The short answer

A VFD is sized by current, not horsepower. Take the full-load amps (FLA) stamped on the motor nameplate, add margin for the load's duty class, correct for ambient temperature and altitude, and then pick the smallest drive whose continuous output current rating at your duty class meets or exceeds that number. Horsepower labels are a shorthand that only holds true for a standard 4-pole NEMA Design B motor on a variable-torque load — the moment your load is a conveyor, a hoist, a compressor, or a rooftop unit in Arizona, the HP label starts lying to you.

Continuous drive amps needed = Motor FLA × Duty margin ÷ (Temperature factor × Altitude factor × Carrier factor)

• Size on ampsThe nameplate FLA is the input that matters
• Match duty classNormal Duty and Heavy Duty ratings on the same drive can differ by two frame sizes
• Then derateHeat and altitude take capacity away before the motor ever spins

Why you size a VFD on amps, not horsepower

What to read off the motor nameplate

Six fields on the nameplate decide the drive. Everything else is useful context.

Uploading a nameplate photo to the calculator above reads fields 1–5 automatically.

Nameplate unreadable, painted over, or the motor is inaccessible? Clamp an ammeter on one motor lead while the machine runs its heaviest normal cycle and record the highest steady reading. A measured amp figure beats a nameplate figure in every case — the nameplate tells you what the motor can draw, and the clamp tells you what your process actually demands. If you can't do either, our application engineers can work backward from the machine, the driven equipment, or the drive that's already in the panel.

Sizing a VFD, step by step

1. Step 1 — Confirm the supply

   Note the voltage and phase at the panel where the drive will land: 208, 230, 240, 380, 400, 460, 480, 575, or 600 V, and single- or three-phase input. Drive voltage classes are hard boundaries, not preferences — a 230 V class drive on a 480 V feeder is destroyed on power-up, and a 480 V class drive on a 208 V feeder will undervolt-fault or fail to produce rated torque. If your input is single-phase, jump ahead to voltage, phase and supply before you do anything else, because the derate is severe.

2. Step 2 — Take the motor's FLA at that voltage

   From the nameplate, matched to how the motor is wired. Where you have a clamp meter and a running machine, use the measured peak steady current instead.

3. Step 3 — Classify the load and pick a duty margin

   Variable torque (fans, centrifugal pumps), constant torque (conveyors, positive-displacement pumps, extruders, compressors), or high inertia and high breakaway (hoists, crushers, centrifuges, mills, reciprocating loads). Duty margin by load type has the margin table.

4. Step 4 — Choose the duty class the drive must satisfy

   Normal Duty gives you roughly 110% overload for 60 seconds. Heavy Duty gives roughly 150% for 60 seconds, and costs you frame size. Normal Duty vs Heavy Duty explains which one your load demands. Getting this wrong is the most expensive mistake on this page.

5. Step 5 — Apply environmental derating

   Ambient temperature above the drive's rated ambient, installation altitude above 3,300 ft (1,000 m), and any carrier frequency above the factory default all reduce the drive's continuous current. Derating has the factors and a worked combination.

6. Step 6 — Select the drive by its continuous current at your duty class

   Open the manufacturer's rating table — not the HP column on the marketing page — and find the smallest frame whose continuous output amps at your duty class equals or exceeds your derated requirement. If you land within about 5% of a breakpoint, go up. The price delta between adjacent frames is almost always smaller than one unplanned production stop.

7. Step 7 — Check the peaks, the panel, and the leads

   Verify the drive's overload profile covers your worst-case transient (braking and inertia). Confirm the enclosure can shed the drive's heat (enclosure and panel heat). Check your motor lead length against the drive's limit and add a filter if you're over (harmonics and cable length). Then size input conductors and protection to code.

Normal Duty vs Heavy Duty: one drive, two ratings

Nearly every modern VFD ships with two published current ratings, and the gap between them is wide enough to change your part number. The industry names vary — ABB uses I_N / I_Hd and light-overload versus heavy-overload, Yaskawa publishes Normal Duty and Heavy Duty, Schneider and Invertek describe it as standard versus high overload — but the concept is identical. The same power stage is certified for a higher continuous current if you promise not to demand much overload from it, and a lower continuous current if you need it to survive hard transients.

