FIW (Fully Insulated Wire) is an alternative wire to build switching transformers typically using TIW (Triple Insulated Wires). Due to the big choice of overall diameters it allows to produce smaller transformers at lower costs. At the same time FIW has better windability and solderability compared to TIW.

FIW is produced in a multiple coating process, which guarantees insulation without any defects.

ELEKTRISOLA FIW is approved as MW85C and according to OBJT2. It has been sold successfully for several years to the automotive industry and for applications which do not require UL approval acc. to UL 60950. As Elektrisola FIW is approved as MW85C it can be used by many Insulation Systems according to UL 1446. The Safety Standard IEC 62368-1 Edition 3 is approved and allows the use of FIW. The old Safety Standard IEC 60950 was withdrawn end of 2020.

The FIW Wire

Elektrisola developed a product based on a modified polyurethane with a life time acc. to IEC 60172 of 20,000 h at 180 °C, short designation P180. It is produced with multiple passes of individual covering of insulation and is in-line tested for high-voltage-continuity to assure the perfect insulation without any insulation defects.

FIW is defined with many Grades specifying different insulation thicknesses. FIW 3 is the smallest defined build, while FIW 9 is the biggest.

Elektrisola Standard FIW Grades ex stock are FIW 4 and FIW 6, as they give a good compromise between good technical performance and affordable costs. 


FIW is specified in different specifications. Basically there are Standards for enamelled wires, such as IEC and NEMA, but there are also Safety Standards, such as the old IEC 60950, replaced by IEC 62368-1, and UL Safety Standards, such as UL 2353.

In addition, some values are also specified in product standards as the Transformer Standard acc. IEC 61558-1.


  • IEC 60317-56 and 60317-0-7

  • NEMA MW85c

  • UL 2353

Test Conditions

  • IEC 60851

  • IEC UL 60950 Annex U

  • IEC 61558-1

  • IEC 61558-2-16

  • UL 2353

Advantages of FIW

  • choice of different insulation builds (with different insulation thicknesses) allows optimization of products like smaller transformers and gives a cost advantage
  • excellent solderability
  • superior windability
  • high temperature class of 180 °C, thermal life time acc. UL 60950 Annex U tested in transformers for temperature class 155 °C/130 °C

  • proven insulation system acc. UL 1446 of enamelled wires also with UL, used over many years 

Example 0.25mm FIW 6, 390°C, 2.4 sec.

Production Process

The basic production process is similar to a normal enamelled wire  Goto  Production Process, but has many more single wire passes - up to 120 - to generate the finally very thick enamel layer, as shown in below photo, where always 3 alternate layers were coloured to demonstrate the big number of layers.

Multiple layers of a 0.25 FIW 7

In addition, every production line is equipped with an In-Line High Voltage Continuity Tester, which checks the insulation of the wire over the complete length permanently to guarantee that there is no insulation fault.


The FIW wire is spooled on standard spools used in Europe and in Asia 

Goto  Spools/Packaging.

Delivery ex Stock

Many FIW wire configurations can be supplied ex stock. In the table  Goto  Dimensions the grey highlighted diameters and builds are typical stock items.

Technical Data

Standardized Properties of FIW

IEC 60317-56 describes a full Enamelled Wire Specification with mechanical, electrical, thermal and other characteristics like soldering

Goto  FIW acc. IEC 60317-56

UL 2353, rather similar to IEC 60950 Annex U, gives mainly electrical properties which mostly are rather short-term tests

Goto  FIW acc. UL 2353

The new Safety Standard IEC 62368-1 continues with the same requirements, taken partly from the Transformer Standard IEC 61558-1 for FIW.

Breakdown Voltage

Breakdown Voltage (BDV) values for FIW will vary depending upon which standard is used to calculate these values.

When using the enamelled wire standards for FIW (IEC 60317-0-7 and 60317-56) the BDV is calculated using the minimum insulation increase per size, (min. insulation increase = min. OD including insulation – nominal bare wire dimension).

Goto  BDV calculation acc. IEC 60317-0-7

The minimum values can be found in IEC 60317-56

Goto  Minimum BDV acc. to IEC 60317-56

The Transfomer Standard IEC 61558-2-16 was finally corrected as the BDV calculation in the old version mistakenly used the minimum insulation increase divided in half, erroneously resulting in approximately half the BDV values as the magnet wire standards.

IEC 61558-1 as a new version of IEC 61558-2-16 has already been released, correcting errors (like the BDV calculation described above) found in the previous version. One major difference: The new version requires the FIW to maintain durability for dielectric strength for one minute at 180 °C with a factor of 0.85 compared to IEC 60317-0-7. Here the dielectric strength is measured by Breakdown Voltage (rms) at room temperature.

Goto  Minimum BDV acc. to IEC 61558-1

IEC 62368 follows the BDV values of IEC 61558, but does not specify 180 °C as test temperature as IEC 61558

Goto  Minimum BDV acc. IEC 62368

Goto  Comments on different BDV standards

Calculation of Breakdown Voltage

The minimum breakdown voltage acc.  to IEC 60317-0-7 can be calculated with the nominal diameter and the FIW Grade by tool 

Goto  Tools for Calculation of BDV


Dimensions of FIW depend on the Insulation Grade, which describes the amount of insulation on the bare wire

Goto  Dimensions FIW

Voltage or necessary FIW Grade by diameter  for a given voltage can be calculated by a calculator 

Goto  Calculator BDV/Grade

Weight / Length

Due to the extraordinary thickness of the insulation of FIW the length of a certain quantity of FIW and the weight of a wire with a certain length deviate remarkably from experiences with normal enamelled copper wire.

The length of 1 kg of FIW 3 - FIW 9 for nominal diameters in the range of 0,071 - 0,710 mm - assuming nominal outer diameters acc. to IEC 60317-56 - can be looked up in the  Goto  FIW Length Table on the next page in km/kg.

The weight of 1 km of FIW 3 - FIW 9 for the same range of nominal diameters - with nominal outer diameters acc. to IEC 60317-56 as before - can be looked up in the 

Goto  FIW Weight Table

The weight or length of a specific FIW wire can be checked by a calculator 

Goto  Calculator Length/Weight


  • UL Insulation Systems acc. UL 1446
  • IEC 61558-2-16 for transformers (SMPS) specifies use of FIW and IEC 61558-1
  • IEC 62368-1 as new Safety Standard, replacing IEC 60950