TL;DR
- Sleeving sizes by the wire, not by habit — pick an inside diameter 10–20% larger than the conductor OD; our sizing table maps common wire sizes to sleeve IDs.
- Tying cord, glass rope, and banding tape are not interchangeable — polyester tying cord for general lashing, polyester-glass rope for high-temperature strength, heat-shrinkable banding tape for pre-stressed winding consolidation.
- VPI compatibility is a specification, not an assumption — confirm varnish/resin compatibility and shrinkage behavior before ordering for impregnation processes.
- Quote with four numbers — ID, wall grade, cut length or spool length, and thermal class get you an accurate same-day quotation.
Lead insulation failures in motors and transformers rarely come from exotic causes — they come from a sleeve that was one size too tight, a tying cord that relaxed after varnishing, or a banding material that was never compatible with the impregnation resin. Choosing insulation sleeving and tying cords correctly is a sizing exercise plus a materials-compatibility exercise, and both are quick once you know the rules. This guide covers where each product is used, how to size sleeving to your conductor, how the three lashing materials compare on strength and process behavior, and what to put in your RFQ.
Where Sleeving and Tying Cords Are Used
Two product families do two different jobs on the same machine:
- Insulation sleeving — braided fiberglass or polyester sleeves (silicone, acrylic, or PVC coated) that slide over motor lead-out wires, transformer lead and tap conductors, and jumper connections. The sleeve provides the dielectric barrier and mechanical protection where a conductor leaves the winding and crosses grounded metal. For coating selection by temperature and voltage, see our silicone vs acrylic vs PVC sleeving selection guide.
- Tying and banding materials — polyester tying cord, polyester-glass rope, and heat-shrinkable banding tape that lash end windings, secure lead wires to the winding body, and consolidate coils before impregnation. These are mechanical products first: their job is to hold everything exactly where the winder put it through curing, shipping, and decades of electromagnetic vibration.
In practice the two families work together on every motor and transformer: sleeving insulates the leads, tying materials anchor them. SIDA supplies both from one factory — see the fiberglass sleeving and tying cords manufacturer page for the combined range.
Sleeving Sizing: Inside Diameter Rules
The sizing rule is simple: sleeve ID = conductor OD × 1.10–1.20. Below 10% clearance the sleeve drags, splits strands, and slows the line; above 20% it slides and bunches. Use this table for common cases:
| Conductor / bundle OD | Recommended sleeve ID | Typical use |
|---|---|---|
| 1.0–1.8 mm (AWG 18–14 wire) | 2.0 mm | Small motor leads, control wiring |
| 1.8–2.7 mm (AWG 13–10) | 3.0 mm | General motor lead-out wires |
| 2.7–3.6 mm (AWG 9–7) | 4.0 mm | Larger motor leads |
| 3.6–4.5 mm | 5.0 mm | Heavy leads, small busbar joints |
| 4.5–5.4 mm | 6.0 mm | Transformer lead conductors |
| 5.4–7.2 mm | 8.0 mm | Parallel lead pairs, tap connections |
| 7.2–9.0 mm | 10.0 mm | Busbars, large transformer leads |
| 9.0–13.5 mm | 12–16 mm | Cable joints, grouped leads |
Two field notes. First, measure the conductor with its own insulation on — the sleeve goes over the insulated wire, not the bare copper. Second, for irregular bundles (two parallel leads plus a thermistor, for example), measure the smallest sleeve that passes the bundle’s widest point, then heat-set or tie the ends so the slack cannot migrate.
Tying Cord vs Glass Rope vs Banding Tape
All three lash windings, but they behave differently in strength, shrinkage, and impregnation compatibility:
| Property | Polyester tying cord | Polyester-glass rope | Polyester banding tape |
|---|---|---|---|
| Construction | Twisted/braided polyester fiber | Glass fiber core, polyester braid | Woven/unidirectional polyester webbing |
| Thermal class | Class B–F (130–155°C) | Class F–H (155–180°C) | Class B–F (130–155°C) |
| Tensile strength | Moderate (size-dependent) | High — glass core carries load | High across the width |
| Heat shrinkage | Low; slight tightening at cure | Very low (dimensionally stable) | High — shrinks at bake to pre-tension windings |
| Varnish/VPI uptake | Good; absorbs resin well | Good on the braid; glass core wicks resin | Excellent; designed for impregnation |
| Best for | End-winding lashing, lead anchoring | High-temperature or high-vibration lashing | Consolidating rotor/armature windings before bake |
Selection in one line each: use polyester tying cord for general lashing and lead anchoring in class B/F machines; step up to polyester glass rope when the lashing itself must survive class H temperatures or severe vibration without relaxing; choose polyester banding tape when you want the banding to shrink during the bake cycle and lock the winding under permanent tension.
