Cable Gland Sizes Guide

Summary

Cable glands are one of those components every electrician fits weekly but rarely thinks hard about — until the wrong size arrives on site and a job stalls waiting for a merchant run. The confusion usually comes from mixing up three different numbers that all look similar: the cable's conductor size (mm²), the cable's overall diameter (mm OD), and the gland's thread size (M-number). These are three separate things, and getting a gland "close enough" on CSA alone is how you end up with an entry that won't seal, won't grip the armour, or physically won't fit through the knockout.

This guide is a comprehensive quick-reference for gland sizing: standard ISO metric thread sizes with their clamping (OD) ranges, SWA gland cone sizing linked to common cable sizes, gland types and when to use each, and the specific considerations for glanding into different enclosure types. It complements swa cable glanding and termination, which covers the full termination procedure, earthing method, and adiabatic CPC calculation in detail — this article is the sizing lookup, that one is the how-to.

Key Facts

• Gland thread size ≠ cable CSA — a gland is selected by the cable's outer diameter (OD) against the gland's stated clamping range, not by the conductor's cross-sectional area
• Standard ISO metric entry threads — M12, M16, M20, M25, M32, M40, M50, M63, M75 (largest gland bodies may go beyond M75 for very large cable)
• PG threads (Pg7 to Pg48) — an older German thread standard still found on legacy equipment and some imported enclosures; not directly interchangeable with ISO metric without an adaptor
• NPT threads — American National Pipe Thread, occasionally found on imported plant/machinery; requires an NPT-specific gland or a thread adaptor, never force a metric gland into an NPT entry
• Gland types by construction — Type A/E1W (indoor, unsealed or basic seal), Type BW (indoor, basic), Type CW (outdoor/damp, sealed with inner and outer washers), Type CX (heavy duty, wire braid armour), E1FW/E1FX (hazardous area, ATEX/UKCA rated)
• SWA cone sizing is separate from thread size — the cone (sometimes suffixed S for "small" armour diameter range within the same thread size) grips the steel wire armour specifically; two cables needing the same thread size (e.g. M20) can need different cone sizes depending on armour diameter
• Gland standard — BS EN 62444:2013, Cable glands for electrical installations for general LV/electrical use (superseded the older BS 6121)
• Hazardous area glands — must be ATEX/UKCA certified to BS EN 60079-14 for the specific zone (Zone 1, Zone 2, Zone 21, Zone 22); never substitute a standard gland
• IP rating achieved by the whole assembly — gland body, sealing washer, shroud, and enclosure entry all contribute; a gland rated IP66 only achieves that in practice with the correct washer and shroud fitted correctly
• Locknut — fitted inside the enclosure, same thread size as the gland body, secures the gland against the enclosure wall
• Reducers/adaptors — available where the enclosure knockout is a larger size than the gland needed (e.g. M25 knockout, M20 gland required) — always use a proper reducer, never wrap tape around a gland thread to force a fit
• Multiple cables through one entry — multi-way glands (or a gland plate with multiple entries) exist for low-voltage data/control cable bundles; never pack multiple power cables through a single-cable gland
• Flexible conduit glands — a different product family (often with a locking collar and a captive nut) sized to the conduit's nominal bore (e.g. 20mm, 25mm, 32mm flexible conduit), not to a cable OD directly
• Weatherproof/UV-stable shrouds — recommended on all outdoor CW-type glands; unprotected rubber seals perish and let water track down the thread within a few years

Quick Reference Table — ISO Metric Gland Thread Sizes & Clamping Ranges

Clamping ranges are typical/indicative — always check the specific manufacturer's data sheet, as ranges vary between brands (CMP, Hawke, Prysmian, Lapp) even for the same nominal thread size.

Quick Reference Table — SWA Gland Sizing by Cable Size (Typical)

Always confirm against the specific cable manufacturer's OD data — SWA construction (and therefore OD) varies between manufacturers for the same nominal conductor size, particularly around the armour wire diameter and bedding thickness.

Quick Reference Table — Gland Type Selection by Environment

Detailed Guidance

Reading a gland's size marking

Gland packaging and the gland body itself typically carry two figures: the thread size (e.g. "M20") and, for SWA-specific glands, a cone/gland number (e.g. "20" or "20S"). The "S" suffix denotes a smaller armour-clamping range within the same nominal thread size family — it exists because two cables can share an M20 outer thread but have different armour diameters underneath the outer sheath. Always check both figures against the cable data sheet, not just the thread size, when the cable is SWA.

Sizing from cable OD, not from CSA

The correct sizing method:

1. Get the actual cable OD from the manufacturer's data sheet for the specific cable being installed — do not estimate from a generic CSA table, because SWA and multicore cable OD varies meaningfully between manufacturers for the same conductor size (a 6mm² 4-core SWA from one manufacturer may be a different OD to a 6mm² 4-core SWA from another)
2. Match the OD against the gland's stated clamping range — most manufacturers publish a minimum and maximum OD for each gland size; the cable OD should sit comfortably within this range, ideally not at the very top or bottom
3. For SWA, additionally check the armour diameter against the cone size — this is a separate check from the outer OD clamping range
4. Check the enclosure knockout size matches (or can be reduced/adapted to) the gland's thread size

A gland at the very top of its clamping range will not seal reliably; a gland at the very bottom will not grip the cable sheath securely enough to prevent pull-out. Where a cable OD falls between two standard gland sizes, size up rather than down, and check whether a reducing washer/adaptor set is needed to seat correctly in the target knockout.

