CTC (Details) Print

Technical details Continuously Transposed Conductors

Continuously Transposed Cable



Continuously Transposed Conductor (CTC) is utilized in powertransformers and chokes for reduced losses and low winding cost. For applications in oil-filled transformers the conductors are insulated with formvar based enamel.

The stranded conductors are covered with various types of insulating paper or tape. For dry transformers there are enamelled conductors (temp. index 180) available and an overall covering of Nomex® 410 or glass-tape.

The mechanical rigidity of a CTC under short circuit conditions can be improved by either using conductors with an increased copper hardness or with a thermobondable overcoat or combination of both.



Calculation of the Axial and Radial CTC dimensions.

The maximum overall dimensions including the papercovering can be calculated using the formula below.


where :

  • n = number of strips in CTC
  • t1= nominal thickness of the conductor
  • w1 = nominal width of the conductor
  • e = total enamel increase,
    PVA = 0.10 mm,
    PVA+EP = 0.16 mm,
    PVA+BCL = 0.14 mm
  • i=intercolumn spacer
  • p = overall paper covering
  • K = graded positive space allowance affecting the radial dimension, given in table 1
  • Δp = tolerance on paper increase, according, to IEC 60317-27, table 1
  • Wd = Windingdiameter
  • Ba = transposition pitch

Table 1:


If requested an intercolumn spacer can be incorporated which may be either:

  • paper of 0.10 mm thickness or
  • presspahn of 0.1 - 0.2 or 0.3 mm thickness 

To calculate the transposition pitch

A good windability is obtained if a full transposition is made within each turn of the winding.
From the client's minimum winding diameter the best pitch for transposition can be calculated with the formula:


If the transposition pitch does not fit the given formula other considerations such as the size of the conductor together with number of strands must be taken into account as well. Please seek our advice!

Size range

To define the full range of possible sizes and their combinations would require extensive tables.

The outer limits are given in the table below.


Hardened conductors with or without a thermobondable overcoat

Hardened copper conductors can be produced according to BS 1432: 1970 or other standards.


whole size range

CPR 1 140 - 170 MPa

whole size range

CPR 2 170 - 220 MPa

max. width 10 mm, cross-section 25 mm2

CPR 3 220 - 260 MPa

max. width 8 mm, cross-section 12 mm2

Silverbearing copper is used from 0.1 or 0.2% proofstress >= 220 Mpa

The thermal bonding of the conductors is achieved by a semi-cured epoxy overcoat. The shelf life of our bondable CTC is at least one year if stored at temperatures below 23°C.

After winding the coil, the single conductors of the CTC are bonded together by heat treatment during vapourphase drying process.

Small samples tested after curing demonstrate a full bond strength after about

  • 80 hours at 100°C

  • 30 hours at 110°C

  • 22 hours at 120°C

  • 15 hours at 130°C

The whole range of CTC can be supplied with a semi-cured overcoat.

Insulation of CTC

The insulation of CTC consists in general of a number of paper tapes, butt wound in the same direction.
The outer two layers are usually interlocked. On request, groups of layers can be wrapped in alternating directions and/ or the last layer can be wound with an overlap.

The available types of paper are:

  • Kraftpaper, standard density

  • Kraftpaper, high density, thermally upgraded

  • Kraftpaper, creped with high elasticity, thermally upgraded

  • Nomex® 410

The properties of the papers used, fully meet the requirements of national and international standards.

The increase due to the paper covering is the difference between the overall size and the size of the CTC after transposing. Adjustments of the paper covering to meet a given overall size are in general only measured on the width of the CTC and are not decisive for the height.

The thickness of the total paper covering is determined by lifting off a piece of insulation and measuring with a micrometer.

Non-standard insulations with glasstape or in combination with other insulating materials are possible on request.

Standard drum-types



The drum types used for deliveries of CTC are given in the pdf document below unless agreed otherwise.  On request we can also supply on non-returnable reels.

Drum dimensions:
Drum dimensions Drum dimensions

Smit Draad Standard CTC Specifications

Ordering data

Whether enquiring or ordering, it is of benefit to both parties to provide the following data:

  • Total length
  • Number of conductors
  • Thickness of single conductor
  • Width of single conductor
  • Increase due to enamel
  • Thickness and quality of intercolumn spacer
  • Paper quality
  • Increase due to paper covering and tolerance
  • Overall dimensions
  • Winding diameter, minimum
  • Makeup / length per drum
  • Customer technical specification (if available)
  • Information sheet CTC for ordering/enquiry 


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