Ignoring duty class is how a correctly-calculated drive still ends up undersized. An engineer sizes a 40 HP drive for a 40 HP conveyor, the datasheet's 40 HP figure is the Normal Duty rating, the conveyor jams on a Monday morning and demands 180% torque, and the drive current-limits, ramps down, and trips. Nothing was wrong with the arithmetic. The wrong column was read.

Rule of thumb: if the load can stall, jam, reverse-drive the motor, or has to break away under full load, size on Heavy Duty. If it can't, Normal Duty is not just acceptable, it's the economical choice — and on a large fan or pump it can save you an entire frame size.

How much margin to add, and why

Margin is not superstition and it is not a safety factor bolted on out of caution. Every percent you add is paying for a specific, identifiable current you expect the drive to deliver that the nameplate FLA doesn't account for: filter loading on a fan, cold product in a pump, a slug on a conveyor, the inrush of a clutch engaging. Decide what you're paying for and the number stops being arbitrary.

These are starting points, not rules. Where you have measured amps from the running machine rather than a nameplate figure, you can safely take the lower end of each band — the measurement has already absorbed most of the uncertainty the margin exists to cover. Where you're specifying a machine that doesn't exist yet, take the upper end.

Derating: what the environment takes back

A drive's published current rating comes with an asterisk, and the asterisk is the conditions it was tested under. Move the drive somewhere hotter, higher, or ask it to switch faster, and the same hardware safely delivers less current. Derating is not optional and it is not conservative engineering — it is the manufacturer telling you the rating is void outside the stated envelope.

Always start from your specific drive's rating conditions, because they differ by family. A typical open-chassis or IP20 unit is rated to 50 °C; put the same power stage in a NEMA 1 / IP21 kit and the rating usually drops to 40 °C because the vents are restricted.

Ambient temperature

The trap here is that "ambient" means the air temperature at the drive's own intake, not the temperature of the room. A drive inside a sealed NEMA 12 enclosure in a 35 °C plant is not living at 35 °C — it is living at 35 °C plus whatever its own losses have added to the air inside that box. Enclosure and panel heat shows how to work that number out.

Altitude

Two mechanisms are at work and they're worth understanding separately. Thin air carries away less heat, which costs you current. Thin air is also a worse dielectric, which costs you voltage withstand. That second effect is why a 480 V drive in Denver is fine but a 600 V drive at 8,000 ft may need to be specified differently.

Carrier (switching) frequency

Raising the carrier frequency makes the motor quieter and the current waveform smoother. It also makes the IGBTs switch more often, and every switching event is a fixed energy loss. Doubling the carrier roughly doubles switching losses.

Most drives will silently reduce the carrier on their own when they get hot, which protects the drive but changes the motor's acoustic signature — a common and puzzling "the drive got noisy in August" complaint. If audible noise is a genuine requirement, size the drive for the carrier you intend to run, not the default.

Motor FLA (460 V, 50 HP)                   65.0 A
Variable torque, real-world margin +15%    74.8 A required at ideal conditions

Rooftop ambient, design peak 50 °C
  10 °C above the drive's 40 °C rating
  at −1.5%/°C                              factor 0.85
Altitude 5,300 ft (1,615 m)
  615 m above 1,000 m at −1%/100 m         factor 0.94
Carrier left at factory default            factor 1.00

Combined derating factor: 0.85 × 0.94 × 1.00 = 0.80

Continuous drive current required:
  74.8 A ÷ 0.80 = 93.5 A

Voltage class, phase, and supply quality

Voltage classes are boundaries, not suggestions

Note that 460 V and 480 V drives are usually the same product with a different label, and that many 400 V class drives will accept 480 V — but many is not all, and the ones that won't fail immediately and without warranty. Confirm the input voltage range on the specific datasheet, not the class name.

Higher voltage is cheaper to distribute: the same horsepower at 575 V draws roughly 80% of the current it does at 460 V, which means smaller conductors, smaller breakers, and less voltage drop over a long run. On any job with a feeder over a few hundred feet, this is worth a conversation before the panel is built.

Single-phase input into a three-phase drive

This comes up constantly on small pumps, dust collectors, machine tools and light commercial work — and it is where most self-service sizing goes wrong.

