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
I would think there would be a slight difference but very little and that would depend a lot on the operating frequency. Two quotes gleaned from Google and appear to be reliable sources:
"Litz wire is used to make inductors and transformers, especially for high frequency applications where the skin effect is more pronounced and proximity effect can be an even more severe problem. Litz wire is one kind of stranded wire, but, in this case, the reason for its use is not the usual one of avoiding complete wire breakage due to material fatigue."
"Litz wire conductors are beneficial for reducing A.C. losses in high frequency winding"
So this is just to make a point that Litz wire is used to improve effeciency, this would be especially so in SMPS like computer PSU's where the transformers operate at high frequencies. It's not used for inductance reasons as far as I am aware and probably makes little difference.
Best Answer
This is not an easy question to answer, as there are many things going on in wire at high frequencies.
While skin effect causes the current to travel nearer to the surface of the strands, the size of the stranding will have been chosen to make that effect tolerable. There is also the proximity effect - the field from a conductor affects nearby conductors and increases their impedance. On a regular spiral lay stranded wire with insulated strands, this would have the effect of greatly reducing the current in the inner strands of the bundle, hence the need for Litz wire, which has the stranding arranged such that each strand spends a more or less equal proportion of its length in the interior and exterior of the bundle, thus evening out the currents carried by each strand. The specific lay pattern of the wire therefore has an effect on the current distribution. The thermal dissipation of the wire is also not easy to determine, since the strands are insulated, the dissipation from the inside of the bundle by radial conduction from contact with other strands is limited, and the heat has to mostly follow the length of the strand to its next turn at the outside layer where it can be cooled by convection/conduction.
There's good write-up here