Cable seal compression starts with geometry, but contact pressure also depends on load, material response and the shape of the assembled insert. A cable-to-opening comparison describes the initial fit. A specified reduction describes how an opening changes. Mean contact pressure describes normal load spread over a defined contact area. Use the quantity that answers your design question.
I begin by labelling each input as free, assembled, loaded or recovered. That simple distinction keeps a dimension measured on a loose sleeve separate from a dimension under a tightened nut. The examples below use assumed dimensions and loads to show the calculations; apply the selected gland’s cable range and assembly instructions when choosing or installing it.
Key takeaways
- Keep the measurement state and tolerance with every cable and seal dimension.
- State the denominator when reporting an overlap or reduction percentage.
- Evaluate the smallest and largest permitted dimensions as well as the nominal fit.
- Calculate mean contact pressure from the normal load and the area carrying that load.
- Check the assembled seal’s local contact and required functions after using these geometric comparisons.
Identify the dimensions behind cable seal compression
The free opening is the insert’s bore before the stated assembly deformation. Its diameter is useful for a fit comparison, while a loaded dimension describes a later state. Write the state beside the value so measurements taken at different stages remain distinguishable.
| Quantity | State to record | What it helps establish |
|---|---|---|
| Cable outside diameter | Jacket condition, measurement location and tolerance | The surface that meets the insert |
| Free opening | Loose insert before assembly deformation | Initial cable-to-opening overlap or gap |
| Assembled opening | Defined closure, with or without a cable as specified | The resulting geometric change |
| Contact area | Chosen interface under a defined load | The surface carrying the normal force |
| Recovered dimension | Unloaded state after stated recovery conditions | Deformation remaining after exposure |
For an oval cable, retain both the small and large dimensions. An average can hide a loose sector on one side and strong interference on another. A slit insert or shaped bore also needs dimensions that describe its actual contact surfaces. I match the measurement to the feature the seal will touch, then use the sealing-ring design guide to follow how the insert closes.
Calculate diameter overlap with a named reference
Diametral overlap is the cable diameter minus the free opening diameter. Let be the cable outside diameter and the free opening diameter. For round, concentric outlines, the nominal radial overlap is half that difference:
Expressing overlap relative to the cable diameter gives:
Worked example: a 12.00 mm cable and 11.40 mm opening
Assume both dimensions describe round features in the stated free condition. Their diametral overlap is 12.00 − 11.40 = 0.60 mm, and nominal radial overlap is 0.30 mm. Dividing 0.60 mm by the 12.00 mm cable diameter gives 5.00% cable-referenced overlap.
If the denominator is instead the 11.40 mm opening, the result is 5.26%. The dimensions are unchanged; the reference is different. I put the denominator beside the percentage so drawings and worksheets can be compared on the same basis.
These outlines superimpose two free-state measurements. During assembly, the insert and possibly the cable deform according to their shape, material response and available space. Use the overlap to describe the starting geometry, then assess that deformation in the complete design.
A negative result describes a free-state gap. Check how the gland closes the insert and whether the cable falls within its declared range. A positive result describes interference before assembly; evaluate insertion, jacket loading and closure with the actual seal design. The cable gland sizing guide explains how to compare the full cable diameter interval with that range.
Carry tolerances through the overlap calculation
Nominal dimensions describe one combination. The smallest cable with the largest opening produces the least overlap; the largest cable with the smallest opening produces the most. For the same circular definition:
Worked example: the same nominal fit with tolerances
Assume the cable is 12.00 ± 0.10 mm and the free opening is 11.40 ± 0.15 mm. These are illustrative tolerances, not recommended manufacturing limits.
| Combination | Diametral overlap | Radial overlap | Cable-referenced percentage |
|---|---|---|---|
| Small cable, large opening: 11.90 and 11.55 mm | 0.35 mm | 0.175 mm | 2.94% |
| Nominal: 12.00 and 11.40 mm | 0.60 mm | 0.300 mm | 5.00% |
| Large cable, small opening: 12.10 and 11.25 mm | 0.85 mm | 0.425 mm | 7.02% |
The nominal 5% therefore represents a possible range of about 2.94–7.02% in this example. I check both ends against the design’s permitted fit and deformation. This is a worst-case dimensional range; it does not assign a probability to either extreme. Temperature, swelling or a change in jacket condition require dimensions for that changed state.
Compare circular areas using the squared diameter ratio
An area ratio compares cross-sectional sizes. For circular cable and opening outlines, area is proportional to diameter squared:
Using the nominal 12.00 mm and 11.40 mm values gives 1.108, or about 110.8%. The cable’s circular cross-section is about 10.8% larger than the free opening’s area. This is an opening-area comparison; material strain uses a change in the material’s own dimensions.
For the seal material, also consider the outside boundary, length and space into which it can move. Two inserts with the same bore can have different wall thicknesses and closing mechanisms. Those differences affect deformation and the load carried at the cable surface. Keep the hole area, material area and loaded contact area as separate quantities.
