Introduction: Round and Sector Conductors
The cross‑sectional shape of a current‑carrying conductor is one of the fundamental parameters of a power cable. It directly affects the external diameter of the product, installation convenience, consumption of insulating materials, and even current‑carrying characteristics. In the cable industry, two main shapes are used: round and sector (sometimes called segmental). A round conductor is a classic cylinder; a sector conductor is a segment with three or four faces that allows the conductors to be packed more tightly within the cable.
The choice of shape is not arbitrary. For single‑core cables, the round conductor is practically the only option. For multi‑core cables, especially three‑ and four‑core types, the sector shape provides significant savings in dimensions and mass. The difference manifests in manufacturing costs, ease of stripping, and insulation reliability. More details on the construction and classification of power cables can be found in the article about power cables.
Comparison Table: Round vs. Sector Conductors
| Parameter | Round Conductor | Sector Conductor |
| Cross‑sectional shape | Circle (cylinder) | Segment, close to a triangle or sector of a circle |
| Use in single‑core cables | The only type used | Not used |
| Use in multi‑core cables | Mainly for cross‑sections up to 10–16 mm² | For cross‑sections from 16 mm² and above, in three‑ and four‑core cables |
| External cable diameter | Larger for the same cross‑section | Smaller for the same cross‑section, saving up to 15–20% |
| Consumption of insulation and sheath | Higher | Lower |
| Cable mass | Higher | Lower for the same cross‑section |
| Ease of stripping | Easier, standard terminals | More difficult; special sector‑shaped terminals required |
| Connection to terminal blocks | More convenient, especially for busbars | May require crimping into a round shape |
| Cable flexibility | Higher, especially with stranded conductors | Lower; sector conductors are usually solid |
| Current rating | Better heat dissipation | Slightly worse when tightly packed |
| Common cable grades | VVG, AVVG in small cross‑sections | AVBbShv, VBbShv in large cross‑sections |
Cross‑Sectional Geometry and Its Effect on the Cable
A round conductor has a uniform electric field distribution and maximum area for a given perimeter. However, when three round conductors are twisted together, significant voids remain between them, which are filled with insulation or screening. This increases the external cable diameter and material consumption. The sector conductor is designed so that three or four conductors form an almost tight circle. Each conductor has a cross‑section shaped like a sector of a circle with an angle of 120 or 90 degrees. When twisted, they form a compact core that is close to a cylinder in shape.
The saving in outer diameter reaches 15–20% compared to a cable with round conductors of the same cross‑section. For example, a four‑core cable with sector conductors of 120 mm² has an outer diameter about 10 mm smaller than the equivalent with round conductors. This is critical when laying in confined cable ducts and pipes. The minimum bending radius requirements also become less stringent because a smaller‑diameter cable can be bent to a smaller radius. A comparison of single‑core and multi‑core constructions is given in the article single‑core or multi‑core.
Design Features: Solid and Stranded Conductors
Round conductors can be solid or stranded. A solid round conductor is rigid and is used up to 10 mm² for copper and up to 16 mm² for aluminium conductors. Stranded round conductors provide flexibility class 2 or 5 and are used in flexible cables. Sector conductors are almost always solid, especially in aluminium versions, because aluminium is more easily formed into a sector. Stranded sector conductors are rarer and are used for large cross‑sections where a combination of compactness and some flexibility is needed.
A solid sector conductor is rigid. It cannot be bent to a small radius, and installation requires effort. Stranded round conductors, on the other hand, are flexible, but their use in multi‑core cables of large cross‑sections increases the diameter and insulation consumption. Therefore, manufacturers compromise: up to 16 mm², stranded round conductors are more common; from 25 mm², solid sector conductors are used. For aluminium cables, the sector shape is particularly advantageous because aluminium is lighter and cheaper than copper, and the savings on insulation and sheath are even more noticeable. More details on conductor material differences can be found in the article copper or aluminium.
Electrical Characteristics and Current Ratings
The resistance of a conductor depends on its cross‑section, material, and length, but not on the shape of the cross‑section. However, the shape affects heat dissipation. A round conductor has the maximum surface area for a given cross‑section, which promotes better cooling. A sector conductor, fitting tightly against adjacent conductors, dissipates heat less effectively, and the permissible continuous current may be 3–5% lower. In practice, this is accounted for in the PUE tables: derating factors may apply when calculating for cables with sector conductors.
Another difference is the skin effect and proximity effect. At alternating current, round conductors are more susceptible to skin effect at large cross‑sections, whereas the sector shape with its increased perimeter reduces this effect. This is one reason why sector conductors are used precisely in large cross‑sections, where proximity effects are already significant. For low‑voltage networks up to 1 kV, the difference in electrical characteristics is small, but when selecting the cross‑section, one should always refer to the standard tables rather than intuitive considerations.
