. Finally, cross-check that the selected size is available in the required material, IP rating, and certification class.
Sizing a cable gland correctly is a matter of matching thread size to the enclosure entry and clamping range to the measured cable outer diameter, while accounting for cable type, tolerances, and operating temperature. A mismatch in either dimension will compromise sealing performance and shorten service life.
Frequently Asked Questions
Q1: How do I know what size cable gland I need?
First, measure the actual outer diameter of your cable with calipers. Then confirm the thread size and standard of your enclosure entry hole. Select a gland that matches the thread size and has a clamping range that covers your measured cable diameter, ideally in the middle of the range for best seal performance.
Q2: Can I use a larger cable gland for a smaller cable?
You should not use a significantly larger gland for a smaller cable. If the cable diameter falls below the minimum clamping range, the seal will not compress properly against the jacket, resulting in leaks and poor strain relief. Reducer inserts are available for limited size adjustments, but they are not a substitute for correct base sizing.
Q3: Does cable gland size refer to thread size or cable size?
In common usage, “cable gland size” usually refers to the thread size (such as M20 or PG11), but this is only half the specification. The equally important cable clamping range defines what cable diameter will actually fit and seal properly. Always check both dimensions.
Q4: What size cable gland for 4 mm SWA cable?
A 4 mm² SWA cable typically has an outer diameter around 9–11 mm depending on manufacturer and armor type. A 20 mm metric or PG13.5 double-compression gland is commonly specified, but always measure the actual cable inner sheath and overall armored diameter before final selection.”
Correct cable gland sizing is a foundational step for reliable sealing, strain relief, and certification compliance. By measuring actual cable diameter, matching it to the clamping range, and aligning thread size to your enclosure, you eliminate the most common cause of premature gland failure. XingWei offers standard, waterproof, explosion-proof, and marine cable glands across a full range of Metric, PG, and NPT thread sizes, with published clamping ranges for every model. To get size recommendations tailored to your specific cable and application, reach out to our technical team or request a datasheet pack.