Common sizing mistakes on site

• Assuming CSA determines gland size directly — two cables of the same conductor CSA but different constructions (e.g. standard PVC/SWA/PVC vs LSF/SWA/LSF) can have meaningfully different OD, and therefore need different gland sizes
• Reusing a gland from a different cable run without checking OD — a spare gland from a previous job is not automatically the right size for the next
• Ignoring the cone/thread distinction on SWA glands — ordering by thread size alone without checking the cone size against armour diameter
• Forcing an oversized gland with excess tape/packing — this defeats the seal and the cable grip; if the gland doesn't fit correctly, get the correct size, don't improvise
• Mixing PG-threaded gland stock with metric knockouts — PG7 and M16 look similar but are not the same thread pitch; forcing one into the other damages both the gland and the enclosure entry
• Under-ordering for a multi-cable job — always order 2-3 spares of each size for a job with several cable runs; a missing gland stalls a whole termination stage

Gland selection for specific scenarios

Domestic garden office/outbuilding sub-main (typically 2.5mm² or 4mm² SWA): M20 CW gland with outer shroud is the standard specification for the outdoor entry point; an indoor BW or A/E1W gland is acceptable at the consumer unit end if that entry is genuinely dry and indoors.

EV charger installation: typically 6mm² or 10mm² SWA depending on run length and charger rating; M20-M25 CW gland at the charger end (usually outdoor, wall-mounted), matching gland at the consumer unit/distribution board end.

Commercial 3-phase sub-main: size per the specific cable — 16mm², 25mm², or larger SWA is common; M32-M40 range; always confirm against the manufacturer's cable data sheet for that specific installation rather than a generic "3-phase = M32" assumption.

Flexible conduit termination (not direct cable glanding): a different product — flexible conduit glands are sized to the conduit's nominal bore (16mm, 20mm, 25mm, 32mm) with a locking collar, not to a cable OD; don't confuse this with cable gland sizing.

Reducers and adaptors

Where the available enclosure knockout doesn't match the gland size needed:

• Reducing washers — a stepped washer that lets a smaller gland seat correctly in a larger knockout, maintaining seal integrity
• Thread adaptors — convert between metric and PG, or between two metric sizes, where a direct-size gland isn't available or the enclosure has a fixed legacy entry
• Never substitute improvised packing (tape, sealant alone, cut washers) for a proper manufactured reducer — this compromises both the mechanical grip and the achieved IP rating

Frequently Asked Questions

How do I find the exact OD of a specific cable?

Check the manufacturer's technical data sheet for that specific cable reference (not just "6mm² SWA" generically) — reputable UK manufacturers (Prysmian, CMP, Doncaster Cables, AEI) publish OD figures per product. If the data sheet isn't to hand, measure the actual cable with callipers before ordering glands, particularly for larger or unusual cable runs.

What's the difference between a gland's "size" and its "thread size"?

For most standard glands they're the same figure (an "M20 gland" has an M20 thread). For SWA glands specifically, the gland/cone number sometimes differs slightly in naming convention between manufacturers, and the "S" suffix variant shares a thread size with the standard variant but has a different armour-clamping range. Always cross-check against the specific manufacturer's sizing chart rather than assuming all "20" or "M20" designations behave identically across brands.

Can I use a smaller gland with packing to fit a slightly undersized cable?

No — this is a common shortcut that compromises both the seal and the mechanical retention of the cable. If the cable OD is below the gland's minimum clamping range, use the next size down, or use a proper reducing insert if the manufacturer supplies one for that gap. Improvised packing (tape, foam) does not achieve a reliable IP rating and can work loose over time.

Do brass and nylon glands come in the same size ranges?

Broadly yes for thread sizes, but clamping ranges and mechanical performance differ — brass glands are the default for SWA armour grip because the cone needs to mechanically bite the steel wire armour, which nylon/plastic glands generally aren't rated to do reliably. Nylon/plastic glands are more commonly used for unarmoured cable (flex, data cable) where there's no armour to grip and the requirement is sealing and strain relief only.

What size gland do I need for a standard UK domestic consumer unit tails entry?

This depends on the incoming supply cable, typically 16mm² or 25mm² single-core (meter tails) rather than SWA — these usually enter via a simple grommet or a compression gland sized to the tails' OD (commonly in the M20-M25 range), not an armoured SWA-type gland, since meter tails are unarmoured.

Regulations & Standards

• BS EN 62444:2013 — Cable glands for electrical installations (the primary UK/EU gland product standard, superseded BS 6121)

• BS 7671:2018+A2:2022 — IET Wiring Regulations; governs cable selection, installation methods, and earthing requirements that the gland must support

• BS EN 60079-14 — Explosive atmospheres — Electrical installations design, selection and erection; governs hazardous-area gland selection and certification

• BS EN 60529:1992+A2:2013 — Degrees of protection provided by enclosures (IP code); the basis for gland/seal IP rating claims

• BS EN 50525 — Electric cables — Low voltage energy cables (the cable manufacturing standard family that determines actual cable OD)

• IET On-Site Guide — practical reference tables for cable and containment selection alongside gland sizing

• CMP Products — technical gland selection guide — major UK gland manufacturer, published OD/gland size tables

• Hawke Cable Glands — technical documentation — manufacturer sizing charts and hazardous-area gland certification data

• Prysmian Cleveland Cable Company — SWA cable data sheets — cable OD reference for accurate gland sizing

• BSI Group — BS EN 62444 — the cable gland product standard

• swa cable glanding and termination — full SWA termination procedure, earthing method, adiabatic CPC calculation

• cable sizing — cable sizing by circuit type with BS 7671 table references

• fuse ratings — related quick-reference format for MCB/fuse selection

• outbuildings regs — SWA burial depth and outbuilding electrical supply requirements