Installation, Stripping, and Termination
A round conductor is significantly easier to install. Standard cable lugs, ferrules, and terminal blocks are designed for round shapes. When crimping a lug, a round conductor uniformly fills the tube, ensuring a reliable electrical contact. A sector conductor requires special lugs with an internal sector‑shaped profile or preliminary compression of the conductor into a round shape using a special die. This complicates installation and requires additional tools.
Stripping a sector conductor is also more labour‑intensive. The insulation on a sector conductor has a variable thickness around the perimeter, and when removing it, care must be taken not to damage the sharp edges of the conductor. When connecting to circuit breakers and busbars, sector conductors often have to be pre‑crimped to a round cross‑section, adding an extra step. In cramped switchgear, round conductors are preferable. Therefore, for indoor distribution boards, designers often choose cables with round conductors even for 25 and 35 mm² cross‑sections, despite the larger diameter.
Material Consumption and Cable Mass
Sector conductors provide noticeable savings in insulation materials. The PVC compound or cross‑linked polyethylene that fills the gaps between round conductors is almost unnecessary in a sector design. The cable sheath also becomes thinner because the outer diameter is smaller. In total, polymer consumption is reduced by 20–30%. Cable mass is reduced not only due to polymers but also due to the smaller overall dimensions, which facilitates transportation and installation.
For the manufacturer, sector conductors are cost‑effective: savings on expensive PVC compounds and rubber are significant. For the user, the benefits in mass and diameter mean the ability to use smaller cable ducts, trays, and joints. Aluminium cables with sector conductors are particularly popular in distribution networks where large cross‑sections are required and weight reduction is important. For example, AVBbShv 4×120 with sector conductors weighs almost a quarter less than the same cable with round conductors, if such a version were produced.
When to Prefer a Round Conductor
- Single‑core cables of any cross‑section – sector shape is not feasible there.
- Multi‑core cables with small cross‑sections up to 10–16 mm², where the diameter difference is small and installation convenience is more important.
- Flexible cables requiring stranded conductors of classes 2 and 5 – sector conductors of these classes are rare and expensive.
- Cables for connection to standard terminals, circuit breakers, and busbars where crimping simplicity is important.
- Concealed wiring in apartments and offices using cross‑sections of 1.5–6 mm².
When a Sector Conductor Is Justified
- Three‑ and four‑core power cables with cross‑sections from 25 mm² and above.
- Aluminium cables for distribution networks and underground laying, where mass savings are important.
- Armoured cables VBbShv, AVBbShv, where a compact core simplifies armouring application.
- Cables laid in cable ducts and pipes with limited diameter.
- Main feeder lines where the electrical losses due to shape are insignificant, but the gain in dimensions is large.
Typical Mistakes in Selection
- Choosing a sector conductor for a flexible cable. Such a conductor is rigid and cannot withstand repeated bending.
- Using a large‑cross‑section round conductor in a cable laid in a narrow pipe. The cable does not fit or is damaged during pulling.
- Attempting to crimp a sector conductor with an ordinary round lug. The contact becomes unreliable and heats up.
- Ordering an aluminium cable with round conductors of large cross‑section. It is more expensive and heavier than a sector version, with no practical benefit.
- Ignoring the conductor shape when calculating currents – one should use the PUE tables that account for the actual construction.
- Using a sector conductor in circuits requiring high flexibility, e.g., for mobile equipment connection.
Frequently Asked Questions
What is the difference between a round conductor and a sector conductor?
The cross‑sectional shape: round is a cylinder, sector is a segment resembling a sector of a circle. The sector shape is used in multi‑core cables to reduce the outer diameter.
Which conductor is better for apartment wiring?
Round. In apartments, small cross‑sections up to 6 mm² are used, where sector shape offers no benefit, and round conductors are easier to install.
Are stranded sector conductors available?
Yes, but rarely. They are used for large cross‑sections where a combination of compactness and some flexibility is required. Most sector conductors are solid.
Why are aluminium cables more often made with sector conductors?
Aluminium is easier to form into a sector, and the gains in mass and dimensions are especially important for aluminium cables used on long feeders.
How do you connect a sector conductor to a circuit breaker?
The sector conductor is pre‑crimped into a round shape using a special die, or special sector‑shaped lugs are used. Connection without pre‑crimping is not allowed.
Does the conductor shape affect the permissible current?
Marginally. A sector conductor has poorer cooling, so the permissible current may be a few percent lower. Exact values are given in the PUE tables.
Conclusion
The shape of a current‑carrying conductor is not just geometry but a carefully engineered decision. The round conductor is universal, easy to install, and indispensable for small cross‑sections and flexible cables. The sector conductor saves materials, reduces cable dimensions and mass, which is especially valuable for heavy aluminium lines. The choice between them is dictated by cross‑section, cable type, installation conditions, and termination method. Understanding these differences helps avoid mistakes during design and installation, ensuring reliable cable network operation throughout its service life.