Many three-phase drives will accept single-phase input, but only at a substantially reduced rating. The reason is straightforward: the input rectifier and the DC bus capacitors were designed for the smooth six-pulse ripple of a three-phase feed. On single-phase, only two of the three input legs are energised — four of the six diodes carry the entire load while two sit idle — and the bus ripple roughly triples, so both the conducting diodes and the bus capacitors run far hotter for the same output power.

Where single-phase supply is the permanent reality, a purpose-built single-phase-input drive is nearly always the better buy. It's rated honestly, needs no derate, and costs less than the double-size three-phase unit it replaces. Our team can identify which families in stock are rated for it.

Supply quality items worth checking before you size

• Voltage imbalance above about 2% between phases causes disproportionate negative-sequence current in the rectifier. Above 3%, add a line reactor and consider a frame up.
• Generator or soft-grid supply. Drives are non-linear loads and a generator's impedance is high. A common rule is to size the genset at 2–2.5× the drive kW; ask about it rather than assuming.
• Utility power factor correction capacitors switching on the same bus can produce resonance and destructive transient overvoltage on the DC bus. If your facility has switched cap banks, a line reactor at each drive is cheap insurance.
• Long feeder / high impedance. Check the available short-circuit current, because the drive's or panel's SCCR must meet or exceed it (enclosure and install specs).

Motor full-load amps and typical drive ratings

The amp figures below are the NEC Table 430.250 full-load current values for three-phase AC induction motors. Two important caveats before you use them:

1. These are code values for sizing conductors and protective devices — not a substitute for your motor's nameplate. Use them to sanity-check a nameplate reading, to spec a panel before the motor arrives, or when the nameplate is genuinely unreadable. Where you have the real nameplate, the real nameplate wins.
2. Drive continuous current ratings vary by family and by duty class. The right-hand columns are typical ranges for planning. Confirm against the specific drive's rating table before you release a purchase order.

Reading the table: find your HP row, take the FLA column matching your supply voltage, apply your margin and derating from margin and derating, then move right and choose the drive column matching your duty class. If your requirement lands between two rows, take the higher one.

Five sizing jobs, worked end to end

Nameplate FLA @ 460 V                             65 A
Load class: variable torque, real-world  +15%     74.8 A
Duty class: Normal Duty                           (110%/60 s is ample)
Design ambient at drive inlet                     50 °C  → factor 0.85
Altitude 5,300 ft                                 → factor 0.94
Carrier: factory default                          → factor 1.00

Required continuous current: 74.8 ÷ (0.85 × 0.94) = 93.5 A

Nameplate FLA @ 230 V                             54 A
Load class: constant torque, cyclic      +20%     64.8 A
Duty class: Heavy Duty                            (150%/60 s needed for breakaway)
Ambient 40 °C, indoor, at drive rating            → factor 1.00
Altitude: sea level                               → factor 1.00
Carrier raised to 8 kHz for noise                 → factor 0.85

Required continuous current: 64.8 ÷ 0.85 = 76.2 A

Nameplate FLA @ 460 V                             40 A
Load class: constant torque, shock       +30%     52 A
Duty class: Heavy Duty                            (150%/60 s for a fully loaded restart)
Ambient 45 °C inside a sealed 4X box in sun       → factor 0.93
Altitude: sea level                               → factor 1.00
Carrier: factory default                          → factor 1.00

Required continuous current: 52 ÷ 0.93 = 55.9 A

Nameplate FLA @ 230 V, 3-phase motor              22 A
Load class: variable torque, real-world  +15%     25.3 A
Duty class: Normal Duty                           → 25.3 A required at output

Single-phase input derate on a 3-phase drive      × 0.50
Equivalent three-phase drive rating needed:       25.3 ÷ 0.50 = 50.6 A

1. Read the OLD DRIVE's nameplate, not the motor's.
   Record: output amps, output voltage, input voltage/phase,
   HP/kW rating, and the duty class if stated.

2. Take the old drive's rated CONTINUOUS OUTPUT AMPS as your floor.
   Whoever commissioned this machine sized it for the load; if it
   ran for 20 years, the rating was adequate.

3. Check the OVERLOAD SETTING in the old drive's parameters if it
   still powers up, or in the machine documentation. A motor overload
   parameter set to 34 A tells you the motor's real FLA is 34 A (check the
   units — some families enter this as a percentage of drive rating) —
   the number you couldn't read off the nameplate.

