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An M20 cable gland does not automatically fit a 20 mm cable. This is one of the most common—and most expensive—mistakes in cable-entry selection. The M20 marking normally identifies the entry thread, while the cable that can be sealed is determined by a separate clamping range.
This guide explains how to choose a cable gland by cable outer diameter, clamping range, entry thread, thread length, material and ingress-protection requirement. It also addresses difficult cases that appear repeatedly in real installations: very thin cables, existing enclosure holes, pre-terminated RJ45 cables, multiple cables through one entry and outdoor boxes that still collect water.
Quick answer
Select the gland from the cable's measured outer diameter first. Then verify the enclosure thread or cut-out, available thread length, sealing method, material, temperature range and the IP rating of the completed installation. Never select a gland from the M, PG or NPT designation alone.
1. Why Cable Gland Sizes Are Easy to Misread
The thread size and cable size are different specifications
A cable gland has two interfaces: one with the enclosure and one with the cable. The entry thread connects the gland to a panel, junction box or machine housing. The sealing insert compresses around the cable jacket. These interfaces are related by the product design, but they are not the same dimension.
For example, M20 × 1.5 describes a metric entry thread with a nominal 20 mm outside diameter and a 1.5 mm pitch. It does not promise that a 20 mm cable will pass through or seal correctly. Depending on the gland series and sealing insert, an M20 product may have a clamping range such as 6–12 mm, 6–13 mm or another manufacturer-defined range.
| Specification | What It Controls | Example | Common Mistake |
|---|---|---|---|
| Entry thread | Compatibility with the enclosure | M20 × 1.5 | Assuming M20 fits a 20 mm cable |
| Clamping range | Cable outer diameters that can be sealed and retained | 6–13 mm | Checking only the maximum diameter |
| Thread length | Usable wall thickness and thread engagement | 8 mm | Buying a short-thread gland for a thick enclosure |
| Clear opening | Whether the cable or connector can physically pass through | Product-specific | Ignoring a large pre-terminated plug |
| IP rating | Tested resistance to solids and water under stated conditions | IP68 | Assuming every installed assembly remains IP68 |
Why a larger thread may contain a smaller sealing hole
A large enclosure entry and a small cable can occur in the same installation. Manufacturers therefore offer reduced clamping inserts, removable inner inserts and different sealing ranges for the same thread size. This lets an installer seal a thin cable in an existing M20 hole without adding a separate thread reducer.
This design is useful, but it means that moving from M16 to M20 does not guarantee a larger cable range. Always read the minimum and maximum cable diameter instead of judging the rubber insert by the thread marking.
Four dimensions to record before ordering
- Cable outer diameter: Measure the finished jacket at the sealing location.
- Entry size and thread: Identify metric, PG, NPT or a plain through-hole.
- Enclosure wall thickness: Confirm enough thread remains for a locknut or full engagement.
- Largest pass-through dimension: Measure any RJ45 plug, terminal, ferrule or molded connector that cannot be removed.
2. How to Choose the Right Cable Gland Size
Step 1: Measure the actual cable outer diameter
Use calipers to measure the cable where the sealing insert will grip it. Do not choose from conductor cross-section, AWG size, core count or a nominal cable-family name. A 5-core power cable and a 5-core control cable can have very different outside diameters.
Measure over the normal jacket—not over a label, heat-shrink repair, corrugated sleeve or distorted section. For oval or irregular cable, measure both major and minor axes and select a gland or insert designed for that cable shape.
Step 2: Match the cable to the published clamping range
The cable diameter must fall within the manufacturer's stated minimum and maximum clamping range. A cable below the minimum may not be retained or sealed. A cable above the maximum may damage the insert, prevent assembly or place excessive stress on the jacket.
When possible, avoid placing the cable exactly at the limit of the range, especially when jacket tolerances, temperature changes or repeated movement are expected. A comfortably centered fit generally gives more installation margin, but the manufacturer's data remains the authority.
Step 3: Match the thread, pitch and enclosure opening
After the cable fit is confirmed, identify the enclosure interface. For a threaded entry, match both designation and pitch. For a plain through-hole, confirm the recommended cut-out diameter and use the correct locknut and sealing washer or O-ring.
Do not force a metric gland into a PG opening or treat a similar-looking NPT size as interchangeable. A partial engagement can feel tight while providing poor retention, damaged threads and an unreliable environmental seal.
Step 4: Verify thread length, material and environment
A thick cast enclosure may require a long-thread gland, while a thin sheet-metal panel may use a standard thread and locknut. Then check the operating environment: UV exposure, oils, chemicals, washdown, salt spray, temperature, impact and electromagnetic compatibility can all change the appropriate material and construction.
- Measure the cable outer diameter.
- Identify round, flat, multiple or pre-terminated cable construction.
- Select a clamping range that includes the measured diameter.
- Match the exact thread system and pitch to the enclosure.
- Check cut-out size, wall thickness and thread length.
- Choose nylon, brass, stainless steel, EMC or application-specific construction.
