Transformer Wire

Looking to purchase some transformer wire, but not sure which kind you need? TEMCo has your guide for picking the right magnet wire for you. Call us at 877.474.8209 or view our online product selection by clicking the links below.

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Soderon 155 » GP/MR-200 »


What is a Transformer?

A transformer transfers electrical energy from one circuit to another circuit through the transformer's coils. Transformers are used to increase voltage before electrical energy is transmitted over long distances through wires.

Transformer Wire

What is Transformer Wire?

Transformer wire is copper or aluminium wire coated with a layer of insulation or enamel.

Transformer Wire Specifications

Conductor Type

Transformer wire is normally made from electrolytic-tough pitch (ETP) copper, also known as oxygen-free copper. Aluminium will be used on larger transformers that are lower voltage and have kilovolt-amperes (kVA) greater than 15.

Since aluminum has lower conductivity than copper, aluminium needs a cross-section that is 66% larger than a copper would require to achieve equal ratings. For this reason, copper wire takes up less space than aluminium wire.


Transformer wire comes in three shapes: square, round or rectangular.

The shape of the wire will be the determining factor in how tightly the wire can be coiled. Square wires can be coiled tighter than round wires because they will have less space between the turns. Square wires are used in confined spaces and applications that require higher power and that have a larger wire gauge.

Rectangular wires are laminated into a flat strips and are the most easily wound since they lay flat. Rectangular wire can be used in larger power transformers that operate at high voltages.

Rectangular Transformer Wire Round Transformer Wire

Pictured above: Rectangular transformer wire (left), round transformer wire (right)

Solid vs. Stranded Winding Wire

Solid wire, which can also be called solid-core or single-strand wire, is one single strand of insulated wire. Solid wires do not bend easily, and are found in applications where the wires not meant to be frequently moved.

Stranded wire,is made up of multiple wire strands braided together, making them more flexible than solid wires and easier to install. The flexibility of the wire increases with the amount of strands the wire has. Stranded wire will have a minimum of seven strands: six wires wrapped around one in the middle.

Wire metal fatigue occurs when a wire is routinely subjected to constant loading and unloading, which can cause the wire material to begin to crack. The flexibility of the stranded wire helps to curb this problem.

Solid wire also has less surface area than a stranded wire, making it is less susceptible to corrosion.

Solid Transformer Wire Stranded Transformer Wire

Pictured above: Solid transformer wire (left), stranded transformer wire (right)

Litz Wire

Litz wire is another form of stranded wire. It contains multiple stranded wires braided together.

High-frequency transformers, that have tens to hundreds of kilohertz, as well as large power transformers, will often us litz wire to minimize the skin-effect and proximity effect losses.

Skin effect: An alternating current can become concentrated in the top layer of a wire, leading to an increase in the wire's effective resistance. The skin depth, or how deep the current is flowing in the wire, will be around a centimeter for a wire running at 60 Hz.

Proximity effect: an alternating magnetic field flowing through a conductor can produce eddy currents in conductors that are adjacent. Eddy currents can result in a change in the distribution of current in a wire. The current in the adjacent wire may pool in the part of the wire that is farthest away from other conductors carrying current in the same direction.


Square or rectangular transformer wires, with thicker insulations, can sometimes come with an additional high-temperature tape made out of polyimide or fiberglass. The windings are also covered with a varnish that extra insulating and is meant to help improve the strength, and the long-term reliability, of the wire.

The turns of wires that are bare, meaning they have no insulation, would not be able to not touch each other as they would short out. The insulation serves to enhance the thermal endurance of the wire and protect the wire. In the case of a wire needing to have hundreds or thousands of turns, the wire will protection provided by the insulation is required. Transformer wire will always be insulated.

Different insulations will come with different thermal capacities, diameters (which are measured in both millimeters or inches), and AWG wire sizes.

Large power transformers that operate at high voltages may be insulated by oil-impregnated paper or pressboard.

TEMCo supplies Soderon 155 wire and GP/MR-200 wire, both of which are double coated.

Double coating wire has two purposes: to strengthen the wire, and increase its durability. Double coating also lets the wire take on the characteristics of each insulation. In the case of GP/MR 200, it has higher dielectric properties and chemical resistance due to a polyester-imide base and a polyamide-imide overcoat. Soderon 155 has higher resistance to solvents and greater windability because of polyurethane insulation with an overcoat of polyamide.