4. Check the FAULT HISTORY. Repeated overcurrent, overload, or
   overtemperature faults mean the original was UNDERSIZED. Do not
   replicate it — go up a frame.

5. Confirm the physical envelope: frame dimensions, conduit entry,
   keypad cutout, and whether the replacement can reuse the existing
   backplate and wireway.

6. Confirm the control interface: analog in/out counts, relay and
   digital I/O, and the fieldbus the PLC expects (Modbus RTU/TCP,
   EtherNet/IP, PROFINET, BACnet). A drive that fits the panel but
   can't talk to the PLC has not solved the problem.

When the load pushes back: braking, regeneration and inertia

An induction motor becomes a generator any time the load turns it faster than the drive's commanded frequency. That happens more often than people expect: lowering a hoist, decelerating a large fan wheel, a downhill conveyor, a centrifuge coasting down, a vehicle test dynamometer, any web tension unwind. The energy flows backward into the drive's DC bus, the bus voltage climbs, and at the trip threshold the drive faults out — which on a hoist means the load is now held only by the mechanical brake.

Sizing a drive for these applications means sizing two things: the power stage for the motoring current, and a path for the regenerated energy.

Where the energy can go

Sizing a dynamic braking resistor

Two numbers matter and they're often confused. The resistance determines peak braking torque and must not fall below the drive's stated minimum, or the chopper transistor is destroyed. The wattage determines how long you can brake for and is set by duty cycle, not by peak.

The peak braking power is roughly the mechanical power you're absorbing. The continuous wattage rating you need is that peak multiplied by the fraction of each cycle spent braking:

Average braking power ≈ kinetic energy absorbed ÷ deceleration time
                      = (0.5 × J × (ω₁² − ω₂²)) ÷ t_decel

Peak braking power    ≈ 2 × average, for a linear deceleration ramp

Resistor W rating     ≈ Average braking power × duty cycle fraction
                        (e.g. braking 6 s in every 60 s → × 0.10)

Resistance value      ≥ the drive's minimum permitted ohms (never below)
                      ≈ V_bus_trip² ÷ Peak braking power

Note that the resistance value comes from the peak and the wattage from the average — confusing the two is how chopper transistors die. For an overhauling load — a hoist lowering continuously, or a decline conveyor — the duty cycle fraction is effectively 1.0 and the resistor must be rated for full continuous braking power. This is usually the point where a regenerative unit becomes the cheaper and cooler option. It's also where installers get caught out: a resistor sized for 10% duty on a hoist that spends its shift lowering will glow, then open, then leave the drive with nowhere to put the energy.

High inertia: the ramp is the sizing constraint

On a large fan wheel, a centrifuge, or a flywheel, the acceleration torque swamps the load torque. The drive sits at current limit for the whole ramp, and the sizing question becomes a race between the ramp time and the drive's overload timer. A drive rated 150% for 60 seconds that needs 150% for 90 seconds will trip on overload every single start.

You have two levers. Lengthen the ramp so the current demand falls below the overload threshold — free, and usually acceptable on equipment that starts twice a day. Or size the drive up so its continuous rating covers the accelerating current — necessary when the process demands a fast start, or when the machine cycles frequently enough that thermal accumulation matters. Where starts are frequent and inertia is high, budget for both a larger frame and a braking solution, and expect to specify a motor with a separately-powered cooling blower.

What the drive does to everything else in the building

A correctly sized drive can still cause problems on both sides of itself. On the input side it's a non-linear load that distorts the supply. On the output side it produces a PWM waveform with rise times fast enough to damage motor insulation. Neither shows up in a horsepower calculation, and both are cheaper to solve at the quote stage than after commissioning.

Input side: harmonics

A standard six-pulse VFD draws current in pulses rather than sinusoidally, generating characteristic 5th, 7th, 11th and 13th harmonics. IEEE 519 is the reference standard for acceptable limits at the point of common coupling. For buses at or below 1 kV it sets a voltage limit of 8% THD, with 5% on any individual harmonic; from 1 kV to 69 kV the THD limit tightens to 5%. Allowable current distortion (TDD) is not a single number — it depends on the ratio of available short-circuit current to maximum demand load current at the PCC, so it has to be calculated for your specific service. Utilities and large facility owners increasingly write it into specifications, and it's the standard your commissioning engineer will be measuring against.

