Skip to main content

Continuously Transposed Conductors

Continuously Transposed Conductors (CTCs) are used in Large Power Transformers, generally more than 50 MVA, Furnace Transformers, and Traction Locomotive Transformers. The current production range of CTC is between 5 to 72 strands with a variety of different insulating materials.

What is CTC?

CTC consists of a group of enamelled rectangular wires, which are connected up parallel to the ends. In the group each strand (enamelled rectangular wire) successively and repetitively takes up every possible position in the whole cross section. The bunch is wrapped with oil compatible pure cellulose paper.

Where is CTC used?

  • Transformer coils wound with CTC
  • Large Power Transformers, generally > 1 MVA
  • Furnace Transformers
  • Traction Loco Transformers
  • Chokes
  • Inductance Coils

What are its advantages?

  • Transformer coil winding with CTC helps in compact design of transformers, compared to Paper insulated copper conductor.
  • Paper less (netted) CTC is used in special transformers such as Furnace and Freight Loco, for improved cooling.

What is Transposing?

The enamelled wire conductors within CTC are arranged in two parallel stacks. One stack having one more conductor that the other. The conductors are continuously transposed by machine action moving them from position to position in discrete steps:

Step 1: Conductor 1 is moved from the top of the left hand stack to the top of right hand stack.

Step 2: The right hand stack is moved down one conductor thickness in relation to the left hand stack.

Step 3: Conductor 3 is moved from bottom of right hand stack to the bottom of left hand stack.

Step 4: The left hand stack is moved up one conductor thickness in relation to the right hand stack.
At the end of four transposing step, as shown in the figure, the CTC conductors have returned to the original geometry, but each conductor advanced one position clock-wise.



  • The copper conductor used is Electrolytic Tough Pitch (ETP) or Oxygen Free Copper With High conductivity, for controlled low losses in electrical machines.
  • The number of strands includes all numbers between 5 to 43; odd numbers can give higher cross section are in CTC. Paper tapes up to 24 wrapping is applied as external insulation over the transposed conductor bunch.
  • Besides fully annealed copper conductor, PRECITRANS CTC is available with copper conductor with controlled proof stress, for increased mechanical strength for Transformer Winding. Standard Proof Stress Grade are as under:


PRECITRANS CTC Current Manufacturing Range

Enamelled Rectangular Winding Wires used in CTC

Enamelled Rectangular Winding Wires are one of the key ingredients of CTC. Wire Enamel being a critical input of Enamelled Rectangular Winding Wires is carefully chosen to give the desire properties like abrasion resistance, transformer oil resistance, temperature resistance, dielectric strength, etc.

Please refer to the following table for guidelines regarding Enamel Selection and increase in dimensions due to Enamel insulation.

Note: The above increase in dimensions can be used as a guideline. Specific requirements can be executed as per Customer request.

Paper Arrangement

Transposed Enamelled Conductors / Strands are wrapped with multiple layers of oil resistant Kraft and/or other special papers to give high dielectric and mechanical strength to the CTC. The paper rolls can be slitted into required width tapes.

pecial papers, depending on customers requirement can be used.

Unless otherwise agreed with the customer, the insulation shall consist of at least 3 layers of 0.080 mm thick Kraft paper.

The paper covering is applied in the following arrangement:

  • Papers are wound up to 8 papers in the same direction
  • Above 8 papers, in groups of maximum 8 papers
  • All inner layers are but lapped and staggered from 25 to 40%
  • The two outermost layers are wound interlocked with about 50% overlap

Agreement with customer is required for any change in thickness of papers and the type of wrapping arrangement, in order to reach required paper insulation thickness.


Calculation of the cable outer dimensions
Maximum dimension in axial direction:

Maximum dimension in radial direction

H = Axial cable dimension
B = Radial cable dimension
h = Axial strand dimension
b = Radial strand dimension
Kh = Maximum plus tolerance 0.10 mm for axial dimension
Kb = Maximum plus tolerance for radial dimension (values in table)
n = Number of strands in the cable
iE = Increase in dimensions due to enamel (0.13 for pva and 0.19 for pva plus epoxy)
ip = Thickness of separator between the strands stacks.
ic = Paper covering (in case of cable without paper ic/2 is the thickness of the plastic wire)

Kb Values

S = Transposing distance (Transposing pitch), distance between two transpositions.

External dimensions are tested under pressure of IN/mm² or the value required by the customer.


An intercolumn separator between the two stacks of CTC conductors is inserted, for all stack heights more than 10 mm, unless otherwise agreed to with the user.

The standard paper thickness for this is 0.100-0.110 mm thick.

Intercolumn paper is generally inserted if:

Standard thickness of intercolumn separator paper, besides 0.100 mm, are 0.20, 0.30, and 0.40 mm

PRECITRANS and PRECITRANS-Bond CTC is generally supplied on wooden non-returnable drums. On agreement with the customer CTC can also be supplied on returnable wooden spools.

CTC is also supplied with separators to permit two or more parallel winding of CTC in transformer coil, for improved winding efficiency at customer.

Note: All dimensions in mm

We need the following information from our valued clients to understand fully your requirement and deliver the right product :

  • Number of Conductors (N)
  • Conductor Width (mm)
  • Conductor Thickness (mm)
  • Corner Radius Conductor (mm)
  • Mechanical Properties (N/mm2)
  • Type of Paper
  • Paper Increase (mm)
  • Number of Papers (N)
  • Type of Enamel
  • Enamel Increase (mm)
  • Type of Intercolumn Paper
  • Thickness of Intercolumn Paper
  • Inner Winding Diameter
  • Overall Maximum Dimensions of CTC (Height in mm & Width in mm)
  • N of Length
  • Meters per Length
  • Reel Type
  • Number of Reels
  • Separator details (if any)
  • Total Copper in Kgs
  • Total CTC in Kgs

For any other special requirement, please get in touch with us via