- Confirm the required IP rating, approvals and installation instructions.
| Material | Typical Strengths | Common Applications | Check Before Selection |
|---|---|---|---|
| Polyamide/nylon | Lightweight, corrosion-resistant and economical | Control boxes, lighting, general machinery and indoor/outdoor enclosures | UV grade, temperature and chemical compatibility |
| Nickel-plated brass | Mechanical strength, durable threads and EMC options | Industrial machines, motors and control cabinets | Corrosion exposure, grounding and shield termination |
| Stainless steel | High corrosion resistance and cleanability | Marine, food processing, chemical and offshore equipment | Alloy grade, sealing material and washdown requirements |
3. Metric vs PG vs NPT Cable Glands
Metric cable gland threads
Metric cable glands commonly use designations such as M12 × 1.5, M16 × 1.5, M20 × 1.5 and M25 × 1.5. The first number identifies the nominal major thread diameter; the second identifies the pitch. Metric entry threads are widely used in modern industrial equipment and are covered in cable-gland applications by IEC-related requirements.
PG cable gland threads
PG, or Panzergewinde, is a legacy thread system still found in existing machinery, junction boxes and products sold in multiple markets. A PG designation should not be treated as a direct metric cable diameter or hole diameter. Use the manufacturer's PG thread table and clamping-range data. European equipment has broadly transitioned toward metric entries, but PG products remain relevant for replacement and retrofit work.
NPT cable gland threads
NPT is a tapered inch-based pipe thread widely encountered in North American equipment. It is dimensionally and functionally different from parallel metric and PG entries. Match the exact NPT designation and follow the specified sealing method; do not substitute a visually similar metric or PG gland.
| Thread System | Typical Marking | General Form | Selection Priority |
|---|---|---|---|
| Metric | M20 × 1.5 | Parallel metric thread | Match major diameter, pitch and clamping range |
| PG | PG9, PG13.5, PG21 | Parallel legacy thread | Use an actual PG dimension table; do not infer millimeters |
| NPT | 1/2" NPT | Tapered inch pipe thread | Match the NPT size and specified thread-sealing method |
Important: A conversion chart can help identify candidates, but it does not make metric, PG and NPT threads interchangeable. Verify the actual enclosure specification before ordering an adapter or replacement gland.
4. Cable Glands for Thin, Multiple and Pre-Terminated Cables
How to seal a thin cable around 3 mm
If a 3 mm cable is below the gland's minimum clamping diameter, tightening harder is not a reliable solution. Select a smaller clamping range, a manufacturer-approved reducing insert or a gland specifically designed for small-diameter cable. For certified or safety-critical equipment, improvised hose sleeves, tape and injected silicone should not be assumed to preserve the original IP rating, strain relief or approvals.
How to route a cable with an RJ45 plug or molded connector
An oversized conventional gland may allow the connector to pass but then leave too much space around the cable jacket. The better solution is usually a split cable gland for a pre-terminated cable, a split cable-entry frame or a purpose-designed bulkhead interface. The split insert closes around the cable after the connector has passed, avoiding field re-termination.
This is also a current direction in cable-entry design: modular split systems accommodate factory-terminated Ethernet, servo, data and power harnesses while supporting easier retrofit and maintenance. Product capabilities vary, so check cable diameter, connector dimensions, strain relief and the rated IP performance of the exact configuration.
How to pass multiple or flat cables through one entry
Do not compress two or more round cables inside a standard single-hole insert. The valleys between cables create leakage paths and uneven clamping. Use a multi-hole sealing insert sized for each cable, a multi-cable gland or a modular cable-entry system. Flat cable similarly requires a profile-matched flat insert rather than a round seal.
| Installation Problem | Recommended Solution | Avoid |
|---|---|---|
| Cable is below the minimum clamping range | Smaller gland or approved reducing insert | Overtightening or relying on tape |
| RJ45 or molded plug is larger than the cable | Split gland, split entry frame or bulkhead connector | An oversized single-hole gland with an open gap |
| Several cables must share one panel entry | Multi-hole insert or modular multi-cable entry | Bundling cables through one round seal |
| Flat or ribbon cable | Profile-matched flat cable insert | Forcing it into a standard round seal |
5. How to Install a Waterproof Cable Gland Correctly
Understand what the IP rating covers
IEC 60529 classifies enclosure protection against solids and water. A gland may be sold with an IP rating, but the finished enclosure depends on the complete installed interface: the correct cable diameter, undamaged seal, compatible enclosure surface, correct cut-out, sealing washer or O-ring, thread engagement and tightening procedure.
IP68 should not be translated into one universal immersion depth or unlimited underwater service. The applicable depth and duration beyond the IPX7 condition are defined for the tested product. Check the manufacturer's stated test conditions and whether the rating applies to the assembled configuration you will use.
Installation workflow for an outdoor enclosure
- Confirm the gland, cable and enclosure are compatible before drilling.
- Make the specified cut-out and remove burrs without oversizing the hole.
