Understanding the Clamping Range of Metric Brass Glands

Introduction

Have you ever ordered a metric M20 cable gland, only to find it won’t seal properly around your 10mm cable? Or worse—discovered moisture inside your electrical enclosure weeks after installation because the gland was slightly oversized for the cable diameter?

The clamping range of a metric brass gland defines the minimum and maximum cable outer diameters that can be reliably sealed within a specific gland size—and selecting the wrong range is the number one cause of IP rating¹ failures in industrial installations.

I’m Samuel, Sales Director at Bepto Connector, and after a decade in the cable gland industry, I’ve seen countless projects delayed because engineers didn’t understand this critical specification. The good news? Once you grasp how clamping ranges work and how to match them to your cables, you’ll never face seal failures or compatibility issues again. Let me break it down in practical terms.

Table of Contents

• What Exactly Is the Clamping Range in Metric Brass Glands?
• How Does Clamping Range Affect Sealing Performance and IP Ratings?
• How to Match Cable Diameter to the Correct Gland Size?
• What Problems Occur When Clamping Range Is Ignored?

What Exactly Is the Clamping Range in Metric Brass Glands?

The clamping range is the span of cable outer diameters that a specific metric gland size can accommodate while maintaining its rated IP protection level and mechanical grip strength.

Every metric brass cable gland consists of several key components that work together to create the seal: the gland body with metric threads² (M12, M16, M20, M25, etc.), a compression seal or O-ring, a compression nut, and often a locknut. When you tighten the compression nut, it squeezes the seal around the cable’s outer jacket, creating both environmental protection and strain relief.

Critical Technical Parameters:

• Metric thread size: Refers to the external thread diameter (M12 = 12mm thread OD, M20 = 20mm thread OD, etc.)
• Clamping range: Expressed as minimum-maximum cable OD (e.g., 3-6.5mm for M12, 10-14mm for M20)
• Seal compression ratio: Typically 15-25% compression of the seal material for optimal performance
• Thread standards: ISO metric threads following DIN EN 60423 / IEC 60423 specifications
• Material composition: CW617N brass³ (58% copper, 39% zinc, 3% lead) for machinability and corrosion resistance
• Nickel plating thickness: 5-10 microns for standard applications, 15+ microns for enhanced corrosion protection

The clamping range exists because the compression seal is flexible—it can deform to grip cables of varying diameters. However, this flexibility has limits. If the cable is too thin, the seal can’t compress enough to create intimate contact. If the cable is too thick, you can’t tighten the nut sufficiently, or you risk damaging the cable jacket.

Why metric sizing matters: The metric system provides standardized thread dimensions recognized globally, making it easier to match glands with enclosure knockouts. However, the thread size doesn’t directly indicate the cable diameter—an M20 gland doesn’t necessarily fit a 20mm cable. This is where understanding the specific clamping range becomes essential.

I recall David, a procurement manager from a UK manufacturing plant, who bulk-ordered M16 glands assuming they’d fit his 8mm control cables. The actual clamping range was 4-8mm, putting his cables at the absolute maximum limit. While technically compatible, the minimal compression resulted in IP65 instead of the rated IP68 performance. After we provided M16 glands with an optimized 6-10mm range, his installation passed all pressure testing.

How Does Clamping Range Affect Sealing Performance and IP Ratings?

The relationship between clamping range, seal compression, and IP rating performance is governed by precise mechanical engineering principles that directly impact your installation’s reliability.

The Seal Compression Sweet Spot

When a cable sits in the middle of the clamping range, the compression seal achieves optimal deformation—typically 18-22% compression of its original thickness. This creates:

Uniform contact pressure: The seal contacts the entire cable circumference evenly, eliminating potential leak paths

Strain relief effectiveness: Proper compression creates friction that prevents cable pull-out under mechanical stress (typically 80-120N pull-out force)

Long-term resilience: The seal operates within its elastic range, maintaining spring-back properties over thousands of thermal cycles

Clamping Range vs. IP Rating Performance

Cable Position in Range	Seal Compression	Achievable IP Rating	Pull-Out Force	Long-Term Reliability
Below minimum (-10%)	<12%	IP54 or failure	<40N	Poor—seal may slip
At minimum threshold	12-15%	IP65	50-70N	Marginal—sensitive to vibration
Optimal middle range	18-22%	IP68	80-120N	Excellent—rated lifespan
At maximum threshold	23-26%	IP67	90-130N	Good—but difficult installation
Above maximum (+10%)	>28%	IP65 or cable damage	140N+	Poor—seal over-compressed, cable crushed

Hassan, a quality manager from a Saudi petrochemical facility, learned this lesson the hard way. His team installed M25 glands (clamping range 13-18mm) on 12.5mm cables—just below the minimum. Initial pressure testing passed, but after six months of thermal cycling between 25°C nights and 50°C days, the seals had relaxed enough to allow moisture ingress. We replaced them with M20 glands (10-14mm range), positioning his 12.5mm cables in the optimal zone. Two years later, those glands still maintain IP68 in one of the harshest environments imaginable.

