Kraft paper
Cellulose paper with high dielectric strength in oil. Standard for mineral-oil transformers designed for conventional hot-spot temperatures.
Winding wire guide
Paper Covered Magnet Wire (Aluminium Conductor)
Paper covered magnet wire is a round or rectangular conductor wrapped in overlapping layers of insulating paper, used mainly for the windings of oil-immersed power and distribution transformers and reactors. Paper-covered rectangular aluminium wire is standardised in IEC 60317-27-4. This guide explains the construction, papers, sizes, aluminium-versus-copper choice and how to specify it; Metline supplies the aluminium versions.
- Conductor
- EC-grade aluminium, round or rectangular
- Standard
- IEC 60317-27-4
- Insulation
- Kraft, upgraded kraft, crepe, aramid paper
- Main use
- Oil-immersed transformer windings
What is paper covered magnet wire?
A paper covered (paper insulated or paper wrapped) winding wire is a bare or enamelled conductor around which strips of electrical paper are wound in several overlapping layers. Once the winding is dried and impregnated with transformer oil, the oil-soaked paper forms an insulation system with high dielectric strength, good resistance to the mechanical forces of short circuits and a long service life.
Paper covered wire is the traditional conductor for windings in oil-filled distribution and power transformers, shunt reactors and some traction transformers. Rectangular (flat) conductors dominate because they fill the winding window better than round wire, which raises the space factor and reduces transformer size.
Aluminium conductors have become widely used in distribution transformers because they cost less and weigh about half as much as copper for the same resistance. IEC 60317-27-4 specifies paper tape covered rectangular aluminium winding wire for oil-immersed transformers, with nominal conductor widths of 2.00 – 16.0 mm, thicknesses of 0.80 – 5.60 mm, and paper tapes of 25 – 125 µm conforming to IEC 60554-1, applied as two or more layers.
Conductor and insulation construction
| Element | Options | Notes |
|---|---|---|
| Conductor material | Electrical-conductor aluminium (1350 / 1060 / 1070 type), annealed | Conductivity about 61 – 62 % IACS; resistivity about 0.028 Ω·mm²/m at 20 °C |
| Conductor shape | Rectangular (flat) with rounded edges; round | Edge radius set by the standard for each thickness to protect the paper |
| Base coat (optional) | Enamel (polyvinyl acetal, polyesterimide and others) | Enamel under paper gives turn insulation and allows combined conductors |
| Paper type | Kraft, thermally upgraded kraft, crepe paper, aramid paper, polyester or polyimide film combinations | Choice depends on thermal class and oil system |
| Wrapping | Two or more layers, overlapped or butt-lapped with stagger | Total insulation build agreed by the transformer designer |
| Combined conductors | 2 or 3 bare, enamelled or paper-covered strands under a common paper wrap | Reduces eddy losses and simplifies winding of large currents |
The conductor is drawn or rolled to size, annealed to a soft temper so that it bends without springback, and then wrapped on a paper-covering line under controlled tension. Edge radii, burr-free surfaces and a uniform paper tension are what prevent paper tearing and partial discharge in service.
Insulating papers and thermal classes
Thermally upgraded kraft
Chemically stabilised cellulose that ages more slowly, allowing higher operating temperature or longer life in the same design.
Crepe paper
Creped for stretch, used on leads and irregular shapes where paper must conform.
Aramid paper
Synthetic aramid insulation (for example meta-aramid papers) for high-temperature designs, suited to thermal classes up to about 220 °C and to ester-fluid transformers.
Film and composite wraps
Polyester or polyimide films, alone or combined with paper, for high turn-to-turn strength in thin builds.
Enamelled then paper-wrapped
An enamel film plus paper wrap combines thin turn insulation with mechanical protection.
Thermal classes and their temperature indices are defined in IEC 60085; transformer temperature limits are set in IEC 60076-2 and related standards. The insulation system, not the paper alone, determines the rating.