- Place the body seal or O-ring flat against a clean, suitable enclosure surface.
- Secure a through-hole installation with the correct locknut; fully engage a threaded entry without cross-threading.
- Pass the cable through and position an undamaged, round jacket in the sealing zone.
- Tighten the gland body and cap to the manufacturer's instructions or specified torque.
- Check retention with a controlled pull, inspect the seal and perform the required enclosure test.
For outdoor boxes, prefer side or bottom cable entry where the equipment design permits, and form a drip loop before the entry. These measures reduce direct water loading but do not replace a correctly rated gland and enclosure.
Common installation mistakes and failure modes
| Mistake | Likely Result | Correction |
|---|---|---|
| Selecting only by M, PG or NPT marking | Cable does not fit or seal | Verify thread and clamping range separately |
| Cable below the minimum diameter | Poor retention and water path | Use a smaller range or approved reducer |
| Several cables in one single-hole seal | Gaps between cable jackets | Use a multi-hole insert |
| Wrong or partially engaged thread | Damaged entry and weak seal | Match the exact standard and pitch |
| Thread too short for the wall | Insufficient locknut or thread engagement | Choose a long-thread version |
| Top entry without water management | Increased exposure to standing or driven water | Use side/bottom entry and a drip loop where possible |
| Using tape or silicone as the primary seal | Difficult maintenance and unverified performance | Use a rated entry solution designed for the cable |
6. Two Real-World Cable Gland Selection Cases
Case 1: An M16 gland is too small, but the M20 insert still looks too small
An installer has a cable that will not enter an M16 gland and orders an M20 replacement. The M20 gland arrives with a reducing insert and a short entry thread. The cable still does not fit, and the gland cannot be secured through the thick enclosure wall.
The error is treating M20 as the cable capacity. The correct process is to measure the cable outer diameter, select a product whose published clamping range includes that measurement and then choose the correct thread length. If the enclosure already has an M20 hole, an M20 gland with the appropriate full-range insert may work; if the wall is thick, a long-thread version may also be required.
Case 2: A pre-terminated PoE camera cable must enter an outdoor box
The RJ45 plug is much wider than the Ethernet cable. A conventional gland large enough for the plug may be unable to seal the jacket afterward. Removing and re-terminating the plug adds tooling, workmanship and link-reliability concerns.
A split cable gland or split entry frame lets the terminated cable pass while a correctly sized split insert closes around the jacket. Mount the junction box so the entry is protected, route exposed cable downward into a drip loop and keep the connection inside the enclosure. Electrical tape alone should not be the primary environmental barrier.
7. Cable Gland Size FAQ
Size and thread questions
Does M20 mean the cable gland fits a 20 mm cable?
No. M20 identifies the nominal metric entry-thread diameter. The cable fit is defined by the product's clamping range and clear opening.
What cable gland should I use for a 3 mm cable?
Choose a gland or manufacturer-approved reducing insert whose published clamping range includes 3 mm. Do not assume PG7 is suitable without checking its actual minimum and maximum cable diameter.
What does PG9 mean?
PG9 is a standardized legacy thread designation, not a reliable statement that every PG9 gland seals a 9 mm cable. Check the PG thread dimensions, required enclosure opening and the specific gland's clamping range.
Can I replace an M16 cable gland with M20?
Only if the enclosure can accept the M20 entry or an appropriate adapter and the selected M20 gland has the correct clamping range. The larger thread alone does not prove cable compatibility.
Special cable-entry questions
Can two cables go through one cable gland?
Use a cable gland with a purpose-designed two-hole insert or a modular multi-cable entry. Do not place two cables in a standard single round sealing hole.
How do I install a cable with an RJ45 plug through an enclosure?
Use a split cable gland, split cable-entry frame or suitable bulkhead interface designed for pre-terminated cables. This avoids choosing an oversized gland that cannot close around the smaller cable jacket.
Waterproofing and function questions
Is every IP68 cable gland suitable for permanent underwater use?
No. Check the manufacturer's test depth, duration, cable range and installation requirements. The completed enclosure must also be designed and verified for the intended environment.
What is the difference between a cable gland and a waterproof connector?
A cable gland seals and retains a continuous cable as it enters an enclosure. A waterproof connector creates a detachable electrical interface. For a static cable entry, a gland may be simpler; for equipment that must be disconnected, a rated connector may be more appropriate. See M12 Not Enough? Choose Waterproof Circular Connectors for a broader comparison.
Conclusion
The correct cable gland size comes from a sequence of checks, not one number. Start with the measured cable outer diameter and clamping range. Then verify thread standard, pitch, cut-out, wall thickness, thread length, material, IP requirements and any connector that must pass through the opening.
For standard round cables, this process prevents most fit and sealing failures. For thin, flat, multiple or pre-terminated cables, select the appropriate reducing, multi-hole or split entry system instead of forcing a standard gland to perform outside its rated configuration.
Browse Elecbee's cable glands, cable supports and fastening solutions to compare metric, PG, nylon, metal, multi-hole and waterproof options.
