Magnet wire or enamelled wire is a copper or aluminium wire coated with a very thin layer of insulation. It is used in the construction of transformers, inductors, motors, speakers, hard disk head actuators, potentiometers, electromagnets, and other applications which require tight coils of wire.
The wire itself is most often fully annealed, electrolytically refined copper. Aluminium magnet wire is sometimes used for large transformers and motors, but because of its lower electrical conductivity, an aluminium wire must have 1.6 times the cross sectional area as a copper wire to achieve comparable DC resistance. Due to this, copper magnet wires contribute to improving energy efficiency in equipment such as electric motors. For further information, see: Copper in energy efficient motors and Copper wire and cable: magnet wire (Winding wire).
Smaller diameter magnet wire usually has a round cross section. This kind of wire is used for things such as electric guitar pickups. Thicker magnet wire is often square or rectangular (with rounded corners) to provide more current flow per coil length.
Modern magnet wire typically uses one to four layers (in the case of quad-film type wire) of polymer film insulation, often of two different compositions, to provide a tough, continuous insulating layer. Magnet wire insulating films use (in order of increasing temperature range) polyvinyl formal (Formvar), polyurethane, polyamide, polyester, polyester-polyimide, polyamide-polyimide (or amide-imide), and polyimide. Polyimide insulated magnet wire is capable of operation at up to 250°C. The insulation of thicker square or rectangular magnet wire is often augmented by wrapping it with a high-temperature polyimide or fiberglass tape, and completed windings are often vacuum impregnated with an insulating varnish to improve insulation strength and long-term reliability of the winding.
Other types of insulation such as fiberglass yarn with varnish, aramid paper, kraft paper, mica, and polyester film are also widely used across the world for various applications like transformers and reactors. In the audio sector, a wire of silver construction, and various other insulators, such as cotton (sometimes permeated with some kind of coagulating agent/thickener, such as beeswax) and polytetrafluoroethylene (Teflon) can be found. Older insulation materials included cotton, paper, or silk, but these are only useful for low-temperature applications (up to 105°C).
For ease of manufacturing, most new magnet wire has insulation that acts as a flux when burnt during soldering.  This means that the electrical connections at the ends can be made without stripping off the insulation first. Older magnet wire is normally not like this, and requires sandpapering or scraping to remove the insulation before soldering.
Breakdown voltage depends on the thickness of the covering, which can be of 3 types: Grade 1, Grade 2 and Grade 3. Higher grades have thicker insulation and thus higher breakdown voltages.
The temperature class indicates the temperature of the wire where it has a 20,000 hour service life. At lower temperatures the service life of the wire is longer (about a factor 2 for every 10 °C lower temperature). Common temperature classes are 105° C, 130° C, 155° C, 180° C and 220° C.
Current density 
In practice maximum current density can vary from 2.5A/mm2 for a wire isolated from free air to 6A/mm2 for a wire in free air. If the wire is carrying high frequency currents (above 10kHz) the skin effect may affect the distribution of the current across the section by concentrating the current on the surface of the conductor.
If active cooling is provided by blowing air or circulating water much higher current densities can be achieved - proportionally to the effectiveness of cooling.