A useful screening rule: if the total connected drive load exceeds roughly 20–25% of the supply transformer's kVA, harmonics deserve a real study rather than a rule of thumb. Below that, a 3% line reactor per drive is usually sufficient and worth fitting regardless — it also protects the drive from line transients and reduces DC bus capacitor stress, which extends the drive's life.

Output side: motor cable length and reflected waves

The drive's output is a train of steep-edged voltage pulses. A long motor cable behaves as a transmission line, and where the cable's surge impedance doesn't match the motor's, the pulse reflects at the motor terminals and superimposes on the incoming pulse. The result can be a peak voltage approaching twice the DC bus voltage — on a 480 V system, transient peaks approaching 1,360 V at the motor terminals.

NEMA MG 1 Part 31 defines the inverter-duty motor insulation standard. The standard expresses the requirement as 3.1 x rated line-to-line volts with a rise time no faster than 0.1 µs — about 1,430 V for a 460 V motor and 1,490 V for a 480 V one, which is why vendor literature usually rounds it to a 1,600 V class limit. A motor built to that standard has real margin. An older motor, or a rewound one, or any general-purpose motor without that rating, may not — and the failure mode is insulation breakdown in the first turns of the winding, months or years after commissioning, presenting as a phase-to-phase or turn-to-turn short that looks like a random motor failure.

Always check the specific drive's published maximum cable length, because it varies widely by family and by carrier frequency — higher carrier frequencies shorten the permissible run considerably. Two secondary effects also bite on long runs: the cable's distributed capacitance draws charging current that the drive counts as output current (so a long run effectively derates the drive), and the same fast edges drive shaft currents through the motor bearings, causing fluting and premature bearing failure. On motors above roughly 100 HP, or on any long-lead installation, an insulated bearing or a shaft grounding ring is cheap compared to a bearing replacement.

EMC and shielding

Use shielded VFD cable, terminate the shield 360° at both ends, keep motor cables physically separated from control and signal wiring, and cross them at right angles where you must cross. Where the installation must meet an EMC standard — IEC 61800-3 is the drive-specific one — confirm whether the drive has an integral EMC filter and which environment category (industrial vs residential/commercial) it satisfies. Retrofitting an external EMC filter after a failed emissions test is significantly more expensive than specifying the right internal one.

Enclosure, panel heat and the install specs that follow from sizing

Picking the enclosure rating

Enclosure choice feeds straight back into sizing, and in two directions at once. A sealed NEMA 12 or 4X box cannot exchange air with the room, so all the drive's losses accumulate inside it — which raises the drive's real ambient and therefore its derate. Meanwhile the drive itself is usually rated to a lower ambient in a NEMA 1 kit than as an open chassis, because the vent kit restricts airflow. It's common for a drive to need a frame size up purely because of how it's being enclosed.

Drive heat output (W) ≈ HP × 746 × 0.04

Worked example — 50 HP drive in a sealed NEMA 12 enclosure:
  Heat load      = 50 × 746 × 0.04       ≈ 1,490 W
  Plus the contactor, PLC, power supply
    and control transformer in the box    ≈   200 W
  Total in-box heat load                  ≈ 1,690 W

Rise above room ambient with no active cooling depends on the
enclosure's radiating surface area. A 60 × 36 × 12 in box has
4.27 m² of total surface area — and only about 2.9 m² of that
radiates usefully, because the back face sits flat against a wall.
At a typical 5.5 W/m²/°C for painted steel in still air:

  Counting all six faces:
    Rise ≈ 1,690 ÷ (4.27 × 5.5) ≈ 72 °C
  Excluding the wall-mounted back (the realistic case):
    Rise ≈ 1,690 ÷ (2.90 × 5.5) ≈ 106 °C

  Room at 30 °C  →  in-box ambient of 100 °C or more

SCCR — the spec that fails inspections

The panel's Short-Circuit Current Rating must equal or exceed the available fault current at its point of installation, and it is determined by the weakest component in the power circuit, not by the drive alone. Many drives have a modest standalone SCCR (5 kA is common on small frames) that rises substantially — often to 65 kA or 100 kA — only when protected by a specific class and rating of fuse or circuit breaker named in the manual. Get the branch protection wrong and the panel's SCCR collapses to the drive's bare figure, which is a documented inspection failure and a genuine arc-flash hazard.

