CalcPanel

VFD Sizing Calculator

Enter motor HP or kW, voltage, phase, CT/VT application, overload, ambient, and altitude to estimate required VFD kW. Default: 10 HP · 480 V · 3φ · constant torque · 150% OL · 40°C · 1000 m → ~11 kW class.

Quick calculator

VFD is upsized for application type, overload, ambient temperature, and altitude derating. Results update instantly.

Advanced calculator

Apply CT/VT duty, overload %, ambient temperature, and altitude derating for drive frame sizing.

About this calculator

See all protection calculators on the hub. This screens a VFD (ASD/VSD) size from motor power plus CT/VT duty, overload, ambient, and altitude derating (IEC 61800-style planning factors—confirm with the drive datasheet). Example: 10 HP (7.46 kW) constant-torque @ 480 V with 150% overload → round up to an 11 kW heavy-duty frame. Next: verify motor current with the Motor FLA Calculator, size shielded cable with the Motor Cable Size Calculator, and review harmonics in the VFD harmonics mitigation guide.

Recommended VFD size

Result explanation

Recommended VFD: 11 kW (14.7 HP)

Assumptions: Screening only. Verify motor nameplate FLA, service factor, starting torque requirements, braking needs, harmonic mitigation, and manufacturer derating curves before specifying. VFD must be matched to motor voltage and phase.

Continue Your Calculation

After drive kVA is estimated, size motor branch conductors and input protection per NEC 430.

Motor cable size →

Standard VFD sizes (IEC / NEMA)

kWHP (approx)FLA @ 480V 3φ (A)Typical Application
0.7512.5Small fan, conveyor
2.236.5Pump, blower
5.57.515Compressor, mixer
7.51020Conveyor, pump
111528Fan, compressor
152038Pump, blower
223055Compressor, extruder
375090Large fan, pump
5575130Crusher, conveyor
75100180Large compressor, mixer
110150260Industrial fan, pump
160200380Extruder, crusher
250350590Large industrial drive

FLA values approximate for 480V 3φ at 92% eff / 0.85 PF. Actual VFD output current varies by manufacturer and model. Consult manufacturer datasheets for exact current ratings.

VFD selection guide — key factors

FactorConstant Torque (CT)Variable Torque (VT)High Overload (HV)
Typical loadsConveyors, compressors, mixers, extrudersFans, centrifugal pumps, blowersCrushers, hoists, punch presses
Overload rating150% for 60 seconds (heavy duty)110% for 60 seconds (normal duty)180-200% for 3-10 seconds
VFD sizingMatch motor HP/kW or +1 sizeMatch motor HP/kW exactly+1 or +2 sizes above motor
BrakingMay need braking resistor for rapid decelUsually coast-to-stop, no braking neededMust have braking resistor/regen
Harmonics6-pulse standard; add reactor if >50HP6-pulse usually sufficient12-pulse or active front-end recommended
Motor cableShielded VFD cable if >50ft; dV/dt filter if >100ftStandard THHN usually OK for short runsShielded cable + sine wave filter

VFD sizing formula

Step 1 — Base VFD kW: VFD_kW_base = Motor_kW × Application_multiplier

Where: CT = 1.10, VT = 1.00, HV = 1.15

Step 2 — Apply derating: VFD_kW_required = VFD_kW_base ÷ (Temp_derating × Altitude_derating)

Temperature derating: ≤40°C = 1.00, 41-50°C = 0.95, 51-60°C = 0.90
Altitude derating: ≤1000m = 1.00, 1001-2000m = 0.97, 2001-3000m = 0.93, 3001-4000m = 0.88

Step 3 — Round up to standard size: Select next standard VFD frame size (0.75, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90, 110, 132, 160, 200, 250 kW...).

Step 4 — Verify current: VFD continuous output current ≥ Motor FLA × 1.1 (10% margin). VFD overload current (60s) ≥ Motor FLA × Overload_factor.

Example: 10 HP (7.46 kW), 480V 3φ, CT application, 150% overload, 40°C, 1000m.

