Busbar Heat Shrink Sleeve Sizing: Bar Section to Supplied ID
Engineer's Guide · 2026-09-16 · 5 min read
Sizing a busbar sleeve looks like a two-number exercise: measure the bar and pick the sleeve. It is not, because a rectangular bar has a diagonal, the sleeve has to pass over the widest point of the assembly rather than the bar, and the wall the part ends up with after recovery is not the wall on the datasheet. Getting those three things wrong produces a sleeve that will not go on, or one that goes on and cannot hold the voltage.
Start with the diagonal, not the width
A rectangular bar is described by its width and thickness, and a round sleeve has to clear the diagonal before it recovers. The supplied inner diameter has to be larger than the diagonal of the bar plus whatever the corner radius takes off, and it has to be large enough that the sleeve slides on without dragging.
| Bar section | Approximate diagonal | Minimum supplied ID to clear it | Sensible ratio |
|---|---|---|---|
| 10 by 3 mm | about 10.4 mm | 12 mm or larger | 2:1 or 3:1 |
| 20 by 5 mm | about 20.6 mm | 22 mm or larger | 2:1 or 3:1 |
| 30 by 10 mm | about 31.6 mm | 33 mm or larger | 2:1 or 3:1 |
| 50 by 10 mm | about 51.0 mm | 53 mm or larger | 2:1 or 3:1 |
| 60 by 10 mm, two bars stacked | the stack diagonal, not one bar | measured on the assembled stack | 3:1 or 4:1 where the stack varies in height |
The three numbers that decide the recovered result
- Supplied inner diameter, which has to clear the assembly before recovery.
- Recovered inner diameter, which has to close onto the bar and grip it without leaving a loose pocket at a corner.
- Recovered wall, which sets the dielectric performance at the working voltage and is the number the datasheet quotes after recovery, not before.
The ratio ties the first two together. A 2:1 sleeve recovers to half its supplied diameter, so a 24 mm sleeve lands at about 12 mm. A 3:1 lands at about 8 mm, and a 4:1 at about 6 mm. Choose the ratio from the gap between what the sleeve has to pass over and what it has to close onto.
The sizing method
- Measure the assembly the sleeve is fed over, at its widest point, including corner radii and any covering already in place. That is the number the supplied ID has to clear.
- Decide the recovered inner diameter from the bar section, adding a small allowance so the sleeve grips rather than stretches to fit.
- Divide to get the ratio you need, then round up to the nearest available ratio rather than down. Under-ratio means the sleeve recovers before it reaches the thin end.
- Check the recovered wall against the working voltage. Wall is the voltage parameter, and it is fixed by the ratio and the supplied wall.
- Check the profile after recovery against every clearance the bar passes through, because the sleeve grows the bar in two dimensions, not one.
- Confirm the colour and any print, since phase identification is usually part of the specification and repainting a recovered sleeve is not an option.
Where the voltage enters the calculation
Ratios and diameters set whether the sleeve fits. Wall sets whether it insulates. Our busbar sleeve is a polyolefin heat shrink with UL 94 V-0 flame behaviour, supplied in 2:1, 3:1 and 4:1, rated to the busbar voltage and held to stable recovered dimensions, with UL 224 and IEC 60684-3 referencing. The 800V platform variant adds the tracking requirement that a high-voltage DC busbar brings with it.
| Duty | What drives the wall | What to state |
|---|---|---|
| Low-voltage control and distribution busbars | Mechanical protection and basic insulation | Bar section, assembly OD, colour coding and the environment |
| 800V EV and ESS busbars | Dielectric withstand plus resistance to tracking under humidity | Platform voltage, the humidity and pollution environment, and the tracking requirement |
| MV switchgear busbars | Anti-tracking performance, partial discharge suppression and creepage | Voltage class, the pollution level and the clearance the sleeve has to help restore |
| Layered or stacked busbars | Uniform wall on the outside of a varying section | The stack dimensions, because the sleeve has to follow a changing profile |
Where the recovered wall is lost
Two common mistakes. A sleeve over-sized to make it slide on easily recovers to a smaller wall than the design assumed, because the same material is spread over a larger recovered circumference. A sleeve recovered over a corner is thin at the corner and full thickness on the flats, so the weakest point is exactly where the field concentrates. Both come from sizing by specification rather than on the actual assembly.
Where the sleeve stops
A sleeve insulates along a continuous run. It does not cover a joint, a tap, a terminal or a bolted connection, where the bar leaves the sleeve and is exposed by definition. Those are the points where a bare busbar remains, and they are handled by a joint or tap box rather than by more sleeve. That is a separate page, and the installation process itself is on the busbar heat shrink installation guide.
In the wider build this belongs to the busbar insulation route, which gathers every part it covers.
Products for this job
These are the busbar insulation parts we make for EV and ESS builds. Send the part and the equipment you recover with, and we will confirm the size and the profile against them.
Related reading
- Busbar Heat Shrink Installation: 8.7/15kV to 35kV Guide
- Busbar Insulation: A Complete Guide
- Why Insulate MV Busbars
- Busbar Insulation Sleeving for 800V EV and ESS
Common questions
How do I calculate the busbar sleeve size?
Start from the diagonal of the bar section plus the corner radius, which sets the minimum supplied inner diameter. Then decide the recovered inner diameter from the bar, work out the ratio that connects the two, and check the recovered wall against the working voltage. Round the ratio up, never down.
Why is the diagonal the number that matters?
Because a round sleeve has to clear the rectangular section before it recovers. A sleeve selected on the width alone will not pass over the corners, and forcing it on damages the wall before the part is even installed.
What shrink ratio should I use for a busbar?
2:1 or 3:1 covers most bar sections where the sleeve is fed over a single bar. Move to 3:1 or 4:1 where the assembly varies in height, where the sleeve has to pass over a widened section, or where it has to reach a smaller diameter after clearing a large one.
Does over-sizing the sleeve reduce the wall?
Yes, if the sleeve is larger than the bar needs. The same material recovers to a lower wall than the design assumed, which reduces dielectric margin. Size to the assembly rather than adding a margin for ease of fitting.
Why is the wall thinner at a corner?
The sleeve cannot reach into a corner as it recovers, so the material thins there while the flats hold full thickness. That puts the thinnest wall where the electric field concentrates. Check the corner as a separate case on any bar with a generous radius.
What colour options are there, and does colour affect performance?
Colour is used for phase identification and for assembly error prevention. Where the specification includes a flame class or a tracking requirement, confirm that the colour compound carries the same classification rather than assuming it follows the base grade.
What temperature does the sleeve see?
Polyolefin busbar sleeve recovers across a moderate band and then operates continuously inside its rated range. Confirm the busbar's own temperature at full load, because the sleeve has to live with that for years rather than survive it once.
What do I send for a quotation?
The bar cross-section, the widest point of the assembly the sleeve is fed over, whether the run is straight or has bends, the platform voltage and whether it is AC or DC, the environment, the colour or print requirement and the annual volume. WELLELE makes busbar heat shrink sleeve in 2:1, 3:1 and 4:1 with UL 94 V-0 flame behaviour and stable recovered dimensions, rated to the busbar voltage.
Specifying this part?
Send the drawing for a manufacturability review.