What the conductor does and how it is classified
The conductor carries the current, and its cross-section sets resistance, heat and voltage drop. IEC 60228 does not define a conductor by its strand count or diameter but by its maximum DC resistance at 20 C. A cable sold as 25 mm2 copper is correct if its resistance is no higher than the limit for that size and class; that single number is the buyer's best check.
The standard groups conductors into four classes. Class 1 is solid, used for small building wire such as H07V-U. Class 2 is stranded, the normal choice for power cable and H07V-R. Class 5 is flexible, made of many fine wires, used in cords, H07V-K and solar cable. Class 6 is more flexible again, for moving applications. The finer the strands, the more flexible the cable and, for the same nominal size, the slightly higher the permitted resistance.
Resistance limits buyers can check
The table lists maximum DC resistance at 20 C from IEC 60228 for common sizes. Compare these with the figure on each drum's test record.
| Size, mm2 | Copper Class 1/2 | Copper Class 5, plain | Copper Class 5, tinned | Aluminium Class 2 |
|---|---|---|---|---|
| 2.5 | 7.41 | 7.98 | - | - |
| 4 | 4.61 | 4.95 | 5.09 | - |
| 6 | 3.08 | 3.30 | 3.39 | - |
| 10 | 1.83 | - | - | 3.08 |
| 25 | 0.727 | - | - | 1.20 |
| 50 | 0.387 | - | - | 0.641 |
| 95 | 0.193 | - | - | 0.320 |
| 120 | 0.153 | - | - | 0.253 |
The partner plant catalogue behind our reference constructions lists 0.727 ohm/km for the 4x25 mm2 copper XLPE cable and 0.320 ohm/km for the 4x95 mm2 aluminium cable, both matching the limits. The tinned Class 5 figure of 5.09 ohm/km for 4 mm2 is the one to check on H1Z2Z2-K solar cable.
Copper or aluminium
Aluminium has higher resistance than copper for the same size, so it is usually specified one or two sizes larger. Reading the table, aluminium 120 mm2 at 0.253 ohm/km sits close to copper 70 mm2 at 0.268 ohm/km, though the final size is set by the project engineer under IEC 60364-5-52, BS 7671 or NEC as applicable. Aluminium is lighter and its metal is cheaper, which is why utilities and large feeders use it. The trade-offs are larger diameter, joints and lugs rated for aluminium, and the fact that plants do not supply small aluminium sizes: the reference catalogue starts at 10 mm2. In the US, NEC sets 14 AWG as the minimum copper size and 12 AWG for aluminium or copper-clad aluminium. See aluminium cables for constructions.
- Plain annealed copper: the default for building wire, power, control and data cable.
- Tinned copper: Class 5 for solar cable, with single wires of no more than 0.31 mm in the reference H1Z2Z2-K; also used where corrosion is a concern.
- Aluminium Class 2: LV and MV feeders from 10 mm2 upward, round stranded in the reference 4x95 mm2 construction.
Short cross-sections and CCA
Two conductor risks recur. The first is an undersized conductor: BASEC has warned of SWA cable whose effective cross-section was up to two sizes below the marked size, with both conductor and armour resistance out of limit. The second is copper-clad aluminium sold as copper. CCA is about 30 percent lighter, shows aluminium when the strand is scraped, and can be identified by XRF. Category network cable requires solid copper conductors, so a Cat6 cable with CCA conductors does not meet the category. The desk does not offer CCA as a data cable option; read more in CCA vs copper.
What changes price, weight and lead time
Copper and aluminium make up roughly 80 percent of product cost according to listed cable makers' filings, and Chinese quotes are normally metal price plus a processing fee. LME three-month copper reached a record of USD 14,875 per tonne intraday on 10 September 2026 (Fastmarkets) and was around USD 14,315 per tonne on 21 October 2026, which is why quote validity is counted in days and long projects use a copper adjustment formula. Weight follows the metal: the reference 4x25 mm2 copper cable weighs about 1,292 kg/km. Lead time starts when copper is booked against the agreed quote; see cable pricing guide and MOQ and lead time.
How the desk checks conductors
- The spec sheet names metal, class, size and, for tinned or compacted conductors, the finish.
- Each drum's test record shows DC resistance at 20 C, compared with the IEC 60228 limit.
- At pre-shipment inspection a sample end is cut, strands are counted and scraped, and weight per metre is compared with the datasheet.
- Any deviation is raised with the plant before loading, not after arrival. Send your sizes through the quote form.