What overload should I choose?

Normal duty (≈110%/60s) for variable torque loads (fans/pumps). Heavy duty (≈150%/60s) for constant torque loads (conveyors/mixers/crushers). When in doubt, select heavy duty or check your drive datasheet.

How do altitude and temperature affect VFD sizing?

Above ~1000 m and 40 °C, most drives require derating. A common rule is 1% per +100 m and 1% per +1 °C, but values vary—always verify in the manufacturer’s datasheet.

Can I feed a 3-phase VFD from single-phase?

Some drives allow it with significant derating (often around 0.58). Check the manual—the calculator lets you set the exact factor.

Selecting the correct VFD power rating is crucial for motor control, automation, safety, and energy efficiency.

This guide explains VFD power calculation with formulas, variable breakdowns, tables, and real-world engineering examples.

VFD Power Rating Calculator (HP/kW)

Q: How do you calculate the minimum VFD size?

We compute motor full-load current from power, voltage, PF and efficiency; apply overload (duty) and site derating (altitude, ambient, single-phase input); then select the smallest standard VFD size whose capacity meets or exceeds the required current/power.

Size your VFD based on motor power, voltage, PF, efficiency and derating (duty, altitude, temperature, single-phase input).

Motor power:

Motor voltage (L-L): Power factor (PF): Efficiency (η): Duty / overload: VFD input phase: Single-phase derate factor:

Typical when feeding a 3-ph drive from 1-ph

Altitude (m):

Derate above 1000 m (default 1% per +100 m)

Ambient (°C):

Derate above 40 °C (default 1% per °C)

Altitude derate per +100 m: Temperature derate per +1 °C: Decimals:

What this calculator does

We compute motor full-load current from power, voltage, PF, and efficiency; apply duty/overload and site derating (altitude, ambient, single-phase input); then suggest a minimum and recommended VFD size in kW and HP. Results are guidance—verify against the chosen VFD datasheet and local codes.

Formulas used

Motor current (3-phase): I = (P_out) / (√3·V·PF·η) with P in watts.
Overload/derating: I_req = I · (overload) / (derate factor).
Minimum VFD power: P_vfd,min = P_motor · (overload) / (derate factor).

Typical derating defaults

Above 1000 m: ~1% per +100 m. Above 40 °C: ~1% per +1 °C. Single-phase feeding a 3-phase drive: ~0.58 factor. Adjust as per the manufacturer’s datasheet.

Normal vs Heavy duty

Normal duty ~110% overload for 60 s (fans/pumps). Heavy duty ~150% for 60 s (conveyors, mixers, crushers). If unsure, choose heavy duty for a conservative pick.

Common Values for VFD Power Rating (HP and kW)

Below is a comprehensive table of commonly used motor power ratings and the corresponding VFD sizing (HP/kW), nominal current (Amps), and typical voltage levels at both 50 Hz and 60 Hz.

Standard Motor Power Ratings and Corresponding VFD Ratings

Motor Power (HP)	Motor Power (kW)	Line Voltage (V)	Typical Current (A) @ 400V	Recommended VFD kW	IEC Frame Size	NEMA Frame
0.5	0.37	230 / 400 / 480	1.2	0.75	71	56C
1	0.75	230 / 400 / 480	2.4	1.1	80	143T
2	1.5	230 / 400 / 480	4.2	1.5	90S	145T
3	2.2	230 / 400 / 480	6.0	2.2	90L	182T
5	3.7	230 / 400 / 480	9.6	4.0	112M	184T
7.5	5.5	230 / 400 / 480	13.5	5.5	132S	213T
10	7.5	230 / 400 / 480	18.0	7.5	160M	215T
15	11	230 / 400 / 480	25.0	11.0	160L	254T
20	15	230 / 400 / 480	32.0	15.0	180M	256T
30	22	230 / 400 / 480	45.0	22.0	200L	284T
50	37	400 / 480	73.0	37.0	280S	324T
75	55	400 / 480	108.0	55.0	315S	365T
100	75	400 / 480	145.0	75.0	355M	405T

Notes:

Based on IEC 60034 and NEMA MG1 standards.
Current values are nominal, based on typical 4-pole motors at 50/60 Hz.
Voltage classes vary by region: 230V/400V (IEC), 208V/480V (NEMA/ANSI).

Key Formulas for Calculating VFD Power Rating

The selection of a VFD power rating is based on a variety of electrical and mechanical factors. The following formulas are essential for precise sizing.

