When designing or selecting an insulated winding wire for use in a wound component, it is important to consider all of the following: 

  • What is the range of operating temperatures that the product will see in your application? 
  • What temperatures will the wire be subjected to in downstream manufacturing processes? 
  • What is the expected/required thermal life for the end-product? 
  • What voltage rating is required for this application (including transient overvoltages)? 
  • What are the regulatory and electrical testing requirements that I should be concerned with? 
  • Is basic, supplementary, or reinforced insulation required for this application? 
  • Is the wire being used flexible enough for the bending radius used in my application? 
  • Is there a UL recognized insulation system out there with both this wire and the bobbins, core tubes, tapes, varnishes, etc. that I would like to use with it? 

This article will focus on last three questions above. The previous articles in the series focused on the other questions dealing with temperature ratings, voltage ratings, and related concerns. Having now trimmed down the list of potentially suitable materials and products, it is a good time to consider how the remaining possibilities may affect your particular application. 

  • Insulation: There are three “levels” of electrical insulation when it comes to insulated winding wires. These insulation levels are basic insulation, supplementary insulation, and reinforced insulation. Regulatory standards should be consulted to determine what level of insulation is required for your application. These levels were originally defined in IEC 60950 Ed. 2.0 as: 
  • BASIC INSULATION: Insulation to provide basic protection against electric shock 
  • SUPPLEMENTARY INSULATION: Independent insulation applied in addition to BASIC INSULATION in order to reduce the risk of electric shock in the event of a failure of the BASIC INSULATION 
  • REINFORCED INSULATION: Single insulation system that provides a degree of protection against electric shock equivalent to DOUBLE INSULATION under the conditions specified in this standard 

To clear up any confusion with the definition for reinforced insulation above, here’s the definition of double insulation per IEC 60950-1 Ed. 2.0: 


To clarify, double insulation is the combination of a basic insulation plus a supplementary insulation. So, here’s what these definitions mean in terms of Rubadue’s products: 

  • Single Insulated Wire (SIW) has Basic Insulation (1 Layer of Insulation) 
  • Double Insulated Wire (DIW) has Supplementary Insulation. (2 Layers of Insulation) 
  • Triple Insulated Wire (TIW) has Reinforced Insulation. (3 Layers of Insulation) 


  • Flexibility: Bending radii are typically very tight for transformer windings. As a result, the flexibility of the product can be a concern that needs to be evaluated. The conductor used in the wire and the insulation material can both contribute to or reduce the flexibility of the overall product. 
  • Stranded conductors are more flexible than solid conductors, and they can be bent more sharply and more easily than an equivalent solid conductor. Where tight bending radii are employed, stranded conductors may prove to be advantageous. The terms “soft bare copper” and “soft tinned copper” can be misleading when you’re working with larger solid conductors like a 12 AWG solid. 
  • Insulated winding wires having extruded insulations can typically be bent around tighter radii than similar wires with taped insulations. Bending a wire with a taped insulation will cause the layers of tape to move and slide over one another, potentially reducing the overlap/coverage of the tape, and thus the number of layers of insulation covering some points. Taped insulations also have a greater tendency to buckle when bent around too tight of a radius.  


  • Insulation System Compatibility: A combination of chemically incompatible materials can lead to accelerated failure in a transformer or other magnetic component. UL created Electrical Insulation Systems to cut down on the amount of testing required to ensure the chemical compatibility of the various components used in transformers.  


Electrical Insulation Systems contain various combinations of filament wires (insulated winding wires), enameled wire (magnet wires), ground and interwinding insulations, tapes, core tubes, bobbins, spacers, wedges, varnishes, etc. that have been tested together to confirm that the components in the system are chemically compatible. Rubadue’s insulated winding wire products have the benefit of being classified as filament wires in applicable electrical insulation systems. This classification means that those products can be used without additional ground and interwinding insulations and allows for reduced creepage and clearance requirements. 


UL allows transformer manufacturers to use or adopt an electrical insulation system when issuing safety certifications, which helps them to reduce their regulatory burdens when qualifying new products. Rubadue Wire maintains two Class F (155°C) Electrical Insulation Systems with UL under file number E188330: RXT-2 and TCA. Our customers can adopt these systems at no charge from us. Rubadue Wire products can also be found in other suppliers’ systems. Contact the factory for more information. 


Need further assistance with determining what insulated winding wires might be appropriate for your application? Contact our knowledgeable sales and engineering teams to discuss your application at sales@rubadue.com or +1(970) 351-6100. 

IEC 60950-1 is a copyrighted standard owned by the International Electrotechnical Commission.