What is the difference between CT and VT VFDs?

CT/heavy-duty and VT/normal-duty labels identify different manufacturer current and overload envelopes. Match the application torque profile, then verify the exact continuous-current rating, overload percentage and duration in the selected product manual; the labels alone do not determine a kW frame.

How do VFD harmonics affect my system?

VFD input harmonics depend on rectifier topology, line impedance, loading and the point of common coupling. Determine limits from the applicable system study and standard, then select reactors, chokes, passive or active filters, multipulse arrangements or active-front-end equipment from verified manufacturer data rather than a universal horsepower threshold.

What cable should I use between VFD and motor?

Cable construction, shielding, grounding and output filtering depend on the drive, switching frequency, motor insulation, cable length and installation. Follow the exact drive and motor manuals for maximum lead length and dv/dt or sine-wave-filter requirements instead of universal distance thresholds.

VFD Sizing Calculator

Enter motor HP or kW, voltage, phase and duty requirements to calculate a motor-current screening value and build the checks needed for a manufacturer catalog. Final VFD selection must use motor nameplate current and the chosen drive’s duty and derating tables.

Quick calculator

Estimate motor current when nameplate current is unavailable. Results update instantly; final selection uses the chosen VFD catalog.

Motor power

Input voltage (V)

Phase

Advanced calculator

Record duty, overload, ambient temperature, and altitude for the manufacturer-catalog verification step; this tool does not apply a generic derating curve.

Application type

Overload requirement (%)

Ambient temperature (°C)

Altitude (m)

About this calculator

See all protection calculators on the hub. This page calculates a motor-current estimate and turns the selected duty, overload, ambient and altitude into a catalog verification checklist. It does not invent a universal VFD derating curve: use the motor nameplate current and the selected manufacturer’s output-current and derating tables. Next: verify motor current with the Motor FLA Calculator, size cable with the Motor Cable Size Calculator, and review VFD harmonics mitigation.

VFD catalog screening result

Result explanation

Calculated motor current: 11.5 A. Verify the motor nameplate and select a drive whose catalog output-current and overload ratings clear the application.

Assumptions: The current estimate uses 92% efficiency and 0.85 PF only when nameplate current is unavailable. Verify service factor, starting torque, braking, harmonics, enclosure, carrier frequency and manufacturer-specific ambient/altitude derating before specifying.

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)

kW	HP (approx)	FLA @ 480V 3φ (A)	Typical Application
0.75	1	2.5	Small fan, conveyor
2.2	3	6.5	Pump, blower
5.5	7.5	15	Compressor, mixer
7.5	10	20	Conveyor, pump
11	15	28	Fan, compressor
15	20	38	Pump, blower
22	30	55	Compressor, extruder
37	50	90	Large fan, pump
55	75	130	Crusher, conveyor
75	100	180	Large compressor, mixer
110	150	260	Industrial fan, pump
160	200	380	Extruder, crusher
250	350	590	Large 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 catalog checks by application

Constant torque: verify continuous output current, torque at low speed, overload percentage and duration, braking duty and thermal limits in the exact drive manual.

Variable torque: verify the manufacturer’s normal-duty current rating, minimum speed/cooling limits, process control needs and the actual fan or pump system curve.

High-overload or overhauling loads: obtain load torque/inertia and deceleration duty, then verify overload, braking/regen, enclosure and derating with the drive OEM. Application labels do not translate into a universal “one or two frames larger” rule.

For every path, use motor nameplate current and product-specific cable-length, dv/dt, filter, EMC, harmonic, ambient, altitude and carrier-frequency limits.

VFD catalog selection method

Step 1 — Use the nameplate: record motor voltage, full-load current, frequency, speed and service factor. If nameplate current is unavailable, this page provides only a temporary formula estimate.

Step 2 — Choose duty: match the application torque profile and required overload percentage/duration to the manufacturer’s normal-duty or heavy-duty rating.

Step 3 — Verify output current: the drive’s catalog continuous output-current rating must clear the motor nameplate current under the actual duty rating.

Step 4 — Apply the selected manufacturer’s data: check ambient temperature, altitude, carrier frequency, enclosure, input phase, braking and any multi-motor conditions against that exact drive manual. Derating curves are product-specific, so this calculator does not substitute generic percentages.

Worked VFD current-screening examples

10 HP, 480 V, three-phase: with the fallback 92% efficiency and 0.85 PF assumptions, estimated motor current is about 11.5 A; a requested 150% overload duty corresponds to about 17.2 A. Select only after the drive catalog clears motor nameplate current and the stated overload duration.

25 HP, 480 V, three-phase: the same fallback assumptions give about 28.7 A. The application label does not automatically choose a kW frame.

50 HP, 460 V, three-phase: the fallback estimate is about 59.9 A. At 50°C and 2000 m, apply the chosen manufacturer’s exact ambient, altitude, carrier-frequency, enclosure and duty derating tables; this page does not invent universal factors.

Frequently asked questions

Can I use a larger VFD than the motor?

Possibly, but do not apply a universal frame-step rule. Verify the catalog’s minimum motor restrictions, continuous output current, overload rating and duration, protection settings, carrier frequency, enclosure and environmental derating against the motor nameplate and application.

What is the difference between CT and VT selection?

CT and VT are manufacturer duty ratings tied to different output-current and overload envelopes. Select the duty row that matches the load torque profile, then verify its exact current and time ratings; the labels alone do not determine a kW frame.

Do I need braking, harmonic mitigation or output filtering?

Those decisions depend on load inertia and deceleration, regeneration, system impedance, point-of-common-coupling limits, cable construction/length, motor insulation and the chosen drive manual. Use the OEM application guide and a system study instead of universal horsepower or cable-length thresholds.