VFD Motor Cable Length: Limits, Effects & Solutions

As a general rule, keep the motor cable from a 400 V VFD under about 50 m without an output filter; beyond that, reflected-wave overvoltage and dv/dt can stress the motor insulation and cause leakage-trip nuisance trips. The exact limit depends on the drive's switching speed, the cable type and the motor's insulation rating – and you can push much further with an output reactor, a dv/dt filter or a sine-wave filter, up to several hundred metres. This article explains the physics, the practical limits, and the solutions.

What Limits VFD Cable Length

A VFD's fast-switching output pulses travel along the cable and reflect back at the motor terminals when the cable is long. The reflected wave can nearly double the voltage at the motor terminals (up to roughly 2× the DC bus voltage), and the high dv/dt (rate of voltage change) stresses the winding insulation, especially the first few turns. This is why long un-filtered cables cause both premature motor failure and mysterious leakage-current trips.

Recommended Cable Lengths

Relevant standards are IEC 61800-8 (EMC and safety for drive systems) and IEC 60034-25 (AC motors for converter supply), which define the motor insulation levels for inverter duty. For how cable issues interact with drive faults, see our overcurrent guide and overvoltage guide.

Effects of Long Cables

• Reflected-wave overvoltage at the motor terminals, stressing winding insulation
• High dv/dt that shortens insulation life, worst on the first turns
• Increased common-mode current through the cable capacitance, causing leakage-trip nuisance trips on sensitive RCDs
• EMI radiating from long un-shielded runs

Solutions

• Add an output reactor close to the drive – the cheapest and most common fix
• Use a dv/dt filter when motor insulation protection is the priority
• Use a sine-wave filter for very long runs or old/wound-rotor motors
• Use shielded, properly grounded motor cable and keep power cables away from signal cables
• Check the drive's rated output frequency vs cable length in the manual – some drives specify a maximum without filters

Frequently Asked Questions

Can I run 200 m of cable from a VFD to a motor? Yes, with the right mitigation – an output reactor or dv/dt filter, and preferably a sine-wave filter for 200+ m. Without any filter, 200 m is risky and can damage the motor insulation. See the low-voltage VFD range for drives with built-in filter options.

Why does my VFD trip when I extend the cable? Longer cable increases capacitance and common-mode current, which trips sensitive leakage protection, and reflected waves can cause overvoltage trips. Adding a filter or using a type-B RCD usually solves both. More in the FAQs.

Planning a long motor run? Our engineers can recommend the right filter and cable type for your drive and motor – contact us through the VFD selection guide.