Sizing
Sizing a heat-shrink sleeve over a wire joint
Two diameters decide it: what the tube has to clear going on, and what it has to grip once it is down.
Two diameters, and the box only shows you one
Every heat-shrink tube is sold under two numbers. Supplied internal diameter is the bore off the spool. Recovered internal diameter is what it closes to with nothing inside it.
The rule is short. Supplied ID must clear the widest point of the made-up joint. Recovered ID must finish below the smallest diameter the tube has to grip.
Nearly every sizing mistake is half that rule used alone. Pick on supplied ID and the tube threads on beautifully, then sits loose on the core. Pick on recovered ID and you cannot get it past the crimp.
Measure the joint, not the wire. The widest point is the crimp barrel, wider again if you have taped it. Take the smallest point too, over the insulated core beyond the cut-back.
Thread the tube on before you make off. Once the crimp is on, the only way back is to cut. Not settled on a cover yet? The four ways to cover a finished joint is the earlier decision.
What 3:1 buys you over 2:1
Shrink ratio is supplied ID divided by recovered ID: the spread of diameters one size can serve.
The manufacturer's thin wall range shows it at a single size. GSC 12.7/6.35 is 2:1 and recovers from 12.7 mm to 6.35 mm. GSC 3X 12.0/4.0 is 3:1, near enough the same bore going on, closing to 4.0 mm.
Both clear a 9.5 mm crimp. Only the 3:1 also grips a 5 mm core either side. That is what the ratio is for.
A 2:1 tube is still right where the joint is barely fatter than the wire, such as a soldered and dressed twist. Once a ferrule is on it, the ratio does the work.
One measured joint, four candidate tubes
| Tube | Ratio | Supplied ID (mm) | Recovered ID (mm) | Recovered wall (mm) | Verdict on this joint |
|---|---|---|---|---|---|
| GSC 12.7/6.35 | 2:1 | 12.7 | 6.35 | 0.56 | Clears the crimp, never closes on a 5.0 mm core |
| GSC 3X 12.0/4.0 | 3:1 | 12.0 | 4.0 | 0.75 ± 0.15 | Both halves of the rule satisfied |
| GDW 12/4 | 3:1 | 12.0 | 4.0 | 1.5 including adhesive | Same fit, and sealed |
| GMW 12/4 | Not printed on the sheet | 12 | 4 | 1.8 | Same fit in medium wall, published to 3.3 kV |
Worked against a crimp measured at 9.5 mm across the barrel, with 5.0 mm over the insulated core either side. Diameters and walls come from the manufacturer's thin wall, high-shrink-ratio, dual wall and medium wall selection charts.
Recovered wall is the mechanical spec
Recovered ID says whether the tube fits. Recovered wall says what it survives afterwards.
Thin wall GSC 12.7/6.35 finishes at 0.56 mm. Medium wall GMW 12/4 finishes at 1.8 mm on the same bore. Same fit, three times the material.
Take 2.7 mm as the reference for a wall doing mechanical work. It is the recovered wall on GMW 40/12 through GMW 75/22, and the wall the wrap-around sleeve holds right across its range, GWS-55/8 to GWS-240/50.
On the tube range the wall climbs with diameter, 1.5 mm at GMW 10/3 up to 3.5 mm at GMW 425/120. On the wrap-around range it never moves. Wall is a fixed design choice there, not a function of size.
The manufacturer splits the duties plainly. GMW insulates connectors on low voltage straight-through joints and splices to 3.3 kV. GHW gives mechanical protection and outer sealing on underground joints to 36 kV. Both conform to IEC 60684-3-247 and are halogen free.
Two published tolerances for the same 2.7 mm
The manufacturer's web page for the wrap-around sleeve prints the recovered thickness as 2.7 mm at ±10%. The datasheet prints the same 2.7 mm at ±20%. The cable ranges disagree too: the page gives GWS-140/35 as 105-45 mm, the sheet gives 122-38 mm. We cannot tell which is current, so get a wall tolerance confirmed before it goes into a specification. Full chart on the manufacturer's wrap-around sleeve page.
Adhesive-lined, and what the lining costs you
Adhesive-lined tube carries a hot melt layer on the bore. It flows as the tube recovers and fills the steps polyolefin cannot bridge on its own: the shoulder at the end of a barrel, the ridge where tape stops.
