Why are conductors sector shaped?
Technical datasheets may make reference to conductor types: RE and RM are round conductors (solid and stranded) whilst SE and SM denote sector shaped conductors (again, solid or Class 2 stranded). Sector shaped conductors will generally only be used on conductors with larger cross-sectional area sizes, upwards of 25mm2. It more often applies to low voltage cables (3 core and 4 core).
When sector shapes are used the cross sectional area of the conductor can be packed into the smallest possible geometry; this in turn allows the overall size (the outer diameter) of the cable to be reduced. It has the added benefit of also therefore reducing the volume of material used in construction.
The table below illustrates this nicely, with the examples coming from the nominal diameters in BS6724 standard. You can see that in all cases, for both 3 and 4 core configurations, there is a minimum of 10% overall size reduction when using the optimised sector shaping compared to circular conductors.
| Cores x Cross-sectional Area size mm2 |
RM conductor (circular) Overall Diameter mm |
SM conductor (sector) Overall Diameter mm |
SM Reduction Difference |
|---|---|---|---|
| 3 x 25 | 26.7 | 23.6 | 12% |
| 3 x 120 | 48.1 | 40.4 | 16% |
| 4 x 25 | 28.9 | 26.1 | 10% |
| 4 x 120 | 52.6 | 47.1 | 11% |
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