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 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 the questions on voltage ratings and electrical testing requirements. This article will also touch upon other electrical properties that you may want to consider. The previous article in the series, linked here, focused on the first three questions regarding temperatures and temperature ratings. The other considerations will be addressed in a future article. 

  • Voltage rating: The winding wire chosen for a wound component should have a voltage rating that is greater than or equal to the normal operating voltage. It may also be necessary to factor in transient overvoltages in some cases. Voltage ratings can be found in the manufacturer’s UL OBJT2 file and are typically found on the manufacturer’s product datasheets as well. 
  • Dielectric voltage withstand test requirements: Insulated winding wires manufactured and tested to UL 2353, IEC 60950-1, IEC 62368-1, IEC 61558-1, or IEC 60601-1 are required to pass a dielectric voltage withstand test at a minimum of these test voltages.  
  • Basic or supplementary insulation: 3,000 Vrms (twisted pair), 1,500 Vrms (single wire) 
  • Reinforced insulation: 6,000 Vrms (twisted pair), 3,000 Vrms (single wire) 

The wires are tested using a test voltage oscillating at 50 or 60 Hz for a period of 60 seconds.  

If your product is required to pass a dielectric voltage withstand test exceeding these values, then it may be necessary to consult the manufacturer to determine if their product is suitable for your application. The product’s breakdown voltage may also provide some insight into which products may be able to meet your testing requirements.  

Additional testing may also be necessary for operating frequencies above 500 kHz. 

  • Overvoltage category:  It is important to consult the applicable product or device safety standard, i.e. IEC 62052-31 for electricity meters, to determine what overvoltage category may apply to your particular application. The insulated winding wire(s) chosen for your application will need to be able to survive the dielectric voltage withstand tests that apply for that overvoltage category.  
  • Dielectric Constant: A material having a low dielectric constant is a better electrical insulator than a material having a high dielectric constant. ETFE, FEP, and PFA resins have excellent dielectric constants. Tables 1 and 2 below have typical dielectric constants for various extruded and tape insulation materials. 

For those concerned about partial discharge, materials with low dielectric constants can also be advantageous. Dakin et al1 established that there is a correlation between corona threshold voltages and the ratio of insulation thickness to the dielectric constant of the insulation material. If the insulation thickness is kept constant, then the resulting corona threshold voltage will generally be higher for insulation materials with lower dielectric constants.  

  • Dielectric Strength: Typical dielectric strength values for fluoropolymer resins are listed in Table 1 below. These values do not constitute voltage ratings and are for reference only. 


Table 1: Extruded Insulation Materials 

  ETFE (Ethylene Tetrafluoroethylene)  FEP (Fluorinated Polyethylene)  PFA (Perfluoroalkoxy) 
Common Trade Name  Tefzel™  Teflon™  Teflon™ 
Temperature Class  F  F  H 
Max. Operating Temperature  155°C  155°C  180°C 
Typical Dielectric Constant  2.7  2.0  2.0 
Typical Dielectric Strength (0.010 in)  1,700 V/mil  2,000 V/mil  2,000 V/mil 

Table 2: Taped Insulation Materials 

  PET (Polyethylene Teraphthalate)  PPS (Polyphenylene Sulfide)  PEN (Polyethylene Naphthalate)  PI (Polyimide) 
Common Trade Name  Mylar  N/A  N/A  Kapton® 
Temperature Class  N/A*  F  H  H 
Max. Operating Temperature  N/A*  155°C  180°C  180°C 
Typical Dielectric Constant  3.0  3.0  3.2  3.6 
Notes  * Typically used under other tape layers      Typically FEP-backed 


Need further assistance with determining what insulation materials 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. 

Tefzel™ and Teflon™ are trademarks of Chemours. 

Kapton® is a trademark of DuPont Electronics, Inc. 



  1. T. W. Dakin, H. M.Philofsky and W. C. Divens, “Effect of electric discharges on the breakdown of solid insulation,” in Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics, vol. 73, no. 2, pp. 155-162, May 1954, doi: 10.1109/TCE.1954.6372131.