VFD vs. THHN: Is VFD Cable A Necessity For Variable Frequency Drives?
A VFD cable for variable frequency drives is a premium product that is significantly more expensive than copper building wire used for industrial purposes, such as THHN. This prompts contractors to pick THHN for their VFD installations. But is this THHN a viable solution? Turns out there are good reasons why THHN is not recommended for VFD machines, and this is not just the marketing strategy of VFD manufacturers.
NFPA 79 Considerations On Cables Used In Motor Machines
While there were no comments on using THHN in motors before, the 2018 edition of the NFPA 79 introduced several corrections. Now, power correction equipment that belongs to adjustable speed drive systems and servo drive systems has to be supplied exclusively by flexible motor supply cables. These cables include RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2, or other wires, based on the instructions given by the manufacturers. What all of these cables have in common is the fact that they have a cross-linked XLPE thermoset insulation as opposed to the plastic PVC insulation of THHN/THWN-2 cables.
For now, these restrictions exclusively deal with the cables in the panel between the drives and protection equipment. However, it is highly logical that the restrictions will concern all cables used in the VFD machines in the nearest future.
What Is Wrong With Using Thermoplastic THHN in VFD machines?
While THHN cables are convenient and low-cost, they are a bad fit for VFD machines despite still being a popular choice among many contractors.
The problem with using THHN in VFDs stems from the fact that THHN/THWN-2 cable has a thermoplastic PVC insulation that does not perform well in these applications. In the simplest of terms, THHN/THWN-2 is a bad fit for a voltage over 600V. In the modern VFD machine, the voltage can reach as high as 1, 3KV, and spikes in voltage are common.
Under high temperatures and a voltage higher than 600 volts, PVC insulation can reverse back into having bubbles that were present at the manufacturing stage when the insulation was formed from the liquid state. Because of these bubbles, the THHN cables in VFD machines are subjected to the corona discharge effect and high voltage. In extreme circumstances, there might be signs of burning on the insulation if the insulation fails to protect the cable from the impact of high temperatures. The insulation can also melt or deform. For all of these reasons, it is not the best option to use THHN in VFD system components.
Why VFD Cable Works Best For VFD Machines
Obviously, it makes sense that a VFD cable is excellent for VFD constructions as it is specifically designed for these applications. They offer consistent performance despite the upscale pricing. When it comes to the long-term industrial performance of the equipment, VFD cables are a worthy investment. Here are the reasons why VFDs work best:
- VFD cables have high-quality foil+ tinned copper braid shielding that protects the cable from EMI.
- XLPE insulation, which works in a manner similar to heat shrink tubing, is the best option when it comes to surviving voltage spikes and high temperatures.
- Stranded conductors protect the cable from voltage drop.
Alternatives to VFD Cables in Variable Frequency Drives
Copper cables with XLPE insulation, such as RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2, are as good as the VFD cable for motor systems, including VFD machines. Some of these options are double-rated as tray cables, which results in an easy and comfortable installation and management. However, VFD cables are still an all-around best option when it comes to voltage fluctuations in variable frequency drives.
