How to Size a Cable for a VFD-Driven Motor: Step-by-Step with IEC 60364

A cable feeding a motor directly from a switchboard carries a clean sinusoidal current. A cable between a VFD and a motor does not. The output waveform of a pulse-width modulated (PWM) drive is a series of high-frequency voltage pulses that approximate a sine wave at the fundamental frequency, but contain significant harmonic content and steep voltage edges (dV/dt) that impose additional thermal and dielectric stress on the cable.

If you size a VFD output cable using the same method as a direct-on-line motor cable, you will undersize it. The harmonics produce additional I2R losses in the conductors and eddy current losses in the cable screen. The high dV/dt can degrade insulation over time. And the cable capacitance interacts with the drive switching frequency to produce charging currents that add to the thermal load.

I learned this the hard way on a 6.3 km conveyor drive system at Batu Hijau. Standard cable sizing gave us a 95 mm2 cable. After accounting for harmonics and cable capacitance limits, we needed 120 mm2 — and a shorter cable run would have needed a different solution entirely. This guide walks through the correct methodology.

Why VFD Output Cables Are Different

Three factors distinguish VFD output cables from standard power cables:

1. Harmonic Currents

A PWM inverter output contains the fundamental frequency component (the useful power) plus a spectrum of harmonic currents centered around the switching frequency and its multiples. These harmonics do not deliver useful power to the motor — they generate heat in the cable conductors, in the cable screen/armour, and in the motor windings.

The total RMS current in the cable is not the fundamental current alone:

Where I_1 is the fundamental current and THD_I is the total harmonic current distortion as a decimal (e.g., 0.05 for 5% THD).

For a modern PWM drive with a switching frequency of 4-8 kHz, the output current THD is typically 3-8% depending on the motor inductance and cable length. Older drives or drives operating at low switching frequencies can produce 10-15% THD.

2. High dV/dt Voltage Transients

The voltage pulses from a PWM drive have rise times measured in microseconds. A typical IGBT-based drive produces voltage edges of 2,000-8,000 V/microsecond. These steep edges create:

• Reflected wave voltage doubling at the motor terminals when the cable is long (the voltage wave reflects at the high-impedance motor terminals and adds to the incoming wave)
• Partial discharge stress on cable insulation, particularly at cable terminations
• Dielectric heating in the cable insulation from the high-frequency voltage stress

3. Cable Capacitance

Every cable has distributed capacitance between its conductors and between conductors and screen/armour. On a standard 50 Hz power supply, this capacitance draws a negligible charging current. On a PWM waveform with switching frequencies of 2-16 kHz and steep edges, the cable capacitance draws a substantial charging current at every switching transition.

This capacitive charging current flows through the drive output stage, adds to the thermal loading of the cable at the drive end, and can trip the drive on overcurrent if the cable is too long or has too high a capacitance per unit length.

Drive manufacturers publish maximum cable capacitance limits — typically 50-150 nF depending on the drive rating. Exceeding this limit can cause nuisance tripping, increased EMI, and excessive drive output filter stress.

The Correct Sizing Methodology

Step 1: Determine the Fundamental Design Current

Start with the motor full-load current from the nameplate:

For a 75 kW motor at 415V, PF = 0.86, efficiency = 0.94:

I_FLC = 75,000 / (1.732 x 415 x 0.86 x 0.94) = 128.6 A

Use the nameplate value if available. For this example, assume the nameplate reads 130 A.

Step 2: Apply Harmonic Derating Factor

IEC 60364-5-52 Annex E (and the equivalent guidance in Table B.52.17 of earlier editions) provides derating factors for cables carrying harmonic-rich currents. The derating depends on the harmonic spectrum, but for practical purposes, a VFD output cable can be derated using the THD approach.

IEC 60364-5-52, Annex E, Table E.52.1 — Current carrying capacity reduction for harmonic currents

For a VFD output with 5% THD (typical modern drive):

At 5% THD, the effect is small — less than 1%. However, if the drive operates at low speed (below 20 Hz fundamental), the motor impedance drops, THD increases to 15-25%, and the effect becomes significant:

For applications where the motor spends significant time at low speed (conveyors, mixers, fans with wide speed range), use the worst-case THD, not the rated-speed THD.

Step 3: Select Installation Method and Base Current Rating

For this worked example: cable tray (perforated), single layer, touching — IEC 60364-5-52 Reference Method E.