Ask two questions on every job: what is the available fault current at this panel, and exactly which fuse or breaker does the drive manual require to reach the SCCR I need?

Conductors, protection and disconnects

VFD circuits are covered by NEC Article 430, Part X (Adjustable-Speed Drive Systems). The essentials:

• Input conductors are sized on the drive's rated input current, not the motor's FLA, at 125%. The two figures differ — on a three-phase supply a drive's input current is often slightly below the motor FLA thanks to the drive's better displacement power factor — but several families publish an input current above their output current, so read the datasheet rather than assuming. On single-phase input the two diverge dramatically.
• Branch-circuit protection must be the type and rating specified in the drive manual, both for SCCR compliance and because standard thermal-magnetic breakers can nuisance-trip on rectifier inrush.
• Motor overload protection is normally provided electronically by the drive, configured with the motor's actual FLA. The exception is critical: one drive feeding multiple motors cannot provide individual overload protection, so each motor needs its own overload relay.
• Bypass and disconnect. If the process cannot tolerate a drive failure, specify a bypass contactor arrangement — but confirm the motor is rated for across-the-line starting and that the bypass can't be closed while the drive output is live. A local disconnect at the motor is required for servicing, and it must be interlocked or clearly labelled, because opening a disconnect while a drive is running can damage the drive's output stage.
• Grounding. Bond the motor frame back to the drive's ground terminal with a conductor run inside the same raceway as the motor leads, not to the nearest building steel. High-frequency ground currents follow the path of least impedance, not least resistance, and a remote ground path is how you get shaft currents and nuisance ground faults.

Mounting and clearances

Respect the manufacturer's clearances above, below and beside the drive — they exist because the heatsink is convection-assisted and blocking the flow triggers the same derate as a hot room. Mount vertically unless the datasheet specifically permits otherwise. Where drives are stacked vertically in a panel, the upper drive lives in the lower drive's exhaust, and its ambient must be calculated accordingly or a baffle fitted.

When sizing goes wrong: symptom to root cause

Most drives that "fail" were never faulty. They were asked to do something the rating didn't cover. This table works backward from what you're seeing on the keypad to the sizing decision that caused it.

Eight sizing mistakes we see every week

Once you have a number, here's what fits

We stock both new and tested refurbished drives across the major platforms, with a 24-month warranty on every part and repair and same-day shipping on in-stock items from Raleigh, NC.

Obsolete, discontinued or end-of-life? We hold surplus and refurbished inventory for legacy platforms and have technicians in-house who test and repair. If the drive you need hasn't been manufactured in a decade, that's a normal Tuesday for us. → New, Refurbished, or Repaired?

Frequently asked questions

Related guides

• VFD Won't Start? 9 Causes to Check Before You Replace the Drive
• New, Refurbished, or Repaired? How to decide
• Allen-Bradley PowerFlex Alternatives
• The Schneider Electric Playbook: Altivar, Modicon & Zelio Explained
• Yaskawa Servo Drives: Field Guide to Sigma Series, SGDH/SGDV & VFDs
• The LS Electric Playbook

Standards referenced on this page

• NFPA 70 (NEC), Article 430 — Motors, Motor Circuits and Controllers; Part X covers Adjustable-Speed Drive Systems. Table 430.250 gives full-load currents for three-phase motors.
• NEMA MG 1 — Motors and Generators; Part 31 defines requirements for inverter-fed motor insulation systems.
• IEEE 519 — Recommended Practice and Requirements for Harmonic Control in Electric Power Systems.
• UL 61800-5-1 — Adjustable Speed Electrical Power Drive Systems, safety requirements (supersedes UL 508C for drives).
• IEC 61800-3 — Adjustable speed electrical power drive systems, EMC requirements and specific test methods.
• IEC 61800-5-1 — Safety requirements, electrical, thermal and energy.
• IEC 61800-9-2 — Ecodesign, energy efficiency indicators for drive systems and motor starters.
• IEC 60034-30-1 — Efficiency classes of line-operated AC motors (IE codes).
• NEMA 250 / IEC 60529 — Enclosure type ratings and IP codes.

Always size and install to the edition of the code adopted in your jurisdiction and to the specific drive manufacturer's manual, which takes precedence over any general guidance on this page.