Step 1: 7.46 × 1.10 = 8.21 kW
Step 2: 8.21 ÷ (1.0 × 1.0) = 8.21 kW
Step 3: Round up to 11 kW standard size
Step 4: Motor FLA ≈ 14 A; VFD current ≈ 20 A (≥ 14 × 1.1 = 15.4 A ✓); Overload 150% = 30 A (≥ 14 × 1.5 = 21 A ✓)

Worked VFD sizing examples

Example 1 — 10 HP conveyor (480V, 3φ, CT)

Given: 10 HP (7.46 kW) · 480V · 3φ · constant torque · 150% overload · 40°C · 1000m

Calculation: 7.46 × 1.10 = 8.21 kW → round up to 11 kW VFD

Selection: 11 kW (15 HP) heavy-duty VFD, 20 A continuous, 30 A overload (60s). Add input reactor if cable length >100ft. Use shielded VFD cable. Braking resistor optional (conveyor usually coasts to stop).

Example 2 — 25 HP centrifugal pump (480V, 3φ, VT)

Given: 25 HP (18.65 kW) · 480V · 3φ · variable torque (pump) · 110% overload · 40°C · 1000m

Calculation: 18.65 × 1.00 = 18.65 kW → round up to 22 kW VFD

Selection: 22 kW (30 HP) normal-duty VFD, 45 A continuous, 49.5 A overload (110%). No braking needed (pump coasts). Standard 6-pulse diode rectifier. Consider PID macro for pressure/flow control. Note: VT drives are often cheaper than CT drives for the same kW rating.

Example 3 — 50 HP crusher (460V, 3φ, HV, 50°C, 2000m)

Given: 50 HP (37.3 kW) · 460V · 3φ · high overload (crusher) · 150% overload · 50°C ambient · 2000m altitude

Calculation: 37.3 × 1.15 = 42.9 kW → derating: 0.95 (50°C) × 0.97 (2000m) = 0.922 → 42.9 ÷ 0.922 = 46.5 kW → round up to 55 kW VFD

Selection: 55 kW (75 HP) heavy-duty VFD, 130 A continuous, 195 A overload. Must include braking resistor for rapid deceleration (crusher inertia). Use shielded VFD cable + dV/dt filter. Consider 12-pulse or active front-end for harmonic mitigation (crushers are high-distortion loads). Verify enclosure rating (IP20 indoor, IP54 if dusty environment).

Frequently asked questions

Can I use a larger VFD than the motor?

Yes, upsizing is common and recommended for constant-torque or high-overload applications. A larger VFD provides more overload capacity, lower stress on IGBTs, and better reliability. However, the VFD must still be configured for the actual motor FLA for proper overload protection. Oversizing by one frame size is typical for CT applications; two sizes for HV applications.

What is the difference between CT and VT VFDs?

Constant Torque (CT/heavy-duty) VFDs are rated for 150% overload for 60 seconds and sized for constant-torque loads (conveyors, compressors, mixers). Variable Torque (VT/normal-duty) VFDs are rated for 110% overload for 60 seconds and optimized for fans and pumps (torque ∝ speed²). VT drives are often less expensive for the same kW rating. Using a VT drive on a CT load may cause nuisance overload trips.

Do I need a braking resistor?Braking resistors are needed when: (1) rapid deceleration is required, (2) the load has high inertia (fans, centrifuges), (3) the load is overhauling (conveyors going downhill, hoists lowering), or (4) frequent start-stop cycles. For fans and pumps that coast to stop, no braking resistor is needed. Calculate braking power: P_brake = (J × ω²) / (2 × t_decel), where J = inertia, ω = angular velocity, t = deceleration time.
How do VFD harmonics affect my system?

6-pulse VFDs draw non-sinusoidal current, creating harmonics (5th, 7th, 11th, 13th...) that can cause transformer overheating, capacitor bank failure, meter inaccuracy, and interference with sensitive equipment. Mitigation options: (1) input line reactors (3-5% impedance), (2) DC link chokes, (3) passive harmonic filters, (4) 12-pulse or 18-pulse rectifiers, (5) active front-end (AFE) drives. IEEE 519 recommends THD <5% for general systems. Add reactors for VFDs >50 HP or when multiple VFDs share a transformer.

What cable should I use between VFD and motor?

For short runs (<50 ft), standard THHN/THWN-2 copper is acceptable. For runs 50-100 ft, use shielded VFD cable (symmetrical design with ground wires). For runs >100 ft, add a dV/dt filter at the VFD output or use sine wave filter. VFD output has high dv/dt (voltage rise time ~0.1-0.5 μs) that can cause reflected wave voltage doubling at the motor terminals, leading to insulation failure, especially for older motors not rated for inverter duty. Always use motor-rated (inverter-duty) motors with VFDs.