Material Science Behind the Seal

The compression seal—typically made from NBR (nitrile rubber), EPDM, or neoprene—has specific mechanical properties:

• Shore A hardness: 60-70 for standard seals (softer seals accommodate wider ranges but wear faster)
• Compression set resistance: Quality seals retain >85% of original thickness after 1,000 hours at 100°C
• Chemical compatibility: NBR resists oils but degrades with ozone; EPDM excels with water/steam but fails with petroleum products

When the cable diameter falls within the proper clamping range, the seal compresses into its designed working zone. Too little compression leaves microscopic gaps; too much compression causes permanent deformation (compression set), where the seal loses its ability to spring back and maintain pressure.

Why Brass Enhances Clamping Performance

Nickel plating⁴ brass offers specific advantages over nylon or stainless steel for clamping applications:

1. Thermal stability: Brass maintains dimensional stability from -40°C to +100°C, ensuring consistent clamping force
2. Thread precision: CNC-machined brass threads provide smooth, controlled compression without binding
3. EMC shielding: Creates 360° electromagnetic continuity when properly bonded to metal enclosures
4. Corrosion resistance: Nickel plating provides protection equivalent to 500+ hours salt spray testing (ASTM B117)

How to Match Cable Diameter to the Correct Gland Size?

Selecting the right metric brass gland requires a systematic approach that accounts for cable specifications, environmental conditions, and installation requirements.

Step 1: Measure Cable Outer Diameter Accurately

This sounds obvious, but it’s where most mistakes originate.

Proper measurement technique:

1. Use a digital caliper, not a tape measure (accuracy to ±0.1mm required)
2. Measure at three points along a 1-meter cable section
3. Take the maximum reading—cables aren’t perfectly round
4. Add 0.3-0.5mm tolerance for manufacturing variations
5. For armored cables, measure over the outer sheath, not the armor layer

Common measurement errors:

• Measuring from cable datasheet nominal diameter (actual cables often run 5-8% larger)
• Compressing the cable while measuring (soft jackets deform easily)
• Ignoring temperature effects (PVC expands ~3% from 20°C to 60°C)

Step 2: Consult the Metric Gland Sizing Chart

Here’s a comprehensive reference for standard metric brass glands:

Metric Thread Size	Thread OD (mm)	Clamping Range (mm)	Typical Cable Types	Panel Hole Size (mm)
M12 × 1.5	12	3-6.5	Sensor cables, thin control	12.5
M16 × 1.5	16	4-8 / 6-10*	Instrumentation, signals	16.5
M20 × 1.5	20	6-12 / 10-14*	Power cables, standard control	20.5
M25 × 1.5	25	13-18	Medium power, multi-core	25.5
M32 × 1.5	32	15-21 / 18-25*	Heavy power cables	32.5
M40 × 1.5	40	22-32	Large industrial power	40.5
M50 × 1.5	50	28-38	Very large power distribution	50.5
M63 × 1.5	63	32-44	Extreme power applications	63.5

*Multiple clamping ranges available depending on seal insert selection

Step 3: Position Your Cable in the Optimal Zone

The golden rule: Your cable OD should fall between 40-70% of the clamping range span.

Example calculation:

• M20 gland with 10-14mm range (4mm span)
• Optimal zone: 10mm + (4mm × 0.4) to 10mm + (4mm × 0.7) = 11.6-12.8mm
• Your 12mm cable? Perfect fit.
• Your 10.5mm cable? Marginal—consider M16 with 6-10mm range instead.

