How can I determine the voltage insulation rating of magnet wire?

The primary purpose of magnet wire insulation is to provide turn-to-turn or layer-to-layer isolation; as such, the voltages involved are relatively low, regardless of the overall component rating. Therefore the actual voltage rating is usually of little interest. For higher withstand voltages - between the winding and the core or other components - other forms of barrier are normally employed.

This must be the normal case : if the wire coating had to withstand high voltages, its thickness would increase; then there would be less copper in a given winding space, compromising the efficiency of transformers and motors, or wasting materials by requiring larger bobbins and cores to maintain the same performance. Thus, high voltage copper wire would have a vanishingly small market if it were available.

If you require an air cored inductor with high withstand voltage ratings, I suggest you plan on means other than the varnish coating to achieve those ratings. For example, multi-section bobbins, silicone potting, heatshrink sleeving, nylon mounting bolts and standoffs, or whatever is appropriate to the mechanical situation.

Quoting from the great Wiki

"Like other wire, magnet wire is classified by diameter (AWG number or SWG) or area (square millimetres), temperature class, and insulation class.

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."

Calculation of breakdown voltage (Test acc. To IEC 60851.5.4.2, cylinder)

The breakdown voltage depends mainly on the thickness of the insulation (see formula below), but also on the bare wire diameter, the application temperature of the coil and the type of enamel.

Calculation of average values Ds:

Ds = t x Vμ [Volt], with

Ds : breakdown voltage

T : increase due to insulation,

t = da – dnom, : wire diameters with and without insulation

Vμm = volts per micron insulation (dependent on type of insulation)

Example:

Test with cylindrical electrode (round wire)

dnom 0.071mm (bare wire nominal diameter)

da = 0.083mm (wire with coating)

t = da – dnom = 0.083 – 0.071 = 0.012mm = 12μm (thickness of insulation between wires)

Vμ = 205 V/μm, therefore

Ds = 12μ x 205 V/μ = 2,460 V

Most of the commonly available stuff is 500 or 600 volts. Here is an example.

To go a little deeper, there are a number of different coating used for depending on temp environment for example. Here is some info about some of the different materials. Many of the materials will have high dielectric withstand ratings, but there are pin holes that effectively lower the in use rating. There are also corona ratings which are often close to 600V, like here.