Typical size ranges
| Product | Typical conductor size range |
|---|---|
| Paper-covered rectangular aluminium wire (IEC 60317-27-4) | Thickness 0.80 – 5.60 mm × width 2.00 – 16.0 mm |
| Paper-covered flat wire, extended range | Thickness to about 16 mm and width to about 25 mm by agreement |
| Paper-covered round aluminium wire | Diameter about 0.6 – 16 mm |
| Enamelled wire with paper wrap | Round to about 6 mm; flat about 1.0 – 3.5 × 4 – 16 mm |
| Combined (bunched) conductors | Two or three strands, each about 1.0 – 4.0 × 4 – 20 mm |
Sizes outside these ranges, special insulation builds and different numbers of strands can be agreed for specific transformer designs. Specify sizes as bare conductor dimensions plus the total insulation increase, because that is how winding dimensions are calculated.
Paper-covered aluminium vs copper conductor
| Property | Aluminium conductor | Copper conductor (for comparison) |
|---|---|---|
| Conductivity (annealed) | About 61 – 62 % IACS | 100 % IACS (resistivity 0.017241 Ω·mm²/m) |
| Density | 2.70 g/cm³ | About 8.9 g/cm³ |
| Cross-section for equal resistance | About 1.6 × that of copper | 1.0 × |
| Mass for equal resistance | About half that of copper | 1.0 × |
| Mechanical strength | Lower; short-circuit forces need careful design | Higher |
| Material cost and price volatility | Lower | Higher |
| Jointing | Welding, cold-pressure welding or crimping; Al-Cu transitions at terminals | Brazing, crimping, welding |
Aluminium windings are larger for the same losses, so the core and tank grow a little, but the total transformer is usually lighter and less costly. Aluminium is common in distribution transformers and is increasingly used in larger units. Copper is preferred where the smallest size or the highest short-circuit strength is decisive.
Tests and quality checks
- Conductor dimensions and edge radius against the standard tables for each size.
- Resistivity of the annealed conductor at 20 °C, reported on the certificate.
- Elongation and softness, so the wire winds without springback or cracking.
- Insulation build and lapping: number of layers, overlap, tension and total increase.
- Paper properties: type, thickness and supplier conformity to IEC 60554 series.
- Surface: no burrs, slivers or paper tears that could cause partial discharge.
Every lot ships with a test certificate listing the conductor dimensions, resistivity, elongation and insulation build, so the transformer maker can trace each spool through production.
What to check when buying
- Specify the standardIEC 60317-27-4 for rectangular aluminium, or the buyer's own specification for other constructions.
- Give bare size and insulationThickness × width or diameter, edge radius, paper type, layers and total increase.
- Define combinationsNumber of strands, enamel type if any, and radial or axial arrangement.
- Choose spool and lengthSpool type, length or weight per spool, and winding direction (flat or edgewise).
- Storage and transportPaper is hygroscopic: wire must be packed sealed, kept dry and protected from impact; dry the winding before oil filling.
References and standards
- IEC 60317-27-4 – Paper tape covered rectangular aluminium wire
- IEC 60554-1 – Cellulosic papers for electrical purposes: definitions and general requirements
- IEC 60085 – Electrical insulation: thermal evaluation and designation
- IEC 60076-1 – Power transformers: general
Paper covered magnet wire: frequently asked questions
What is paper covered magnet wire used for?
Mainly for the windings of oil-immersed distribution and power transformers and reactors, where oil-impregnated paper gives high dielectric strength and good resistance to short-circuit forces.
Which standard covers paper-covered aluminium wire?
IEC 60317-27-4 specifies paper tape covered rectangular aluminium winding wire for oil-immersed transformers, covering widths of 2.00 – 16.0 mm and thicknesses of 0.80 – 5.60 mm with papers of 25 – 125 µm.
Is paper-covered aluminum wire as good as copper?
It does the same job with about 1.6 times the cross-section for equal resistance, at about half the weight and lower cost. Copper gives a smaller winding and higher mechanical strength.
What temperature can paper-covered wire withstand?
Kraft paper in mineral oil suits conventional transformer temperatures; thermally upgraded kraft allows higher hot-spot temperatures; aramid paper systems are used up to about 220 °C. The complete insulation system sets the rating.
How should paper-covered wire be stored?
Keep it sealed, dry and protected from impact and dust. Paper absorbs moisture, so windings are dried before oil filling.
Do you supply paper-covered copper wire?
No. Metline supplies aluminium conductors only, including paper-covered aluminium wire and strip and enamelled aluminium wire.
Need paper-covered aluminium winding wire?
Send the conductor size, paper type, insulation build and quantities. We will reply with options and a delivered price.
