Construction grade cables and high-performance variable frequency drive (VFD) cables differ in several key areas.
High performance VFD cables are constructed with high strand count flexible tinned conductors. Construction grade cables usually have only 7 or 19 bare copper strands as required by construction standards
High performance VFD cables are more stable thermally at the connection points.
High performance VFD cables are more flexible.
High performance VFD cables are preferred for high frequency drive output components due to the much larger conductor surface area compared to construction grade cable.
Construction grade cables are usually insulated with cross-linked polyethylene (XLPE). This allows them to be sufficiently insulated but only at the minimum wall thickness requirements.
High performance VFD cables usually exceed the minimum wall thickness specifications. This results in significant reductions in capacitance and increases to permissible cable distances.
Construction grade cables use a single helically applied copper tape shielding in contact with three segmented grounds. These meet the minimum requirements of the National Electric Code; however, they may contain less than one equivalent full-sized ground.
High performance VFD cables have more copper at ground potential. This allows them to attract and contain common mode currents. High performance four-conductor cables shielded with foil braid and heavy drains may have more copper in the ground and shielding system than at the circuit potential. This can be the equivalent of three full-sized grounds exceeding the NEC requirement by over 300 percent. The extra ground copper allows the lowest ground impedance and the lowest common-mode emissions.
With larger drive systems, the ratio of common-mode current to working-current is reduced. As a result, the ratio of ground potential copper to circuit conduct may be reduced even as the size of the cable increases. Shielding changes from foil braided cables to dual copper tapes in contact with full sized equivalent symmetric ground conductors. The dual copper tapes provide the maximum surface area and outstanding high-frequency performance compared to the thicker single copper tape.
According to Belden, the use of symmetric design to prevent internally generated ground currents is a design factor in motors over 50 hp with long runs. It is a more significant factor in 100 hp or larger motors.