VFD Selection Calculator
A practical guide to sizing and selection per IEC & NEC standards.
VFD Selection Calculator
Compliant with NEMA MG 1 Standards
Results:
Min. VFD Output Current (A): 0.00
VFD Power Rating (kW): 0.00
Verification Results
How To Use VFD Selection Calculator
1. Enter Motor Data
Input your motor's nameplate values: Rated Power (kW), Voltage (V), Power Factor (PF), and Efficiency (η).
2. Define Sizing Factors
Enter your desired Service Factor (e.g., 1.15) for overload, a Safety Margin (e.g., 1.1), and your planned OCPD (Breaker) Rating.
3. Calculate & Verify
Click "Calculate". Use the NEC/IEC toggle to see verification checks for conductor ampacity and EGC (ground wire) size.
Step-by-Step VFD Selection & Sizing (IEC + NEC)
1. Collect the Right Inputs
- Motor Nameplate: Rated power (kW/HP), voltage, base frequency, rated current, speed, insulation class, service factor (SF), duty (e.g., S1), enclosure, temp rise.
- Supply Data: System voltage, short-circuit current (SCCR) / MVA, grounding system, harmonic limits, ambient temperature, altitude.
- Application: Duty cycle (continuous/intermittent), overload demand (e.g., 150% for 60s), starting torque, braking (coast, dynamic, regen), environment (IEC EMC), cable lengths.
2. Choose Drive Power & Current Rating
- Match Current: Select a drive with an output current rating ≥ motor rated current (at the intended switching frequency and ambient). Select by amps, not just kW/HP.
- Overload Class: Verify the drive’s short-time overload capability (e.g., 150% for 60s) meets the load's requirements.
- Derating: Apply the manufacturer’s derate factors for ambient temperature > 40°C or high altitude. Upsize the drive if needed.
3. Determine Currents for Upstream Sizing
A. For NEC Designs:
- Motor FLC (for motor protection): Use NEC Tables 430.247–430.250, not the nameplate (per NEC 430.6(A)).
- VFD Feeder Conductors: Use 125% × the drive’s rated input current from its datasheet (per NEC 430.122(A)).
B. For IEC Designs:
- Follow local wiring rules (e.g., IEC 60364 adoptions) and the drive manufacturer’s installation instructions.
4. Size Branch/Feeder Conductors (NEC)
- Compute Ampacity: Required Ampacity = 1.25 × VFD Input Rated Current (NEC 430.122(A)).
- Select Conductor: Use NEC Table 310.16. Apply ambient and bundling corrections (310.15).
- Output Conductors (VFD to Motor): Use PWM-rated/VFD cable. Heed NEC 2020 informational note on insulation stress.
5. Size OCPD (Short-Circuit Protection)
The OCPD on the line side protects the VFD. Always follow the drive manufacturer's manual (per NEC 110.3(B)) for the specified fuse/CB type and maximum size.
Do not use motor-only rules from NEC 430.52 unless explicitly permitted by the VFD manual.
6. Size Equipment Grounding Conductor (EGC)
Use NEC Table 250.122. Size the EGC based on the rating of the upstream OCPD (from Step 5).
7. Check Short-Circuit Ratings (SCCR)
Verify the drive's SCCR (from its datasheet, per UL/IEC 61800-5-1) is greater than the available fault current at the installation point. Use listed series-rated combinations if necessary.
8. EMC & Harmonic Compliance (IEC)
Select the correct EMC category (per IEC 61800-3) and emission class (C2/C3). This dictates filters, shielded motor cable, and bonding. Add reactors or active front ends if harmonic limits (e.g., IEEE 519) apply.
9. Output Cabling & Motor Insulation
For long leads or older motors, use VFD-rated cable (symmetrical grounds) and consider dv/dt or sine-wave filters to limit reflected-wave voltage peaks and reduce insulation stress (see NEC 2020 informational note).
10. Braking & Stopping Method
For controlled stops or overhauling loads, specify a dynamic brake chopper + resistor (sized for duty/energy) or a full regenerative (AFE) drive.
11. Environmental & Mechanical Details
Ensure enclosure (NEMA/UL Type or IEC IP code) is suitable for the location. Maintain cooling clearances and apply ambient/altitude derates as required.