Some transformers have windings that have an epoxy resin for protection. This seals the windings, preventing dirt or water from being absorbed. These transformers can be used in environments that are damp or dirty environments.

Wire Gauge

Wire diameter can be determined by the American wire gauge (AWG) rating of the wire. AWG is used in the U.S. and Canada to represent specific characteristics of round wire, such as diameter, resistance and current.

Diameter and AWG number are inversed: as one decreases, the other number increases. A wire that has a diameter of 5.827 millimeters will have a correlating AWG size of 3, while a wire with a diameter measuring 0.101 millimeters will have an AWG size of 38. The smallest gauge wire will have a diameter of 11.684 mm, while the largest gauge wire has a diameter of 0.0799 mm.

In total, there are 44 standard wire gauges. They range from 0000-40.

Transformer Wire Gauge


Like the wire diameter, wire weight will be based on AWG standards and will have an inverse relationship with the gauge, meaning as the gauge becomes greater, the weight of the wire will decrease.

The largest wire weight is 640.5005 lbs per 1000 feet, with a gauge of 0000. For a 40 gauge wire, the weight is 0.0299 lbs per 1000 feet.

Wire is calculated by weight, not length. This is for accuracy, since it is much quicker and easier to weigh a wound wire than it is to determine the length of long distances of wire.

Temperature Rating/Thermal Class

The thermal class, or temperature rating, is the temperature at which the wire will have a service life of 20,000 hours. The service life of the wire will be extended if the wire is used at a lower temperature.

The thermal class will depend on what insulation is being used. 130°, 155°, 180° and 200° are common temperature ratings found in different types of insulation. The maximum thermal class is 250°.

Thermal class is measured in degrees celsius.

Bondable Transformer Wire

Bondable wire contains an extra adhesive film on top of the usual insulation that activate when heated up. It will bond the turn to turn windings of the wire into a self-supporting coil. eliminating the need for bobbins, the spindles around which the wire is wound.

Bonding agents include epoxy, polyester and polyamide.

Solvent, oven and resistance are the three main types of bonding.

  • Solvent bonding: will be applied either while the wire is winding, or the coiled wire will be dipped in the solvent after the winding has completed.
  • Oven bonding: the wire will be put into an oven after it is fully wound in order for the turns to bond. Oven bonding can take 10 to 30 minutes, depending on the size of the coiled wire.
  • Resistance bonding: requires an electric current to heat up the completed coil, which will then cause the wire turns to bond. The amount of voltage and time necessary to complete the process are dependent on the size of the wire and the design of the coil. Resistance bonding is typically used for wire sizes 34 gauge or higher.

Soldering Process

The transformer wire will eventually need to be soldered to a circuit board or a second wire. A metal with a lower melting point, probably lead or tin, will be melted down so it can fuse the wire when the metal solidifies.

Sometimes, wire insulation will need to be taken off before the wire can be soldered, but this is not always the case. Some wires, like the GP/MR-200 wire that TEMCo offers, need their insulation to be removed, while others, like the Soderon 155, have insulation that acts as a flux, meaning it will not need to be removed.

Soldering aluminum can ultimately lead to an increase in the likelihood of corrosion and failure since it does not solder well.

Breakdown Voltage

Breakdown voltage indicates the dielectric strength of the enamel insulation of the wire. The breakdown voltage can be of 3 types: Grade 1, Grade 2 and Grade 3. Insulations with higher grades will have higher breakdown voltages. Higher ambient temperatures result in a reduction of breakdown voltage.

The insulation thickness, or build, is the measurement of the enamel that has been added to the circumference of the bare wire. It can be determined by subtracting the total diameter of the wire from the diameter of the insulation and wire added together.

Insulation build can be single, heavy, triple or quadruple, with single and heavy being the most common.

Wires with smaller gauges will have higher breakdown voltages.

Where can I purchase my Transformer Wire?

Click the link below to view TEMCo's transformer wire selection. We offer wires in numerous different gauges, temperature ratings and insulations. Purchase your TEMCo transformer wire today!

View Soderon 155 Wire Selection View GP/MR-200 Wire Selection

Call us at 877.474.8209