The Complete Guide to Magnet Wire: Types, Coatings, and Temperature Ratings

What Is Magnet Wire?

Magnet wire is a conductor (usually copper or a copper alloy) coated with a thin layer of electrical insulation. It is designed specifically for winding electromagnetic coils in motors, transformers, inductors, generators, and other devices where many turns of wire must be packed into a tight space. The insulation allows adjacent turns to sit directly against each other without shorting, while remaining thin enough to maximize the amount of conductor that fits within a given volume.

The term "magnet wire" can be confusing because the wire itself is not magnetic. The name refers to its use in creating electromagnets and magnetic components.

Conductor Materials

Copper

Copper is by far the most common conductor material for magnet wire. It offers the second-best electrical conductivity of any metal (after silver), excellent thermal conductivity, and good mechanical properties for winding. Copper magnet wire is available in a wide range of gauges and insulation types, making it the default choice for the vast majority of inductors and transformers.

Key properties of copper conductor:

• Electrical resistivity: 1.72 micro-ohm-cm at 20°C
• Temperature coefficient of resistance: 0.393% per °C
• Density: 8.96 g/cm3
• Tensile strength: adequate for machine winding at most gauges

Nickel and Nickel-Clad Copper

Nickel wire and nickel-clad copper wire are used in applications where copper alone cannot meet the requirements. Nickel offers superior corrosion resistance and maintains mechanical integrity at higher temperatures than copper. It is commonly found in aerospace, military, and high-reliability applications.

The tradeoff is higher electrical resistivity (approximately 7x that of copper), which means higher DC resistance for the same gauge and length. In applications where resistance is less critical than durability, such as certain current-sensing transformers, nickel wire can be the appropriate choice.

Aluminum

Aluminum magnet wire is sometimes used as a cost-saving alternative to copper. It is lighter (about one-third the density of copper) and less expensive per pound. The disadvantage is its conductivity, roughly 61% that of copper, which means a larger cross-section is needed for the same current-carrying capacity. Aluminum is more common in large power transformers where weight and material cost savings are significant.

Insulation Types

The insulation coating on magnet wire is what makes it possible to wind hundreds or thousands of turns in close proximity without electrical shorts. Several insulation chemistries are available, each with different temperature ratings, mechanical properties, and chemical resistance.

Polyurethane

Polyurethane insulation is popular for its solderable properties. The coating can be melted through by a soldering iron tip, allowing termination without stripping. This makes it convenient for prototype work and automated soldering. Temperature rating is typically Class 130 (130°C) or Class 155 (155°C) for modified formulations. It offers good windability and a smooth, uniform film.

Polyester (THEIC-Modified)

Polyester insulation provides good chemical resistance and mechanical toughness. Modified polyester formulations using THEIC (tris-hydroxyethyl isocyanurate) achieve Class 180 (180°C) ratings. This insulation type is widely used in motors and transformers where operating temperatures exceed the capability of polyurethane.

Polyamide-Imide (AI)

Polyamide-imide provides the highest temperature rating commonly available, typically Class 200 (200°C) or higher. It also offers excellent resistance to chemicals, solvents, and mechanical abrasion. This makes it the insulation of choice for demanding applications including automotive under-hood components, high-performance motors, and industrial equipment operating in harsh environments.

Polyester-Imide with Polyamide-Imide Overcoat

Dual-coat insulation systems combine a polyester-imide basecoat with a polyamide-imide topcoat. The basecoat provides good adhesion to the conductor and thermal endurance, while the topcoat provides mechanical toughness and chemical resistance. This combination achieves Class 200 ratings with better overall performance than either coating alone.

Single-Coat Enamel (NEMA MW80-C)

The NEMA MW80-C specification defines a single-coat polyester or polyurethane enamel insulation rated to Class 155 (155°C). This is one of the most commonly specified insulation types for general-purpose inductors and transformers. It provides a good balance of temperature performance, dielectric strength, and cost.

NEMA Temperature Classes

The National Electrical Manufacturers Association (NEMA) classifies magnet wire insulation by its maximum continuous operating temperature. These classes are defined in NEMA MW 1000 and correspond to thermal endurance test results.

