VFD Cable Sizing Calculator
Enter the motor current, cable length, rise time and carrier frequency to size the cable and check reflected-wave stress.
Step-by-step
- Enter valid values to begin.
Ampacity: 125% of motor FLC (NEC 430.122(A)) from Table 310.16, then a conservative allowance for carrier-frequency heating (not an NEC factor). Reflected-wave estimate: critical length = tr × 150 m/µs / 2, peak = Vdc(1 + 0.9 × min(1, L/Lcrit)). The drive manufacturer’s maximum cable length and filter guidance always override these estimates.
VFD Cable Sizing: Why Drive Cable Is Not Just Motor Cable
Core Engineering Principles
A variable-frequency drive doesn’t send a sine wave to the motor. It sends a train of fast voltage pulses, with rise times of 50 to 200 ns on modern IGBTs, switching at a carrier frequency of 2 to 16 kHz. Each pulse travels down the cable at about half the speed of light. When it reaches the motor, the motor’s surge impedance is much higher than the cable’s, so most of the pulse reflects back. If the cable is longer than the critical length, the reflected wave arrives at the motor terminals before the rise is complete and the voltage doubles. A 480 V drive has a 680 V DC bus, so you can see 1,300 V or more on a motor wound for 1,000 V peaks.
Those spikes puncture winding insulation, usually at the first turns of the first coil, and show up as repeated motor failures a year or two after commissioning. Current also looks different: the harmonic content and the high-frequency common-mode currents mean a cable runs warmer than its fundamental current suggests, and the common-mode current flows through bearings, which is why shaft-grounding rings and symmetrical three-ground cable have become standard on larger drives.
Vpeak ≈ Vdc × (1 + Γ), Vdc = 1.414 × VLL, Γ ≤ 1
NEC & Standard References
NEC 430.122(A) says conductors supplying power conversion equipment must be rated at least 125% of the rated input current to the drive, and 430.131 covers the disconnecting means. NEMA MG-1 Part 31 defines inverter-duty motors rated for 1,600 V peak voltage and rise time of 0.1 µs or longer, and Part 30 covers general-purpose motors rated for only 1,000 V peak. IEEE 518 and IEEE 1566 cover noise and drive performance, and NEC 250.122 and the manufacturer’s EMC guide determine the grounding conductor. Use VFD-rated, XLPE-insulated, shielded cable with three symmetrical ground conductors and a 1,000 V rating for 480 V drives, and keep the drive-to-motor cable separate from control wiring.1. Conductor: 65 × 1.25 = 81.3 A. With a 0.95 carrier-heating allowance, required ampacity = 81.3 / 0.95 = 85.5 A → 3 AWG copper at 75°C (100 A).
2. Critical length = 100 ns × 150 m/µs / 2 = 7.5 m. The run is 45.7 m, well beyond it, so full reflection.
3. Vdc = 1.414 × 480 = 679 V; Vpeak ≈ 679 × 1.9 = 1,290 V, above the 1,000 V general-purpose limit and below the 1,600 V inverter-duty limit.
4. Use an inverter-duty motor, or install a dv/dt filter at the drive.
- Shield and ground the right way. Terminate the shield 360° at both ends. A pigtail ground defeats the shield at the frequencies that matter.
- Don’t put drive and control cable in the same tray. Separate by at least 12 in. or cross at 90°.
- Bearing currents are real. Fluting on a bearing race is the telltale sign. Use a shaft grounding ring and an insulated bearing on large motors.
- Don’t over-extend the cable. Check the drive manual for the maximum motor-cable length at your carrier frequency.