12. Prepare Documentation
Include datasheets (showing IEC 61800-5-1, IEC 61800-3 compliance) and all calculations for: conductor ampacity (NEC 430.122(A)), OCPD sizing, EGC sizing (NEC 250.122), SCCR check, and EMC plan.
VFD Sizing Chart (Example)
Illustrative examples for 400V, 3-Phase motors (Heavy Duty / Next Size Up). Always verify with manufacturer datasheets.
| Motor (kW) | Typical PF | VFD Size (NEC) | VFD Size (IEC) |
|---|---|---|---|
| 5.5 kW | 0.83 | 7.5 kW | 7.5 kW |
| 7.5 kW | 0.84 | 11 kW | 11 kW |
| 11 kW | 0.85 | 15 kW | 15 kW |
| 15 kW | 0.85 | 18.5 kW | 18.5 kW |
| 22 kW | 0.86 | 30 kW | 30 kW |
| 30 kW | 0.87 | 37 kW | 37 kW |
Quick Code Reference
Key standards to cite in your calculation sheet.
NEC (NFPA 70)
- NEC 430.6(A) / Tables 430.247-250: Motor FLC values.
- NEC 430.122(A): Conductors supplying VFD (≥ 125% of VFD input current).
- NEC 430.52: Max ratings for motor branch OCPD.
- NEC 310.16: Conductor ampacity tables.
- NEC 250.122: EGC sizing based on OCPD rating.
IEC Standards
- IEC 61800-5-1:2022: Safety requirements for power drive systems.
- IEC 61800-3:2022: EMC requirements and test methods.
- IEC 60364 (and local adoptions): Sizing cables, coordination, and earthing.
Worked Mini-Example (NEC-Centric)
1. Given Data
Motor: 30 HP, 460V, 3-phase
VFD: Nameplate input current of 48 A at 480V.
Supply: Ambient 40°C, THHN 90°C insulation.
Plan: Select a 70A OCPD (per VFD manual).
2. Sizing Calculations
Line-Side Conductors (NEC 430.122(A)):
Ampacity = 1.25 × VFD Input Current
Ampacity = 1.25 × 48 A = 60 A
Per NEC Table 310.16, #6 Cu THHN (75°C) is 65A. PASS.
EGC (NEC Table 250.122):
Based on 70A OCPD, the required ground is #8 Cu EGC.
3. Key Takeaway
With a VFD, we size conductors based on 125% of the VFD's input current (NEC 430.122(A)).
If this were a standard motor (no VFD), we would use 125% of the motor's FLC from Table 430.250 (NEC 430.22).
Frequently Asked Questions
What is the formula for VFD selection?
The basic formula for selecting a VFD is:
I_VFD ≥ I_motor × (Service Factor or Load Factor)
That means the drive’s rated current must be equal to or greater than the motor’s full-load current (FLA) after applying correction factors for ambient temperature, overload, and altitude. In practice, you choose the VFD by matching or exceeding the motor’s rated kW/HP and current.
How to choose VFD size?
Select a VFD with an output current rating ≥ the motor’s full-load current at the system voltage and operating conditions. Consider derating for temperature or altitude, overload requirements (e.g., 150% for 60 s for heavy-duty loads), and ensure compliance with IEC 61800-5-1 or NEC 430.122(A) for conductor sizing. Always prioritize current rating over kW/HP for accurate sizing.
How to calculate VFD RPM?
The motor speed (RPM) under VFD control is given by:
RPM = (120 × f) / P
where f is the VFD output frequency (Hz) and P is the number of motor poles. For example, a 4-pole motor at 50 Hz runs at ~1500 RPM; at 25 Hz, it runs at ~750 RPM. The VFD adjusts speed linearly with frequency.
What are the criteria for VFD selection?
Key selection criteria include:
- Motor power, voltage, and full-load current
- Overload capacity and duty cycle
- Environment (temperature, altitude, dust, humidity)
- Harmonic and EMC compliance (IEC 61800-3)
- Control method (V/F, vector, or torque control)
- Braking or regenerative needs
- Compatibility with supply and protection devices (per NEC 430/IEC 60364)
How to calculate the power of VFD?
VFD power (input or output) is calculated using:
P = √3 × V × I × PF × η
where V = line voltage, I = current, PF = power factor, and η = efficiency.
Usually, VFD output power should equal or slightly exceed the motor’s rated kW to ensure reliable operation under full load.