NEMA Class	Max Temp (°C)	Common Insulation Types	Typical Applications
Class 105 (A)	105°C	Oleoresinous enamel, Formvar	Low-temperature general purpose
Class 130 (B)	130°C	Polyurethane, some polyesters	Small transformers, hobby motors
Class 155 (F)	155°C	Modified polyurethane, polyester, MW80-C	Industrial inductors, power transformers
Class 180 (H)	180°C	THEIC-modified polyester, polyester-imide	Motors, high-temp transformers
Class 200 (N)	200°C	Polyamide-imide, dual-coat systems	Automotive, aerospace, harsh environments
Class 220 (R)	220°C	Polyimide (Kapton film wrap)	Extreme-temperature, aerospace
Class 240+	240°C+	Ceramic, fiberglass wrapped	Furnace, foundry, extreme industrial

How Insulation Affects Winding

Build (Film Thickness)

Magnet wire insulation is available in different build thicknesses: single, heavy, triple, and quad. The build specification (defined by NEMA standards) specifies the minimum and maximum increase in diameter over the bare conductor. Heavier builds provide higher dielectric strength between adjacent turns but reduce the amount of copper that fits in a given window area.

For most custom inductor applications, single or heavy build insulation provides adequate turn-to-turn voltage withstand. Triple and quad builds are specified for high-voltage applications where the potential between adjacent turns is significant.

Impact on Fill Factor

Thicker insulation means each turn takes up more space. For a toroidal inductor where window area is limited, the choice between single and heavy build can determine whether the required number of turns fits. In extreme cases, moving from heavy to single build can add 5-10% more turns in the same space, or allow the use of one gauge size larger for lower DCR.

Breakdown Voltage

Dielectric breakdown voltage is the voltage at which the insulation fails and allows current to flow between adjacent turns or between the winding and the core. For single-build Class 155 insulation on 26 AWG wire, the typical minimum breakdown voltage is approximately 5,000 to 7,000 volts. This is far more than needed for most inductor applications (where turn-to-turn voltage is typically millivolts to a few volts), but the margin provides assurance against insulation defects, mechanical damage during winding, and degradation over the product's service life.

Thermal Aging and Service Life

Insulation aging is a chemical process: the enamel slowly degrades through oxidation and thermal decomposition. The rate of degradation doubles for approximately every 8-10°C increase in operating temperature (following the Arrhenius relationship). This means that an insulation system rated for 20,000 hours at its rated temperature might last 40,000 hours if operated 10°C cooler.

For buyers, the practical implication is straightforward. Specify a temperature class that provides adequate margin above the expected maximum operating temperature of the winding (including self-heating from I-squared-R losses). A 20-30°C margin between maximum expected winding temperature and insulation class is a common design practice that significantly extends service life.

Wire Gauge Overview

The American Wire Gauge (AWG) system defines the diameter of the bare conductor. In the AWG system, larger numbers indicate thinner wire. The most common gauges for custom inductors range from 18 AWG (relatively thick, for high-current applications) to 34 AWG (very fine, for high-impedance, low-current designs).

AWG	Bare Diameter (inches)	Resistance (ohm/ft at 20°C)	Typical Use
18	0.0403	0.00639	High-current power inductors
22	0.0254	0.01614	General-purpose transformers
26	0.0159	0.04081	Medium-impedance inductors
30	0.0100	0.10320	Signal transformers
34	0.0063	0.26100	High-impedance current sensors

For a detailed treatment of wire gauge selection, including current capacity and the relationship between gauge, DCR, and core window area, see our dedicated article on AWG wire gauge selection.

Special Considerations

Solderability

Terminated leads must be stripped of enamel insulation and tinned with solder. For polyurethane-coated wire, the enamel can be thermally stripped by the soldering iron. For polyester, polyamide-imide, and other higher-temperature coatings, mechanical or chemical stripping is required before soldering. The solder type matters for RoHS compliance: lead-free solders such as SN100 (99.3% tin, 0.7% copper) have replaced traditional tin-lead solders in most applications.

RoHS and Environmental Compliance

Modern magnet wire is generally RoHS compliant by default. The conductor (copper, nickel, or aluminum) and common enamel insulation chemistries do not contain restricted substances. The area to watch is the termination: lead-free solder and compliant flux must be used. For more on this topic, see our article on RoHS compliance in magnetic components.

Moisture Absorption

Some insulation types absorb moisture from the environment, which can reduce dielectric strength and accelerate degradation. Polyamide-imide is particularly susceptible. For components operating in humid environments, conformal coating of the finished winding or hermetic sealing may be necessary to maintain long-term reliability.

Selecting the Right Wire for Your Application

When specifying magnet wire for a custom inductor or transformer, consider these factors in order:

1. Maximum operating temperature: This determines the minimum insulation class. Add margin.
2. Current requirement: This determines the minimum wire gauge (cross-sectional area).
3. Space constraints: The core window area limits the gauge and insulation build.
4. Conductor material: Copper unless the application specifically requires nickel or aluminum.
5. Termination method: Solderability, lead length, and compliance requirements.
6. Environmental exposure: Chemicals, moisture, vibration, and thermal cycling.