Step 4: Consider Special Application Requirements

High-vibration environments (conveyors, mobile machinery):

• Select glands where cable sits in the upper 50-70% of the range for maximum grip
• Consider glands with extended thread engagement (long-body variants)

Frequent cable replacement:

• Choose the larger clamping range option to accommodate future cable variations
• Specify glands with captive seals that don’t fall out during disassembly

EMC-sensitive applications:

• Ensure cable sits mid-range for optimal 360° shield termination
• Use glands with integrated earthing features for braided shield cables

Step 5: Account for Environmental Factors

Temperature extremes: Cables expand/contract with temperature. If your application sees wide temperature swings, position the cable at the measured diameter at maximum operating temperature.

Chemical exposure: Some chemicals cause cable jacket swelling. If cables will contact oils, solvents, or cleaning agents, measure the cable after exposure or add 5-10% to your diameter measurement.

UV exposure: Outdoor cables may become brittle over time, requiring easier installation. Choose mid-range sizing to avoid excessive installation torque that could crack aged jackets.

What Problems Occur When Clamping Range Is Ignored?

Ignoring clamping range specifications creates predictable failure modes that compromise safety, reliability, and compliance. Here are the three most common—and most expensive—mistakes.

Problem #1: Undersized Cables in Oversized Glands

What happens:
The compression seal can’t deform enough to contact the cable surface uniformly. Microscopic gaps remain, creating leak paths for moisture, dust, and gases.

Real-world consequences:

• IP rating drops from IP68 to IP54 or lower
• Moisture ingress causes corrosion on terminal connections
• In hazardous areas, loss of Ex rating creates safety violations
• Cables can pull out under mechanical stress

The fix:
Use reducing inserts or step-down adaptors that include a smaller seal matched to your cable diameter. At Bepto, we offer reducer sets that allow M25 glands to seal cables down to 8mm while maintaining IP68.

Problem #2: Oversized Cables Forced into Undersized Glands

What happens:
Installers over-torque the compression nut trying to achieve seal, crushing the cable jacket and potentially damaging internal conductors.

Warning signs:

• Visible deformation or color change in cable jacket
• Difficulty rotating compression nut (requires excessive force)
• Cable insulation extruding from gland ends
• Reduced flexibility at cable entry point

Real-world consequences:

• Conductor damage leading to increased resistance and heating
• Insulation breakdown causing short circuits
• Premature cable failure (often months after installation)
• Voided cable warranties due to mechanical damage

The fix:
Never force a cable into an undersized gland. Always step up to the next metric size. If panel holes are already drilled, use reducing washers on the larger gland rather than damaging the cable.

Problem #3: Ignoring Seal Insert Options

What happens:
Many metric sizes offer multiple clamping ranges using different seal inserts. Installers often use whatever insert came pre-installed without checking if it’s optimal for their cable.

Example scenario:
An M20 gland might ship with a 10-14mm seal insert, but your 7mm cable requires the 6-12mm insert option. Using the wrong insert positions your cable outside the optimal compression zone.

The solution:
Always specify the exact clamping range when ordering, not just the metric thread size. Our Bepto product codes include the range designation (e.g., M20-10/14 vs. M20-6/12) to eliminate confusion.

Installation Best Practices Summary:

1. Measure cable OD with calipers at operating temperature
2. Select metric size where cable falls in middle 40-70% of clamping range
3. Verify seal material compatibility with environment
4. Hand-tighten compression nut, then add 1/4 to 1/2 turn with wrench
5. Check for cable deformation—if visible, you’ve over-tightened
6. Perform IP rating verification testing before commissioning
7. Document gland sizes and cable diameters for maintenance records

Conclusion

Understanding clamping range isn’t just technical knowledge—it’s the foundation of reliable cable sealing that prevents costly failures and ensures long-term system integrity. By measuring accurately, consulting proper sizing charts, and positioning your cables in the optimal compression zone, you guarantee IP68 performance and eliminate the most common installation mistakes.

At Bepto Connector, we manufacture metric brass cable glands with precision-machined threads and multiple clamping range options for every application. Our technical team provides free sizing consultations and can supply sample glands for testing before bulk orders. Contact us today for detailed sizing charts, material certificates, and competitive factory-direct pricing on metric brass glands from M12 to M63.

FAQs About Metric Brass Gland Clamping Range

1. Learn more about the international standards for Ingress Protection (IP) ratings in electrical equipment. ↩

2. Explore the ISO metric thread standards used for electrical and industrial cable management. ↩

3. Discover the chemical composition and mechanical properties of CW617N brass used in industrial hardware. ↩

4. Understand how nickel plating protects brass components from oxidation and environmental corrosion. ↩

5. Find out how the Shore A hardness scale measures the durometer of flexible elastomer seals. ↩