How to Size Inverter Duty Cabling for Industrial VFDs (Shielding, Cores & Output Choke Rules)

Variable Frequency Drives (VFDs) generate fast-switching PWM waveforms that stress cable insulation, create EMI/RFI noise, and can cause reflected wave overvoltages at the motor. Choosing the correct inverter duty cable prevents nuisance trips, bearing currents, premature insulation failure, and communication interference.

Recommended Cable Types (XLPE, PVC, Shielded)

• XLPE insulated copper cable: High dielectric and thermal endurance; preferred for most industrial VFD feeders.
• PVC insulated: Acceptable for small drives and short runs; check temperature and derating limits.
• Shielded VFD cable (copper braid or Al-PET tape + drain): Controls common-mode noise and radiated EMI; improves EMC compliance.

Tip: Use cable types explicitly marked “inverter duty” with low capacitance, symmetrical grounds (or dedicated PE), and robust insulation for high dv/dt.

Core Selection: 3-Core vs 3.5-Core

For a three-phase motor on a VFD, specify a 3-core power cable plus a separate protective earth (PE). Do not use 3.5-core on VFD outputs—there is no neutral in a 3-phase motor circuit, and the extra “half-neutral” offers no benefit. Ensure a low-impedance PE path via a dedicated earth conductor or symmetrical grounding per manufacturer guidance.

When You Must Use Shielded Cable

• Cable length > 30–40 m (more radiated noise with longer runs).
• Parallel routing near PLC/SCADA, analog instruments, Ethernet, or encoder cables.
• Facilities with strict EMC requirements or documented EMI issues.

Bond the shield 360° at both ends (drive and motor) using glands or clamps; avoid “pigtails” which reduce shielding effectiveness.

Output Choke / dV/dt Filter Rules by Cable Length

Fast switching edges from the VFD reflect on long cables and can produce high peak voltages at the motor terminals. Apply these practical rules:

• ≤ 100 m: Typically no output choke required (confirm with drive manual).
• 100–150 m: Use a dV/dt reactor (output choke) to limit rise time and reduce reflected peaks.
• > 150 m: Use a sine-wave filter for best protection of motor windings and to reduce audible noise.

Also check: Switching frequency, cable capacitance, motor insulation class, and bearing protection (e.g., shaft grounding) for long runs.

Quick Sizing & Installation Checklist

• Select 3-core inverter duty cable (XLPE preferred) with a separate PE conductor.
• Size by VFD output current with derating for ambient, grouping, and installation method (tray, conduit, underground).
• Use shielded VFD cable when runs are long or near sensitive control wiring.
• Apply dV/dt reactor for 100–150 m, sine-wave filter for >150 m runs.
• Bond shields 360° at both ends; separate power and control cable routes.
• Verify terminal torque, gland sizes, bending radius, and earthing continuity.

FAQs

Is 3.5-core cable allowed on VFD outputs?

No. VFD outputs to three-phase motors do not use a neutral; specify 3-core plus a dedicated PE conductor.

When is shielded cable mandatory?

Use shielded cable for runs longer than ~30–40 m, near instrumentation/communication wiring, or whenever EMC compliance or noise issues demand it.

Do I need an output choke?

At 100–150 m, fit a dV/dt reactor. Beyond 150 m, use a sine-wave filter for maximum protection against reflected waves and insulation stress.

Which insulation is better—XLPE or PVC?

XLPE is preferred for most industrial VFD applications due to higher thermal and dielectric strength. PVC can be used for small drives and short, benign installations.