Input Side Wiring
VFD input conductors are sized using NEC 430 Part X. The input current is typically equal to the output current (minus small efficiency losses). Size conductors at 125% of VFD rated input current. The input breaker sizes per NEC 430.52 — typically 150-175% of rated input current for VFDs with built-in electronic protection.
Line reactors (3-5% impedance): Strongly recommended on VFD inputs to reduce harmonic current distortion, limit inrush current during power-up, and protect against voltage transients. Required by IEEE 519 when total harmonic distortion (THD) at the point of common coupling exceeds limits.
Input contactor or disconnect: Required per NEC 430.102. Can also serve as the means for the VFD to disconnect from power during critical faults (drive uses a control relay to open the contactor).
Output Side Wiring
Maximum cable length: VFDs produce high-frequency PWM voltage pulses. Long cables create voltage reflections (reflected wave phenomenon) that can double the peak voltage at motor terminals — potentially exceeding motor insulation rating. Typical limits: 150 feet without filter, 300 feet with output reactor, 1000+ feet with dV/dt or sine wave filter.
Cable type: Use shielded VFD-rated cable (e.g., Belden VFD cable) or install conductors in metallic conduit (EMT, RMC). The shield or metallic conduit provides a low-impedance return path for high-frequency common-mode currents, reducing EMI emissions.
CRITICAL: Never run VFD input and output cables in the same conduit. The high-frequency PWM output can couple into the input cables, causing upstream equipment interference. Maintain minimum 12-inch separation between input and output cable routes.
Do NOT install power factor correction capacitors on VFD output — capacitors resonate with the PWM frequency and will be destroyed within minutes.
Grounding and EMC
The VFD chassis must be grounded with a low-impedance connection — ideally a flat braid or wide conductor (not a long, thin wire). High-frequency common-mode currents flow through the VFD ground, and impedance matters more than resistance at these frequencies.
Motor grounding: An equipment grounding conductor must run with the output cables from VFD to motor. If using shielded cable, terminate the shield at both ends (VFD ground bus and motor frame) with 360° circumferential clamps, not pigtails.
Shaft grounding: Motors ≥ 25 HP driven by VFDs require shaft grounding rings (e.g., Aegis, SHAFT-GROUNDING) to divert bearing currents. Without shaft grounding, the common-mode voltage from the VFD induces shaft voltage that arcs through the bearings, causing fluting (pitting) and premature bearing failure — typically within 3-12 months.
EMC filters: In sensitive environments (hospitals, data centers, broadcast facilities), install an EMC input filter rated for the VFD. This prevents conducted emissions from propagating back to the electrical system.
Control Wiring
Use shielded twisted-pair cable for all analog signals (speed reference, current feedback). Ground the shield at the VFD end only (single-point grounding) to avoid ground loops.
Digital I/O (start/stop, fault relay) can use unshielded cable but should still be routed separately from power cables with minimum 12-inch separation.
Maintain minimum 90° crossing angles when control cables must cross power cables. Never run them parallel in the same tray or conduit.
Use the VFD's internal relay outputs for external fault indication — do NOT wire the motor contactor in the VFD output circuit for start/stop control (this damages the VFD output transistors).
Common Installation Problems
Motor bearing failure: The #1 VFD-related failure. Caused by common-mode voltage from the PWM output creating shaft voltage that arcs through motor bearings. Solution: shaft grounding ring + insulated bearing on the non-drive end.
Cable length violations: Reflected wave voltage at motor terminals can reach 2× to 2.8× DC bus voltage (up to 1600V peak on a 480V system). Exceeding manufacturer cable length limits without output filters destroys motor insulation — first winding-to-ground, then winding-to-winding.
Ground fault tripping: VFDs produce inherent ground leakage current (common-mode current through the motor cable capacitance). This can trip upstream GFCI devices or ground fault relays. Solution: set ground fault trip level above expected leakage, or use a dedicated isolation transformer.
Overheating: VFDs are heat-generating equipment. Verify enclosure ventilation provides adequate airflow per manufacturer specifications. Typical rule: 3% of VFD kW rating is dissipated as heat. A 100 HP VFD generates approximately 2.2 kW of heat.