What compaction does
An ordinary round stranded conductor is built from round wires laid in layers around a centre wire. Round wires cannot fill a circle, so there are gaps between them, and the overall diameter is larger than a solid wire of the same area. In a compacted conductor each layer, or the whole conductor, is passed through dies or rollers during stranding. The wires deform into shapes that fit against each other, the gaps shrink, and the finished conductor is noticeably smaller and smoother than a non-compacted one of the same nominal size.
The amount of metal does not change. IEC 60228 treats round non-compacted, round compacted and shaped stranded conductors all as Class 2, with the same maximum DC resistance for each nominal size. A compacted 95 mm2 copper conductor must still be at or below 0.193 ohm/km at 20 degrees C, exactly as a non-compacted one. Compaction is a way of arranging the copper, not of saving it.
Why cable makers compact conductors
Compaction is common on power cables for several reasons, each of which also matters to buyers:
- Smaller overall diameter: every layer applied over the conductor, from insulation to sheath, is applied over a smaller circle, so the finished cable is smaller and lighter for the same rating.
- Less material in the other layers: a smaller circumference needs less insulation, bedding and sheath compound for the same thicknesses, which affects cost and weight per kilometre.
- Smoother surface: a compacted conductor presents a more even surface to the layer above it. In medium voltage cable this matters a great deal, because the conductor screen and insulation must sit on a smooth, round base to keep the electric field even.
- Fewer voids: smaller gaps between wires leave less space along the conductor for moisture or compound to migrate; where longitudinal water blocking is required, the construction is still specified separately.
- Handling and packing: a smaller, lighter cable means more metres per drum and easier pulling, within the limits of the drum and the route.
The reference 8.7/15 kV Cu/XLPE/CTS/PVC 3x95 mm2 cable on the MV power cables page uses a compacted copper conductor under its conductor screen, 4.5 mm of XLPE insulation and an insulation screen, with an overall diameter of about 61 mm and about 5,403 kg/km per the partner plant catalogue. The MV semiconductive layers article explains why the smooth conductor surface matters under the screens.
Round, compacted and shaped conductors
| Form | Construction | Effect on cable | Typical use |
|---|---|---|---|
| Round, non-compacted | Round wires in layers, natural gaps | Largest diameter for the size | Building wire, smaller power cores, flexible handling |
| Round, compacted | Layers pressed through dies or rollers | Smaller, smoother conductor | LV and MV power cable, aluminium power cable |
| Shaped (sector) | Stranded and formed to a sector profile | Compact multicore cable | Some larger multicore LV power cables |
Exact diameters for each form vary between plants and are given on each datasheet; this site does not publish a general compaction figure. Not every plant compacts every size either: some compact only larger power conductors and supply smaller sizes as round non-compacted strands, and the reference 0.6/1 kV 4x25 mm2 LV cable is listed simply as round stranded copper. The datasheet, not the designation, tells you which form you are buying, so ask for it with every offer. When the desk compares offers, it compares the conductor diameter and the overall diameter stated by each partner plant for the same size and form.
What buyers need to check
A compacted conductor changes the details at both ends of the cable, so three points belong on the checklist before ordering.
Lugs and connectors. Compression lugs and connectors are designed for a conductor size and often for a conductor form. Connector makers state whether a lug suits compacted, non-compacted or both, and the correct die depends on it. Mismatches cause loose crimps or overstressed barrels. Order lugs against the conductor form actually supplied, and keep the conductor datasheet with the termination kit.
Glands and accessories. Cable glands, MV terminations and joints are selected by diameter over specific layers, such as the insulation or the outer sheath. If a design was based on one plant's non-compacted construction and the delivered cable is compacted, or the reverse, the accessories may not fit. Confirm diameters from the datasheet of the plant that will make the cable.
Resistance, not diameter. A small conductor diameter is not proof of compaction, and it is not proof of short copper either. The only way to tell them apart is the DC resistance test. A conductor that is small because it is compacted passes the IEC 60228 limit; a conductor that is small because copper was left out fails it. The conductor resistance testing article explains what the record should show.
Compaction and aluminium
Aluminium power conductors are also commonly compacted. Because aluminium needs a larger cross-section than copper for the same resistance, as explained in the copper vs aluminium article, reducing the diameter helps keep aluminium cables manageable. The reference 0.6/1 kV Al/XLPE/PVC 4x95 mm2 cable, about 40 mm overall, is checked against the aluminium Class 2 limit of 0.320 ohm/km whatever its strand form. Aluminium lugs for compacted conductors follow the same rule as copper: match the lug and die to the form.
How to specify compaction
- State the form: for example "compacted circular stranded copper conductor, Class 2 to IEC 60228", or leave the form to the plant if accessories are not yet chosen.
- State the resistance limit: the IEC 60228 Class 2 value for the size and metal, verified per drum by test report.
- Freeze diameters with accessories: once glands, lugs or MV terminations are selected, write the diameter range over the relevant layers into the specification.
- Keep the form consistent: on multi-drum projects, all drums of a size should share the same conductor form so that one termination kit fits all.
Partner plants confirm the conductor form and diameters on the datasheet before production, and the desk checks them against the specification at pre-shipment inspection. Send the designation, the accessory constraints and quantities through the quote form.