Its published job is a permanent waterproof barrier and environmental seal to IP 68. It is a seal, not spare wall.
The dual wall chart prints both figures, so the arithmetic is visible. GDW 12/4 recovers to 1.5 mm including adhesive, of which 0.6 mm is the adhesive. That leaves 0.9 mm of polyolefin.
GSC 3X 12.0/4.0 gives 0.75 mm of polyolefin and no adhesive at all. GDW 12/4 is barely thicker in the material that insulates, and far better sealed.
Specify it where water reaches the joint: a duct, a pit, a trench, anything buried or outdoors. Do not specify it to make up a wall you are short of.
Overlap: the only figures the manufacturer publishes
| Repair sleeve sizes | Cable OD covered (mm) | Minimum overlap each side (mm) |
|---|---|---|
| GMRS-19 to GMRS-70 | 5 to 55 | 25 |
| GMRS-115 and GMRS-160 | 40 to 122 | 50 |
| GMRS-180 to GMRS-250 | 60 to 192 | 75 |
Condensed from the cable repair sleeve selection chart, which is where these minimums come from. The bands overlap because the sleeve sizes overlap. Full chart on the manufacturer's cable repair sleeve page.
No overlap minimum is published for plain tube
Read the pattern before you borrow it: 25, 50 and 75 mm track cable diameter, not joint length. Nothing equivalent appears on the thin wall, high-shrink-ratio, dual wall or medium wall sheets, which give supplied ID, recovered ID and recovered wall and then stop. So we have no published overlap minimum for plain tube over a wire joint. The example below borrows the repair-sleeve figure and says so. If that number has to survive a technical schedule, ask the manufacturer for it in writing.
This sequence is ours, not the manufacturer's
What follows is reconstructed from the published selection charts, for one ordinary crimped joint. Where a kit or a reel of tube arrives with its own instruction sheet, that sheet is the authority and this is not.
Worked example: a crimped LV joint
Measure the assembled joint
Dry-assemble if you can, then take two figures with a vernier. Widest point across the crimp barrel: say 9.5 mm. Smallest point the tube has to grip, over the insulated core past the cut-back: say 5.0 mm. Include any tape you intend to put on.
Set the two limits
Supplied ID must clear 9.5 mm with room to slide. Recovered ID must finish under 5.0 mm. GMW 10/3 fails the first test on half a millimetre of clearance, which is not enough to get past a barrel without snagging.
Ratio first, wall second
A 2:1 tube at this bore recovers to 6.35 mm and never touches the core, so go to 3:1. Then choose wall by duty: 0.75 mm on GSC 3X 12.0/4.0 inside a panel, 1.8 mm on GMW 12/4 where the joint gets handled or pulled.
Cut for the overlap
Joint length plus overlap at both ends. At 5 to 55 mm cable OD the repair sleeve chart asks 25 mm each side, so a 60 mm joint wants 110 mm of tube as a floor. Repair sleeves ship in 100, 200, 300 and 400 mm lengths, which is a useful check against your own cut.
Park it clear, then centre it
Slide the tube down the tail while you crimp, then bring it back and centre it against the overlap you cut for.
Shrink from the centre outwards
Start over the middle of the crimp and work out to each end, driving air ahead of the tube. Keep the heat moving. On adhesive-lined tube, a bead of hot melt at both ends says the bore is full. The wrap-around sleeve answers the same question differently, with a temperature-sensitive paint that changes colour once shrinking is complete.
Check it cold
Let it go cold before handling. The tube should follow the crimp with no bridging over the shoulder, no wrinkle standing off the core, and no dull unshrunk patch.
Questions from the bench
Can I size the tube from the cable size in sq mm?
The joint is longer than the tube I have. Can I butt two together?
Is adhesive-lined always the safer choice?
Does this rule size the parts inside a joint kit?
How far up in voltage does a tube over a joint go?
Sizing a kit, not a sleeve?
If the cable is screened, armoured, or working above 3.3 kV, the tube stops being the job and becomes one line on a manifest. The manufacturer's cable joint range runs 1.1 kV to 36 kV, and every part in the box is named and counted here.