How to Wire a VFD

Page review information

Edition covered: NEC Art. 430 / NFPA 79 / IEC 60204-1 — NEC 2023, NFPA 79:2024
Content status: Reviewed
Coverage: Sizing, protection, power and control wiring, grounding/EMC at chapter/article level; STO design and vendor-specific values deliberately excluded.
Last reviewed: August 2026
Primary reference: The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

Safety. This guide is educational reference material, not a work instruction. Electrical work is performed de-energized and verified by qualified personnel under your site’s LOTO procedures, following the drive manufacturer’s manual and the authority having jurisdiction. VFD DC bus capacitors hold a lethal charge after power removal — always observe the drive’s marked discharge wait time.

Overview

A variable frequency drive presents four distinct wiring zones with different rules; treating them the same is the root cause of most drive installation problems.

Line side (input) — three-phase supply from the branch-circuit OCPD; ordinary power wiring, sized from the drive’s rated input current.
Load side (output) — the PWM output to the motor; the noisiest circuit in the panel, which gets the special cable and shield discipline.
Control terminals — 24 V digital I/O, analog speed reference, relay outputs, safety (STO) inputs, fieldbus; signal-level wiring protected from the load side.
DC bus / brake terminals — DC link access and braking resistor/chopper connections; a stored-energy hazard and a high-energy switched circuit.

flowchart LR
    SUP[Three-phase supply] --> DISC[Disconnect + OCPD]
    DISC -->|input conductors| VFD[VFD]
    VFD -->|shielded motor cable| M((Motor))
    BR[Braking resistor] -.->|DC bus / brake terminals| VFD
    CTRL[Control I/O<br/>24V DI/DO · analog ref · STO · fieldbus] -.-> VFD

This guide covers wiring a single low-voltage drive feeding one induction motor. Parameter setup and startup live in the VFD commissioning workflow; STO circuit design belongs to the safety function (see below); multi-motor and bypass arrangements appear only where they change the rules. Terminal designations, torque values, and wire-range limits are vendor-specific — they come from the drive manual, never from a guide, including this one.

Before You Start

Have on hand before pulling wire:

Motor nameplate data — voltage, FLA, frequency, speed, power, service factor (motor fundamentals). Two currents do different jobs: the nameplate FLA feeds the overload function, while the NEC table FLC (Tables 430.247–430.250) governs conventional branch-circuit sizing (NEC 430.6 principle). For a VFD the drive’s rated input current takes over the conductor-sizing role, but the FLA/FLC distinction still applies to overload settings and any bypass.
Drive rating match — drive rated for the motor voltage and current, margin decided upstream. Confirm the duty / overload class (normal vs heavy duty in vendor terms) matches the application; the same frame carries different ratings per class.
Environment — ambient temperature, altitude, and enclosure type all derate the drive; the curves are vendor-specific — consult the drive manual rather than assuming the catalog rating applies.
Drawings — the branch-circuit design (OCPD type/size, conductor sizes, disconnect location) is decided upstream; this guide assumes you are implementing it, not inventing it at the panel.

Sizing & Protection

The governing framework is NEC Article 430 (Part X covers adjustable-speed drive systems) alongside NFPA 79 Chapters 6 and 7 for machinery panels.

Input conductors — NEC 430.122 requires ampacity of at least 125% of the drive’s rated input current, not the motor current: the drive is the load on the branch circuit. cst motor-branch computes the conventional motor branch chain and cst voltage-drop checks the run; see the wire sizing walkthrough for the full worked chain.
Short-circuit / ground-fault protection — the drive’s listing typically specifies the permitted OCPD type and maximum rating (often specific fuse classes, sometimes specific breakers). Deviating from the listed combination can invalidate the drive’s SCCR contribution and with it the panel’s marked SCCR (NFPA 79 Ch. 6; UL 508A SCCR methodology) — verify the pairing against the panel’s SCCR documentation.
Motor overload — a listed drive with an integral electronic overload typically serves as the motor overload device under NEC Article 430 Part X, so a separate overload relay is usually unnecessary; verify against the current NEC edition and the drive’s listing. One drive feeding several motors still needs individual overloads per motor.
Disconnecting means — a disconnect within sight of the drive/controller location is required (NEC Art. 430 Parts IX/X; NFPA 79 Ch. 5). Never open an output-side disconnect or contactor under load — see Common Mistakes.
Output conductors — sized for the motor circuit per Article 430; the drive manual may impose minimum/maximum cross-sections and lengths.

Power Wiring

Segregate input from output. Route input and output power wiring apart; never bundle the PWM output cable with input, control, or signal wiring. Generally accepted practice — verify for your installation.
Use inverter-duty motor cable. Shielded, symmetrical VFD-rated cable — three phase conductors plus symmetrical grounds under a continuous shield — supports the bonding intent of NFPA 79 Ch. 8 and mitigates bearing currents and radiated noise (NFPA 79 Ch. 12 recognizes VFD cabling). Ordinary tray cable is not equivalent at PWM frequencies.
Terminate the shield 360 degrees at both ends. A full-circumference gland or clamp at the drive end and the motor end is the drive-vendor consensus; a pigtail largely defeats the shield at PWM frequencies. Generally accepted practice — verify against the drive’s EMC installation instructions.
Respect motor lead length. Long leads produce voltage overshoot at the motor terminals (reflected-wave effect) that stresses winding insulation. The threshold length and the remedy — output reactor, dV/dt filter, or sine filter — are vendor- and voltage-class-specific: take them from the drive manual, not from a universal number.
Braking resistor wiring is a high-energy, PWM-switched circuit: short runs, shielded or tightly twisted conductors, routed away from control wiring, thermal contact wired into the control/safety chain. Generally accepted practice plus the vendor manual.
Torque discipline — terminal torque values and wire ranges come from the drive manual; record the values used.

