How to Choose Cable for a VFD Motor

Why is a normal ampacity check not enough for a VFD motor cable?

A conventional power-cable calculation checks load current, installation derating, voltage drop, short-circuit withstand and protective coordination, and those checks still apply. For conductor sizing, follow the normal thermal method described in our guide to ampacity and derating. A PWM drive output adds fast voltage transitions and high-frequency common-mode current, which the conventional check does not cover.

Reflections along the motor lead can raise the peak voltage at the motor terminals. Cable capacitance creates charging current that the drive must supply. These effects depend on the drive topology and settings, cable geometry and length, motor insulation and installation, not on conductor size alone.

IEC 61800-3:2022 is the product EMC standard for power-drive systems. Its scope places EMC performance at the level of the drive system, not the cable considered in isolation. It does not replace installation rules, safety requirements or the product-specific instructions for the chosen drive and motor.

What must the drive, motor, cable and installation documents confirm?

Build a four-part compatibility check and request evidence for each interface before the cable is quoted.

• Drive output: exact drive model and frame, output voltage, switching-frequency setting, cable-type and maximum-length tables, permitted parallel runs and output-filter instructions. These control the charging-current limit, reflected-wave mitigation and EMC conditions.
• Motor insulation: the nameplate and the manufacturer's statement for converter duty, including the permissible terminal peak voltage and rise-time conditions where provided. These decide whether the motor can tolerate the PWM waveform or needs a filter or reinforced insulation.
• Cable and PE system: conductor size and ampacity, insulation system, capacitance data if the drive manual uses it, shield material and coverage, and PE cross-section and symmetry. These control thermal duty, the high-frequency return path, common-mode current and bearing-current risk.
• Installation: one-way route length, number of motors and parallel cables, glands and termination hardware, tray or conduit layout, and separation from control and instrumentation cables. Actual installed conditions, rather than catalog EMC claims, determine the real EMC performance and whether the terminations are accessible.

What problems does a VFD motor cable cause, and how are they handled?

Each problem has a cause in the drive, the cable or the installation. The mitigations below are starting points for the specification; the drive and motor manufacturers' tables set the actual limits.

Mitigations are starting points for the specification. The drive manual, motor manual and project EMC requirement control the final design.

How should the shield and protective earth be terminated?

The cable shield and the safety protective conductor have related but different duties. The protective earth must satisfy the applicable electrical safety rules and fault-current duty. For high-frequency common-mode current, the installation also needs a short, low-impedance return path.

ABB's drive-system grounding manual and Siemens' motor instructions both describe symmetrical shielded motor cables with large-area termination of the shield at the drive and motor ends. Siemens illustrates 360-degree contact and short unshielded ends. Where equipotential bonding is inadequate, the project designer must solve that bonding problem rather than assume the shield can safely carry unintended low-frequency equalizing current.

Do not turn this guidance into a universal construction rule. Some drive families permit specified unshielded installations, metal conduit or other arrangements, and the exact drive manual, motor manual, applicable installation code and project EMC requirement control the design. For compatible glands and clamps, see our cable gland selection guide.

How does lead length change the cable choice?

Record the electrical path from the drive terminals to the motor terminals, including detours, junctions and flexible tails. Then check the manufacturer table for the selected drive, because a generic distance cannot be transferred between drive families. Rockwell Automation's PWM-drive wiring guide, for example, treats motor-cable length restrictions and reflected-wave mitigation as product-specific tables.

Long leads may require one or more of the following. Each option changes voltage stress, losses and permissible switching frequency, and sometimes the allowed cable length.

• an output reactor
• a dV/dt filter
• a reflected-wave device
• a sine filter

Parallel motor cables and multiple motors add capacitance and change current distribution, so declare them at design and quotation stage. If the drive manual states a total cable-length limit, check how branches and parallel runs are counted instead of applying the limit separately to each cable.

What does a routing and termination audit cover?

• At the drive: confirm cable-entry space, bending radius, shield-clamp location and PE termination before choosing conductor size and gland.
• Along the route: keep motor output cables separate from analog, encoder, communication and other sensitive circuits, as the equipment manuals require. Document unavoidable crossings and shared trays. Our control and instrumentation cable guide covers the separation question in more detail.
• At junctions: preserve shield continuity and enclosure bonding, and do not leave an unreviewed length of exposed cores.
• At the motor: verify that the terminal box and gland accept the proposed cable, shield termination and PE arrangement without breaking the bend radius or clearance.
• Before energization: inspect termination contact, PE continuity, phase identification and cable condition, and record the installed length, cable type and any output filter for future drive replacement.

This guide is a specification and coordination aid. It does not replace the drive maker's application review, the motor maker's insulation limits or a project-specific electrical design.

Frequently asked questions