VPI and Varnish Process Compatibility
If the machine goes through varnish dipping or vacuum pressure impregnation (VPI), your tying and sleeving materials become part of the resin system. Four checks prevent the classic failures:
- Chemical compatibility — polyester-based cords and tapes are compatible with the common polyester, epoxy, and polyesterimide impregnating resins; confirm against your specific resin datasheet, especially with solvent-borne varnishes.
- Shrinkage timing — heat-shrinkable banding must shrink during your bake cycle, not before it. Verify the tape’s shrink onset temperature against your preheat schedule.
- Extractables — low-quality cords carry spinning oils that contaminate resin tanks. Specify low-extractable, insulation-grade material — not textile-grade substitutes.
- Sleeving coating cure — acrylic and silicone coatings on sleeving are already fully cured, but confirm the coating does not soften or swell in your resin at dip temperature. A 24-hour bench immersion test in your actual resin settles this definitively.
RFQ Checklist
A complete RFQ gets quoted same-day; an incomplete one starts an email chain. The most common omissions are wall grade (suppliers default to the cheapest) and impregnation process (which changes the recommended material). Include:
- For sleeving — inside diameter (from the sizing table above), wall grade or target breakdown voltage (1.5 / 2.5 / 4 / 7 kV), coating system (silicone / acrylic / PVC), cut length or spool length, and quantity in meters.
- For tying materials — product type (cord / rope / banding tape), width or diameter, tensile requirement if known, thermal class of the machine, and your impregnation process (dip or VPI, resin type, bake temperature).
- Logistics — destination port and whether you need sample lengths for process trials before the bulk order (samples are free).
Frequently Asked Questions
Q: Can I use polyester tying cord in a class H motor?
Not for the lashing itself — use polyester-glass rope, whose glass core stays stable at 180°C. Polyester cord is fine for class B/F; at class H it loses strength and embrittles over time.
Q: Does banding tape replace tying cord on end windings?
They do different jobs. Banding tape consolidates the winding body under shrink tension; tying cord anchors individual leads and groups at points. Most machines use both.
Q: How much extra sleeve length should I allow per lead?
Lead length plus 10–15 mm per end for anchoring under the tying cord. If the lead is bent sharply at the terminal, add a further 10 mm — sleeves shorten slightly when bent around a radius.
Q: Can SIDA match an existing sample?
Yes — send a short length of your current sleeving or cord and we will match ID, wall, coating, and color, then return free production samples for your approval before bulk production.
Q: Do I need different wall grades for the same wire size?
Only when voltage or abrasion demands it. A 4 mm ID sleeve in standard wall suits most 400 V motor leads; step up to thick-wall (or a higher kV grade) when the lead crosses grounded laminations, exits through a sharp terminal-plate hole, or the winding runs above 1 kV. Oversizing wall thickness on every lead wastes money and makes terminal boxes harder to pack.
Summary
Size sleeving to the insulated conductor at +10–20% ID clearance; pick tying materials by thermal class and process — polyester cord for general lashing, glass rope for class H and high vibration, banding tape for shrink-consolidation; and treat VPI compatibility as a documented requirement, not a hope. Four numbers in your RFQ — ID, wall/voltage grade, length, thermal class — turn selection into a same-day quotation.
Send your conductor sizes and process details for free samples matched to your line.
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References
- IEC Webstore — IEC 60684, Flexible Insulating Sleeving Specifications
- NEMA — Electrical Insulating Sleeving Standards (VS 1)
- UL Solutions — UL 224 Extruded Insulating Tubing and Component Recognition
- SIDA — Silicone vs Acrylic vs PVC Fiberglass Sleeving: Complete Selection Guide
- SIDA — What Is Fiberglass Sleeving Used For?
- SIDA — Motor Winding Insulation Repair: Technician Guide