Control / Signal Wiring

Keep 24 V digital I/O, analog reference, and relay wiring separated from each other and from all power wiring. The analog speed reference (0–10 V / 4–20 mA) is the most corruption-prone circuit on the drive.
Use screened control cable for analog signals; ground the screen per the drive manual — commonly at the drive end only for low-frequency signal screens, unlike the both-ends rule for the motor cable.
Do not mix commons. Check the drive’s internal reference topology (PNP/NPN sourcing, isolated vs non-isolated commons) before tying drive I/O common to other system commons. Consult the drive manual.
STO wiring exists on most modern drives; its design belongs to the machine’s safety function per ISO 13849-1 and IEC 62061, with wiring depth deferred to the planned servo drive guide.

Grounding, Shielding & EMC

Device-specifics here; the deep treatment is owned by the noise & EMC mitigation guide.

PE first. Terminate the protective earth conductor before anything else, sized on the NFPA 79 Table 8.2.2.3 basis (largest upstream OCPD) — procedure per the table, values not reproduced here. See panel grounding & bonding.
The motor cable is the ground path. Its symmetrical grounds and shield provide the dedicated high-frequency return for PWM common-mode current back to the drive PE terminal; the building ground path alone is not sufficient. Generally accepted practice.
EMC filter bonding — ground the integral or external EMC filter with wide, short bonds to the mounting plate; mask or scrape paint at bond points. A filter bonded through paint is decorative.
Bearing currents — common-mode voltage drives current through motor bearings; mitigation (insulated bearings, shaft-grounding rings, common-mode chokes) is generally accepted practice for larger machines — verify per drive and motor vendor guidance.
Separation from signal cabling — the drive output cable is the worst offender for parallel network runs; separation classes and distances are covered in the EMC guide and the copper Ethernet page.

Common Mistakes

Opening an output contactor or disconnect under load. The drive sees a sudden open circuit at full current and can fault or fail — sporadic output faults that “only happen sometimes.” Interlock the output to open only with the drive stopped.
Unshielded motor cable routed beside analog runs. The PWM output couples into the 0–10 V reference; the drive hunts and the analog signal is noisy only while the drive runs.
Shield pigtails instead of 360-degree terminations. A few centimeters of pigtail is a significant impedance at PWM frequencies — the shield measures continuous but its EMC performance is gone.
Sizing input conductors from motor nameplate FLA. The 430.6 discipline uses table FLC for conventional motor conductors — and for a VFD, 430.122 uses the drive’s rated input current, which normally exceeds the motor current. Undersized input wiring runs hot.
Long unfiltered motor leads. Reflected-wave overshoot roughly doubles the voltage stress at the motor terminals; winding insulation fails months later and the drive gets the blame. Fit the reactor or filter the manual specifies.
Mixed control commons. Tying drive I/O common to another supply’s common without checking isolation topology produces phantom inputs and ground loops — intermittent starts/stops nobody can reproduce.
Megger through the drive. An insulation tester applied to the motor circuit with the drive output still connected destroys the output stage. Disconnect the drive first — see Verification Checks.

Verification Checks

Before and during first energization (evidence-retaining checklists in templates):

Insulation-resistance test the motor and motor cable with the drive output disconnected — never megger through a connected drive
PE and motor-cable ground/shield terminations complete; 360-degree glands at both ends
OCPD type and rating match the drive listing and panel SCCR documentation
Input/output segregation and control-wiring separation match the routing plan
Terminal torques per the drive manual, recorded
Discharge wait time observed before any terminal re-work (NFPA 79 Ch. 7)
Rotation verified by a low-speed bump under drive control, not by swapping leads under power
Hand off to the VFD commissioning workflow for motor data entry, first power-up, and functional checks

Standards References

NEC (NFPA 70), 2023 — Art. 430 Part X (adjustable-speed drive systems, incl. 430.122 input conductors and overload provisions); Parts IX/X (disconnecting means); 430.6 with Tables 430.247–430.250 (FLC vs FLA)
NFPA 79:2024 — Ch. 5 (supply and disconnect), Ch. 6 (protection, SCCR), Ch. 7 (stored-energy discharge), Ch. 8 (grounding and bonding, Table 8.2.2.3 basis), Ch. 12 (conductors and VFD cabling)
UL 508A — SCCR methodology for the drive/OCPD combination in panels
IEC 60204-1 — equipotential bonding and wiring practices for machine electrical equipment (international counterpart)
ISO 13849-1 / IEC 62061 — govern the STO safety function (outside this guide)

Trust Boundary — Engineering Judgment Required

This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.

Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.