VFD Selection Calculator
Find the drive size and the output current a variable frequency drive needs for your motor, using IEC kW ratings or NEC HP ratings and Table 430.250 motor current.
VFD Selection Calculator
How to Use the VFD Selection Calculator
- 1Choose the standard. IEC gives the drive size in kW. NEC gives it in HP and, if you leave the FLA blank, uses the NEC Table 430.250 motor current for 200, 208, 230, 460 and 575 V (and the 220–240, 440–480 and 550–600 V ranges).
- 2Enter motor power, supply phase and voltage. The drive always feeds a three-phase motor; for single-phase input the motor voltage is normally the same as the supply (for example 230 V).
- 3Enter the nameplate FLA if you have it. Drive output current must cover the motor's actual current, and nameplate FLA is more accurate than a calculated value.
- 4Pick the application and sizing factor. 1.00 for fans and centrifugal pumps, 1.25 for constant-torque loads, 1.50 for high overload or high inertia.
- 5Check the current. Use the size as a guide, then confirm in the catalogue that the drive's output current is at least the "Required Drive Output Current".
How VFD Sizing Works
A drive is selected on output current, not on kW or HP alone. Two motors of the same power can have different currents (pole number, efficiency, voltage), and the drive's current limit and overload capability are what protect it.
Required drive output current = motor FLA × sizing factor
Drive power rating ≥ motor kW (or HP) × sizing factor → next standard size
Normal Duty vs Heavy Duty
Most drives carry two ratings. The overload values below are typical catalogue values, not values fixed by a standard; check the drive's data sheet.
| Rating | Typical overload | Typical loads | Sizing factor in this calculator |
|---|---|---|---|
| Normal duty (light overload) | About 110 % for 60 s | Centrifugal fans and pumps, HVAC | 1.00 |
| Heavy duty (high overload) | About 150 % for 60 s | Conveyors, positive-displacement pumps and compressors, mixers | 1.25 (the HD rating is usually about one frame below the ND rating) |
| Above heavy duty | Above 150 % or long overloads | Crushers, high-inertia fans, hoists | 1.50, then check with the manufacturer |
IEC Example: 15 kW, 400 V Centrifugal Fan
Sizing factor 1.00 → drive ≥ 15 kW, normal duty
Select a 15 kW drive whose normal-duty output current is at least 28.3 A
For a 15 kW conveyor (factor 1.25) the calculator gives 18.5 kW and a required normal-duty current of 35.4 A, or a heavy-duty current of at least 28.3 A.
NEC Example: 20 HP, 460 V Conveyor
Sizing factor 1.25 → 25 HP; required normal-duty current = 27 × 1.25 = 33.8 A
Select a 25 HP drive (ND current ≥ 33.8 A) or a drive whose heavy-duty current is ≥ 27 A
Input conductors ≥ 1.25 × the drive's rated input current from its nameplate (430.122(A))
Selection Checklist (IEC and NEC)
| Check | What to do | Reference |
|---|---|---|
| Output current | Drive current at the chosen duty ≥ motor nameplate FLA × factor | Drive data sheet; IEC 61800-2 ratings |
| Derating | Reduce the drive's current for ambient above its rated value (often 40 °C), altitude above 1000 m and higher switching frequency | Drive manual |
| Single-phase supply | Use a drive rated for 1-phase input; a 3-phase-input drive on 1-phase needs a large derating | Drive manual |
| Input protection | NEC: conductors ≥ 125 % of drive input current (430.122(A)); disconnect ≥ 115 % (430.128); SCPD per 430.130 and the drive marking. IEC: the fuses or MCCB listed by the manufacturer | NEC Art. 430 Part X; IEC 60364-4-43 |
| Motor overload | Drive electronic overload can be used if the drive is marked for it (NEC 430.124). Several motors on one drive each need their own overload relay | NEC 430.124; IEC 60364-4-43 |