1. Power Conversion Between HP and kW

Common Values:

1 HP ≈ 0.746 kW
10 HP ≈ 7.46 kW
100 HP ≈ 74.6 kW

2. Output Power of the VFD

3. Motor Current (for 3-phase motors)

This formula is essential when calculating the VFD rating for motors of non-standard ratings or under unique loads.

4. VFD Oversizing (Service Factor)

A safety or service factor is often applied:

Typical SF (Service Factor): 1.1 to 1.25
Required when:
- Motor is running continuously at high load
- Ambient temperature exceeds 40°C
- High starting torque is required

Real-World Examples of VFD Power Rating Calculations

Example 1: Conveyor System in a Packaging Plant

Motor: 5 HP (3.7 kW)
Line Voltage: 400 V
Power Factor: 0.9
Efficiency: 0.96

Step-by-step:

1.Convert HP to kW:

2.Calculate motor current:

3.Select VFD:

VFD must be rated at least 4 kW (next standard IEC rating)
Choose VFD rated for ≥ 6.5 A output

Result: Select 4 kW VFD, 400V class, with current rating ≥ 6.5 A.

Example 2: HVAC Fan in a Commercial Building

Motor: 15 HP (11 kW)
Voltage: 480 V (NEMA)
PF: 0.88
Efficiency: 0.94
Ambient Temp: 45°C (Requires oversizing)

Step-by-step:

1.Determine current:

2.Apply service factor:

3.Choose next standard rating:

15 kW VFD, 480 V, ≥ 18 A output.

Result: Select 15 kW VFD, 480V, current ≥ 18 A.

Factors Influencing VFD Sizing Beyond Motor Power

1. Load Type (Torque Profile)

Different mechanical loads impose varying torque demands. This affects how a VFD should be sized and whether oversizing is needed.

Load Type	Torque Demand	Typical Applications	VFD Sizing Factor
Constant Torque	Torque remains constant	Conveyors, mixers, compressors	1.0× to 1.1×
Variable Torque	Torque ∝ speed²	Pumps, fans (centrifugal)	1.0×
Shock/High Torque	Sudden torque spikes expected	Crushers, grinders, hoists	1.15× to 1.25×

Why It Matters:For high torque applications, oversizing the VFD is critical to avoid thermal overload and premature failure.

2. Ambient Temperature and Cooling

VFDs are typically rated for 40°C ambient. If installed in hotter environments:

Above 40°C: Apply a derating factor (~2–3% per °C over 40°C).
Above 50°C: May require external cooling or relocation.

Where:

α=0.02 to 0.03 per °C above rated
ΔT: Difference from 40°C

3. Altitude Compensation

VFDs must also be derated for high-altitude installations (above 1000 m or 3300 ft)

Where:

H: Installation height in meters

Example:
At 3000 m altitude → derating of 10% ⇒ size a VFD 10% higher.

4. Starting Method & Duty Cycle

Frequent starts/stops or acceleration ramps may require higher current capacity.
Duty cycle exceeding 60% continuous torque operation → oversize by at least 10–20%.

Standards and Compliance in VFD Sizing

IEC Standards

IEC 61800-2: Defines general requirements for VFDs
IEC 60034-1: Covers motor ratings and permissible loading
IEC 61000-4-2 to 61000-4-5: Covers EMI/EMC compatibility

🇺🇸 NEMA/ANSI Standards

NEMA MG1 Part 31: Motors for use with adjustable-speed drives
IEEE 519: Harmonic distortion limits in power systems

Harmonic Distortion (THD)

VFDs generate harmonics that must be filtered to protect the power system and meet IEEE 519 guidelines:

Use line reactors or harmonic filters if:
- The motor is >15 HP
- THD > 5% at PCC (Point of Common Coupling)

Summary Checklist for Proper VFD Sizing

Before selecting your VFD, confirm the following:

✔️ Know the motor’s full load power in HP or kW
✔️ Identify the line voltage and frequency
✔️ Determine the motor current
✔️ Check the load type (constant or variable torque)
✔️ Factor in ambient temperature and altitude
✔️ Include service/oversizing factor
✔️ Ensure compliance with harmonic and EMC standards
✔️ Cross-check with manufacturer-specific VFD data sheets

Final Thoughts

Selecting a properly rated VFD is not simply a matter of matching the nameplate motor power. A variety of factors—load dynamics, environmental conditions, voltage levels, and operational duty—must be considered to ensure reliability, efficiency, and compliance.

By applying the formulas and best practices outlined in this article, electrical engineers, maintenance teams, and system designers can confidently select VFDs that maximize motor performance while minimizing risk and cost.

Calculation of the VFD’s power rating (in HP or kW)