In enamelled conductor orders, the third parameter after dimensions and thermal class is the build class. Called Grade 1, Grade 2 and Grade 3 in IEC terminology, these classes define the thickness of the enamel film applied over the conductor — more precisely, the diameter increase the film causes.
What Does "Build Increase" Mean?
Instead of measuring film thickness directly, standards take the difference between the outer dimension of the coated conductor and the dimension of the bare conductor. In round conductor this corresponds to a "diameter increase", and in flat conductor to an increase defined separately over width and thickness. The minimum and maximum increase values allowed for each dimension step are given in the tables of IEC 60317-0-1 for round copper conductor and IEC 60317-0-2 for flat copper conductor.
This approach has a practical reason: what concerns you in the winding calculation is not the film thickness in microns, but the total space the conductor occupies within the winding.
Classes and Their Effects
| Class | Common name | Relative film thickness | Breakdown voltage | Effect on fill factor |
|---|---|---|---|---|
| Grade 1 | Single build | Thinnest | Lowest | Best fill |
| Grade 2 | Heavy build | About double | Markedly higher | Medium |
| Grade 3 | Triple build | Thickest | Highest | Lowest fill |
The word "relative" in the table is deliberate: the exact increase values change with the dimension step and must be read from the tables of the relevant IEC 60317 section. At small diameters the absolute difference stays in the micron range, while the same class difference visibly changes the winding height at large diameters.
Five Questions That Decide the Choice
1. How high is the adjacent-turn voltage in the winding?
If the voltage difference between turns is high (especially at the entry turns and in windings exposed to impulse voltage), a thicker film is needed. Grade 2 has become the default choice in the industry; Grade 1 is used mostly in low-voltage small coils.
2. Is the machine inverter-fed?
In frequency-converter drives, fast-rising impulses can start partial discharge in the turn insulation. In these applications a thick film or special products with enhanced partial-discharge resistance are preferred.
3. How tight is the winding window?
Choosing Grade 3 raises the breakdown strength but lowers the fill factor. If the window is at its limit, the build increase comes back directly as fewer turns or lost section.
4. How much is the conductor stressed during winding?
A narrow mandrel radius, hard guides and high tension increase the risk of film damage. A thick film is more tolerant of mechanical damage.
5. What are the impregnation and thermal cycle like?
Stresses form on the film during varnish absorption and thermal cycling; in critical applications one class higher improves long-term reliability.
Effect on the Winding Line
A change in build class is not just a decision on paper; it produces concrete results on the winding line. Because a thick film enlarges the outer diameter of the conductor, the number of turns fitting the same die decreases. At layer transitions the conductor flexes less, so the mandrel radius and guide geometry must be reviewed. In return, a thick film is more tolerant of friction and scratching during winding.
When moving to a thin film, fill factor is gained but winding precision must rise. On lines with misadjusted tension, worn guides or high speed, the risk of film damage rises. Before lowering the build class, the safe path is to verify with a trial batch on the winding line.
There is an effect on the impregnation side too: a thick film narrows the voids through which varnish or resin can penetrate the winding. In systems using vacuum impregnation this may require readjusting the process parameters.
NEMA Equivalent
Manufacturers working for the North American market encounter NEMA MW 1000 terminology. Here, instead of Grade 1/2/3, the terms "single build", "heavy build" and "triple build" are used. Although the conceptual correspondence largely overlaps, the tolerance values are not exactly the same; the dimension tables must be compared when switching between the two standards.
Is Grade 2 Always the Safe Choice?
Grade 2 has become the industry default and is a reasonable balance in most applications. But the "thicker is safer" reflex misleads in two situations. First, in designs where the winding window is at its limit: unnecessary thickness directly affects electrical performance by reducing the number of turns. Second, in applications working at very thin diameters; here the film thickness reaches a significant share of the conductor section and the fill loss grows quickly.
The right approach is to choose the build class according to the winding's real electrical and mechanical stress, then verify it with a trial batch on the winding line. A class-change decision should not be made without measurement and testing.
Summary
- Grade defines the film thickness through the diameter/dimension increase.
- Grade 2 is the default choice in most industrial applications.
- A thick film raises breakdown strength and lowers the fill factor.
- Binding increase values must be read from the IEC 60317-0-1 / 60317-0-2 tables.