| Motor insulation and cable length | Long motor cables cause reflected-wave overvoltage; use an inverter-duty motor or a dv/dt or sine filter as the manufacturer advises | NEMA MG 1 Part 31; IEC TS 60034-25 |
| EMC and safety | Choose the EMC category for the site and install as the manual says | IEC 61800-3; IEC 61800-5-1 / UL 61800-5-1 |
| Harmonics | Check current distortion at the point of common coupling; add a line reactor, DC choke, multi-pulse or active front end if needed | IEEE 519; IEC 61000-3-12 |
| Panel SCCR | Drive and its protective device must be rated for the available fault current | UL 508A; IEC 61439-1 |
VFD Size Chart
IEC: 400 V, PF 0.85, Efficiency 90 %
| Motor (kW) | Motor current (A) | Fan / pump drive (factor 1.00) | Constant torque drive (factor 1.25) | Required ND current at 1.25 (A) |
|---|---|---|---|---|
| 0.75 | 1.4 | 0.75 kW | 1.1 kW | 1.8 |
| 1.5 | 2.8 | 1.5 kW | 2.2 kW | 3.5 |
| 2.2 | 4.2 | 2.2 kW | 3 kW | 5.2 |
| 4 | 7.5 | 4 kW | 5.5 kW | 9.4 |
| 5.5 | 10.4 | 5.5 kW | 7.5 kW | 13.0 |
| 7.5 | 14.2 | 7.5 kW | 11 kW | 17.7 |
| 11 | 20.8 | 11 kW | 15 kW | 25.9 |
| 15 | 28.3 | 15 kW | 18.5 kW | 35.4 |
| 22 | 41.5 | 22 kW | 30 kW | 51.9 |
| 30 | 56.6 | 30 kW | 37 kW | 70.8 |
| 37 | 69.8 | 37 kW | 55 kW | 87.3 |
| 55 | 103.8 | 55 kW | 75 kW | 129.7 |
| 75 | 141.5 | 75 kW | 110 kW | 176.9 |
| 110 | 207.5 | 110 kW | 160 kW | 259.4 |
NEC: 460 V, Motor Current from Table 430.250
| Motor (HP) | FLC (A) | Fan / pump drive (factor 1.00) | Constant torque drive (factor 1.25) | Required ND current at 1.25 (A) |
|---|---|---|---|---|
| 1 | 2.1 | 1 HP | 1.5 HP | 2.6 |
| 3 | 4.8 | 3 HP | 5 HP | 6.0 |
| 5 | 7.6 | 5 HP | 7.5 HP | 9.5 |
| 10 | 14 | 10 HP | 15 HP | 17.5 |
| 15 | 21 | 15 HP | 20 HP | 26.3 |
| 20 | 27 | 20 HP | 25 HP | 33.8 |
| 30 | 40 | 30 HP | 40 HP | 50.0 |
| 40 | 52 | 40 HP | 50 HP | 65.0 |
| 50 | 65 | 50 HP | 75 HP | 81.3 |
| 75 | 96 | 75 HP | 100 HP | 120.0 |
| 100 | 124 | 100 HP | 125 HP | 155.0 |
| 150 | 180 | 150 HP | 200 HP | 225.0 |
Values match the calculator. Drive power steps are not evenly spaced, so a 1.25 factor sometimes jumps two sizes (for example 50 HP to 75 HP); in that case compare the current of the smaller drive's heavy-duty rating with the motor FLA before buying the larger one.
VFD Selection FAQs
Take the motor nameplate FLA, decide whether the load is normal duty or heavy duty, and choose a drive whose output current at that duty is at least the FLA (times any extra margin). Then apply the manufacturer's derating for ambient, altitude and switching frequency.
For fans and centrifugal pumps, a drive with the same normal-duty rating as the motor is usually right. For constant-torque or high-overload loads, use the drive's heavy-duty rating, which normally means the next larger drive.
Yes, with a drive rated for single-phase input and a 230 V three-phase motor. The input current is higher than the motor current. If you use a three-phase-input drive on single phase, the manufacturer's derating applies and is often large.
Yes, if they run at the same speed. The drive's output current must cover the sum of the motor currents, the drive must be set to V/f control, and each motor needs its own overload relay because the drive cannot protect motors individually.
NEC 430.122(A) requires the input conductors to have an ampacity of at least 125 % of the drive's rated input current, taken from the drive nameplate, not from the motor current.
IEC 61800-2 (ratings of low-voltage drives), IEC 61800-3 (EMC) and IEC 61800-5-1 (safety; UL 61800-5-1 in North America). Installation follows NEC Article 430 Part X or IEC 60364. IEEE 519 and IEC 61000-3-12 cover harmonics.