Calculate a specified opening reduction
An opening reduction compares the same opening before and after a defined change. If a design instruction specifies a fractional reduction from its initial diameter, then:
Worked example: a specified 5% diameter reduction
Use for a 5% reduction. Starting from a circular 10.00 mm opening gives 9.50 mm. Its area changes from 78.54 mm² to 70.88 mm², a 9.75% area reduction. The diameter and area percentages differ because area follows the square of diameter.
The 5% input here is a stated design assumption. Obtain the closure position or resulting opening from the relevant drawing, measurement or validated deformation model. Nut torque acts through thread friction and the gland’s closing mechanism, so use the prescribed torque to assemble the product and the appropriate dimensional information for this calculation.
When a cable occupies the bore, include its size and deformation in the geometry. A calculated empty-bore reduction describes an empty-bore condition; the loaded interface with a cable may take a different shape.
Calculate mean pressure from normal load and contact area
Mean contact pressure is the normal load distributed over its corresponding contact area. If that load and area are independently established:
A hypothetical 600 N normal load over 300 mm² gives 2 N/mm², equal to 2 MPa. The NIST SI unit table defines pressure in pascals as N/m². One square millimetre is one millionth of a square metre, which makes 1 N/mm² equal to 1 MPa.
I check the direction and surface definition of the load before using it. Around a cylindrical cable, is the sum of local normal-force magnitudes over the selected area. Opposing radial forces may cancel as vectors even though both sides carry pressure. Keep an axial nut force separate from this circumferential contact load unless a suitable load-transfer model relates them.
For an explicitly assumed full cylindrical band, the apparent area is circumference multiplied by axial contact length: . Use diameter and length from the defined loaded state. Confirm that the force refers to that same band before dividing. A projected disk area, an annular face and a cylindrical band describe different surfaces.
The average can conceal high-pressure patches and low-contact sectors. Trace the contact around the cable and along the sealing band when evaluating a leak path. The microscopic sealing guide connects surface contact with these connected leakage routes.
Choose the percentage that matches the physical change
The word “compression” can refer to several measurements. I compare the definitions first, then the values. That prevents a useful geometric result from being mistaken for a material test result.
| Quantity | Reference and meaning |
|---|---|
| Cable-referenced overlap | Cable OD minus free opening, divided by cable OD |
| Opening reduction | Change in one opening relative to its original dimension |
| O-ring squeeze | Cross-section height reduction relative to the original cross-section |
| Gasket compression | Thickness reduction under a defined closure or load |
| Compression set | Residual deformation after specified compression and recovery |
| Mean contact pressure | Normal load divided by the surface carrying that load |
Compression set describes residual deformation after a defined test and recovery, whereas squeeze describes imposed deformation in the installed geometry. ASTM D395-18(2025) covers rubber compression-set methods under specified force or deflection conditions. Keep the method and recovery conditions with a reported set value. For changes while an insert remains loaded, consider the sustained-loading behaviour described in the seal cold-flow guide.
The practical sequence is to calculate the intended geometric quantity, evaluate the resulting loaded contact, then test the required functions of the assembled entry. IEC 62444:2010 addresses complete-gland construction and performance. Relate the calculations to the selected gland, seal and cable, and use its installation and performance requirements for acceptance.
Conclusion
Cable seal compression calculations become useful when the state and reference of each input are clear. Overlap compares the cable and free opening; tolerance limits show the range around the nominal fit. Opening reduction and area ratios describe different geometric changes, so keep their denominators visible.
I use normal load and its matching contact area when pressure is the question. Then I examine the contact around the cable and the functions the assembled entry must retain. This connects the worksheet to the physical seal while keeping overlap, squeeze, compression set and pressure distinct.
FAQs about cable seal calculations
Is a 5% diameter overlap the same as 5% seal compression?
A cable-referenced overlap divides the cable-to-opening difference by cable OD. O-ring squeeze and gasket compression instead describe changes in a material cross-section or thickness. Identify the reference dimension and loaded state before comparing the percentages.
Does a negative overlap mean the gland cannot seal?
A negative value describes a free opening larger than the cable. For a gland that closes the insert during assembly, evaluate its closing travel and declared cable range. Assess the resulting loaded geometry and sealing function after assembly.
Can tightening torque be converted directly to contact pressure?
The conversion needs a model of thread friction, geometry and the closing mechanism. Mean contact pressure then uses the resulting normal load and its matching contact area. I use independently justified load data or a validated model for the selected assembly, and follow its installation instruction for tightening.
Why is a 5% opening-diameter reduction a larger area reduction?
Circular area depends on diameter squared. Reducing diameter to 95% of its original value leaves 90.25% of the original area, a 9.75% reduction. This result describes the opening geometry; the seal material’s deformation also depends on its outside boundary, length and available space.
Sources and technical references
- NIST — Guide to the SI, Chapter 4: The Two Classes of SI Units and the SI Prefixes, accessed 2026-10-28.
- ASTM D395-18(2025) — Standard Test Methods for Rubber Property—Compression Set, accessed 2026-10-28.
- IEC 62444:2010 — Cable glands for electrical installations, accessed 2026-10-28.