The short answer
In a multicore cable carrying a balanced three-phase load, the currents in the cores add up to roughly zero at any instant, so their magnetic fields largely cancel out at the armour. In a single-core cable there is nothing to cancel the field: the whole alternating magnetic field of the conductor passes through the armour around it. If that armour is steel, which is magnetic, the field induces eddy currents and causes hysteresis losses in the wires. The armour heats up, the cable runs hotter than the designer assumed, and in heavy-current circuits the losses can be large.
Aluminium is not magnetic, so aluminium wire armour (AWA) avoids the hysteresis loss and greatly reduces the induced heating. That is why the general rule, followed in IEC 60502-based and BS-based specifications alike, is simple: multicore armoured cables use steel wire (SWA) or steel tape (STA); single-core cables in AC circuits use AWA.
Where single-core cables are used
Single-core power cables appear wherever conductors get too large to handle as one multicore cable. Typical cases include:
- Transformer to main switchboard connections, where several large single cores per phase are run in parallel.
- Generator and large motor feeders in industrial plants.
- Medium voltage feeders, where single-core cables in the 25 to 630 mm2 range under IEC 60502-2 are common alongside three-core types; see the MV power cables page.
- Utility and substation connections, often in aluminium, covered on the aluminium cables page.
In many of these places the cable also needs mechanical protection, either because it is buried, run on open trays in a plant, or installed where it may be struck. That is when the armour question comes up.
Single cores are chosen for handling as much as for electrics. A three-core cable with very large conductors becomes so heavy and stiff that it cannot be pulled or bent into a switchboard, and the drum it needs may be too large to transport easily. Splitting each phase into its own single core, sometimes with two or more cores per phase in parallel, keeps each cable manageable. The trade-off is that the installer now has three or more separate cables per circuit, each with its own armour, gland and earthing point, and the magnetic behaviour of each core has to be considered on its own. Buyers who are used to ordering multicore SWA tend to carry that habit over to single-core lines on the same schedule, which is exactly where the armour mistake appears.
AWA, SWA and unarmoured single cores compared
| Option | AC circuits | DC circuits | Notes |
|---|---|---|---|
| Aluminium wire armour (AWA) | Standard choice where armour is needed | Usable | Non-magnetic; bonding arrangement set by the designer |
| Steel wire armour (SWA) | Avoid: magnetic losses and heating | Sometimes used; designer's decision | The DC case has no alternating field, but check the project specification |
| Steel tape armour (STA) | Avoid, same reason | Designer's decision | Common on multicore cable only |
| Unarmoured with metallic screen | Common at MV | Not typical | Copper tape or wire screen carries fault current; mechanical protection from ducts or tiles |
| Unarmoured, no screen | Common at LV in ducts and trays | Common (for example solar DC cable) | Mechanical protection from the installation method |
Solar DC string cable such as H1Z2Z2-K is single-core and normally unarmoured; see the solar PV cables page. The heating problem described here belongs to alternating current.
Bonding and induced voltages
Even with aluminium armour, a single-core cable's armour and metallic screen sit in the conductor's magnetic field. If the armour is bonded to earth at both ends, a circulating current can flow along it and back through earth or the neighbouring cables' armour, which adds losses. If it is bonded at one end only, the circulating current stops, but a standing voltage appears at the open end. Which arrangement is used, and whether special bonding methods apply on long runs, is decided by the project engineer and affects the current rating calculation.
For buyers the practical points are two. First, the cable's armour and screen dimensions on the datasheet are inputs to that calculation, so the final spec sheet should reach the engineer before the order is confirmed. Second, laying formation matters: single cores of a circuit are usually laid in trefoil or flat formation together, never with phases split across different ducts, and steel parts such as gland plates around a single core need the same care as the armour. Gland plates for single-core AC cables are normally non-magnetic for the same reason.
Sourcing AWA cable
AWA cable is a narrower product than STA or multicore SWA. The partner plant catalogue behind our armoured cables page covers steel tape armour only, so AWA single-core constructions are matched to plants that run aluminium wire armouring lines and have type test records for the construction. The desk checks three things before shortlisting a plant:
- Construction and standard: conductor size and class, insulation, bedding, aluminium wire diameter and outer sheath per IEC 60502-1, IEC 60502-2, BS 5467, BS 6724 or the national standard named.
- Type test coverage: a type test report covering the same voltage, conductor size range and armour type.
- Routine records: conductor resistance per drum and, where required, armour resistance; for MV cores, partial discharge per IEC 60885-3 and the routine voltage test.
Large single cores are heavy and stiff, so drum length and drum size must be planned with the installer: how many metres per drum, how many drums per phase, and whether lengths must be matched for parallel runs. Drum options are on the packaging and drums page.
Checklist for the enquiry
A complete single-core armoured line on an RFQ states: conductor material and size, voltage rating, insulation, 'AWA' explicitly, sheath material (PVC or LSZH), the standard, circuit type (AC or DC), total metres per size and drum lengths. The SWA vs STA article covers the multicore side of the same decision. MOQ and lead time for matched AWA plants are confirmed in writing for each order; the typical ranges are on the MOQ and lead time page, and the cable list can go straight into the quote form.