IEC 60364-5-52, Table B.52.1 — Installation methods

From IEC 60364-5-52 Table B.52.5 (XLPE, copper, Reference Method E, 3-core), the base ratings are:

At standard conditions (30 degC ambient, no grouping), 35 mm2 at 137 A appears sufficient for 130 A. But we are not at standard conditions.

Step 4: Apply Derating Factors

Ambient temperature correction (40 degC): From IEC 60364-5-52, Table B.52.14, for XLPE insulation (90 degC max), at 40 degC ambient:

IEC 60364-5-52, Table B.52.14 — Ambient temperature correction factors

C_temp = 0.91

Grouping correction: Assume three circuits on the same cable tray, single layer, touching. From Table B.52.17:

IEC 60364-5-52, Table B.52.17 — Grouping correction factors

C_group = 0.82 (for 3 circuits, method E, touching)

Combined derating:

Now 35 mm2 (137 A) is clearly insufficient. 50 mm2 (163 A) is also too small. We need 70 mm2 (201 A) minimum.

Step 5: Check Voltage Drop Under PWM Waveform

Voltage drop for VFD cables is calculated at the fundamental frequency, but you must account for the fact that the VFD compensates for cable drop by boosting output voltage. The critical check is at low speed (low frequency), where the VFD output voltage is low and the motor torque is most sensitive to terminal voltage.

For the fundamental frequency at rated speed:

Where L = 80 m, R_c = 0.268 ohm/km and X_c = 0.0803 ohm/km for 70 mm2 XLPE. This is 4.9/415 = 1.2% — well within the typical 5% limit.

Step 6: Check Cable Capacitance

For 70 mm2 3-core XLPE screened cable, typical capacitance is approximately 0.3 microfarad/km (300 nF/km). For an 80 m run:

Most 75 kW drive manufacturers specify maximum output cable capacitance of 100-150 nF. At 24 nF, we are well within limits. However, if this were a 500 m run, the capacitance would be 150 nF — right at the limit, and an output reactor would be required.

Step 7: Final Selection

Selected cable: 70 mm2 4-core Cu/XLPE, Reference Method E

Common Mistakes

1. Ignoring Harmonics at Low Speed

Most engineers size the cable for rated-speed current. But a VFD running a conveyor at 30% speed has dramatically higher THD — 20-30% is common. The cable still carries near-rated current (constant torque load), plus the harmonic content. If the application involves extended low-speed operation, the worst-case condition is NOT rated speed.

2. Using Standard Voltage Drop Calculations

Standard voltage drop formulas use 50 Hz (or 60 Hz) reactance values. On a VFD output, the fundamental frequency varies from 0 to the rated frequency. At low frequencies, the reactance drops (X = 2 x pi x f x L), and the voltage drop is almost purely resistive. This can affect motor performance at low speed. Some drives compensate automatically; others require manual IR compensation programming.

3. Exceeding Manufacturer Cable Length Limits

Drive manufacturers publish maximum cable lengths for a reason. Beyond the published length, cable capacitance causes excessive charging current, reflected voltage waves stress motor insulation, and common-mode current through the cable screen increases EMI. These limits are typically 50-100 m for unscreened cables and 100-300 m for screened cables, depending on drive voltage and switching frequency.

4. Forgetting the Screen/Armour Losses

In screened or armoured VFD cables, the high-frequency harmonic currents induce eddy currents in the metallic screen and armour. These losses generate heat that reduces the cable's effective current capacity. For steel wire armoured (SWA) cables, this effect is more pronounced than for copper-wire-screened cables. Some cable manufacturers publish specific VFD derating factors for their armoured cable ranges — use them.

5. Not Specifying VFD-Rated Cable

Standard PVC or XLPE cables are designed for 50/60 Hz sinusoidal voltage. VFD output cables experience high dV/dt and high-frequency voltage stress. Purpose-designed VFD cables (sometimes marketed as "inverter duty" or "VFD-rated") have enhanced insulation systems, symmetrical construction for EMC performance, and defined capacitance characteristics. For any installation over 10 m, specifying a VFD-rated cable is strongly recommended.

Bottom Line

VFD cable sizing is standard cable sizing plus three additional checks: harmonic current derating, cable capacitance limits, and dV/dt considerations. Miss any one of these and you have either a cable that overheats, a drive that trips on overcurrent, or motor insulation that degrades prematurely. The methodology is straightforward — it just requires more steps than most engineers are accustomed to for a "simple" motor cable.