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VFD Sizing Calculator
Size the drive on amps, not horsepower.
Enter the nameplate, or photograph it. We return the continuous current the drive needs, the duty class, any derating for your site, and the drives on our shelf that match.
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Sized on amps, not HP
Drives trip on current. We size on FLA × duty margin; horsepower is shown for reference only.
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Derated for your site
Ambient above 40 °C and altitude above 1,000 m reduce a drive’s rating. We apply both before matching.
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Matched to stock
Results link straight to new and IAC-certified refurbished drives on the shelf in Raleigh, NC.
Size your VFD from the motor nameplate
Photograph the nameplate or type the values. Nothing is sent to sales until you ask. The example is a 20 HP rooftop fan at 460 V.
VFDs in stock that match this motor
Drives at or above 29.7 A continuous on the 460 V class, sorted by fit. Same-day shipping from Raleigh, NC on orders placed by 4:00 PM Eastern.
How VFD sizing works: amps first, horsepower second
A drive protects itself on current. Two motors with the same horsepower can draw very different amps, so the nameplate FLA decides the size.
Start from FLA, not HP. Use the nameplate or NEC Table 430.250.- Add the duty margin for the load, then derate for heat, altitude and carrier frequency.
- Treat service factor as 1.0 on an inverter-fed motor; the drive limits the current.
Start from FLA, not HP. Use the nameplate or NEC Table 430.250.Drive amps = FLA × duty margin ÷ (temperature × altitude × carrier factors)
Then pick the next standard rating up. Single-phase input leaves about 60% of a drive’s three-phase rating.
Duty margins and derating
Typical values · check the drive manual
| Condition | AdjustmentAdjustment | Duty classDuty |
|---|---|---|
| Fan or pump · variable torqueFan or pump | +10% margin+10% | Normal |
| Conveyor or compressor · constant torqueConveyor or compressor | +15% margin+15% | Heavy |
| Crane or mill · high inertiaCrane or mill | +25% margin+25% | Heavy |
| Frequent starts · over 10 per hourFrequent starts | +5% margin+5% | — |
| Several motors on one driveSeveral motors | +25% margin+25% | — |
| Ambient above 40 °CAmbient above 40 °C | −1% per °C−1% / °C | — |
| Altitude above 1,000 mAltitude above 1,000 m | −1% per 100 m−1% / 100 m | — |
| Carrier frequency raisedCarrier frequency raised | −10 to −35%−10 to −35% | — |
Normal duty or heavy duty?
Most drives carry two ratings on the same frame. The load type decides which one applies, and the wrong choice is the most common sizing mistake.
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Normal duty
110% overload for 60 seconds. Centrifugal loads that start light and never shock-load the motor.
- Fans and blowers
- Centrifugal pumps
- HVAC and cooling towers
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Heavy duty
150% overload for 60 seconds. Loads that start under full torque or see sudden overloads.
- Conveyors and compressors
- Mixers and positive-displacement pumps
- Cranes, hoists and mills
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Worked example · Albuquerque fan
20 HP, 27 A FLA, 1,620 m and 45 °C in the enclosure.
- 27 A × 1.10 = 29.7 A
- ÷ (0.95 ambient × 0.94 altitude) = 33.4 A
- Next rating up: 34 A · ACH580-01-034A-4
VFD derating for temperature and altitude
A drive’s rating holds at 40 °C and 1,000 m. Hotter or thinner air carries away less heat, so the same drive delivers less current. The calculator applies both before it matches a drive.
- Ambient means the air at the drive’s own intake. A sealed enclosure in a 35 °C plant can run well above 40 °C inside.
- Above 2,000 m, thin air insulates as well as cools worse, so check the drive’s voltage class too.
- Raising the carrier frequency for a quieter motor costs another 10 to 35%. Size for the carrier you will run.
Usable amps = rated amps × temperature factor × altitude factor
Factors multiply, they do not add. At 45 °C and 1,620 m: 0.95 × 0.94 = 0.89, so a 34 A drive delivers about 30 A.
Derating factors the calculator applies
1% per °C above 40 °C · 1% per 100 m above 1,000 m
| Site condition | FactorFactor | What it meansNote |
|---|---|---|
| Ambient up to 40 °C (104 °F)Up to 40 °C | × 1.00× 1.00 | Full rating |
| Ambient 45 °C (113 °F)45 °C | × 0.95× 0.95 | Often one frame larger |
| Ambient 50 °C (122 °F)50 °C | × 0.90× 0.90 | Upper limit for most NEMA 1 drives |
| Ambient above 50 °CAbove 50 °C | × 0.85 at 55 °C× 0.85 at 55 °C | Use a high-ambient drive or cool the enclosure |
| Altitude up to 1,000 m (3,300 ft)Up to 1,000 m | × 1.00× 1.00 | Full rating |
| Altitude 1,500 m (4,900 ft)1,500 m | × 0.95× 0.95 | Denver, Albuquerque |
| Altitude 2,000 m (6,600 ft)2,000 m | × 0.90× 0.90 | Check the voltage class too |
| Altitude 3,000 m (9,800 ft)3,000 m | × 0.80× 0.80 | Many families need a special rating |
Combined derating below about 0.70 usually calls for a different enclosure or forced cooling, not just a bigger drive.
Motor full-load amps chart (NEC Table 430.250)
Full-load current for three-phase induction motors at the common supply voltages. Use it to check a nameplate reading or to size before the motor arrives. When you have the nameplate, the nameplate wins.
- Find the horsepower row and read the column for your supply voltage.
- Add the duty margin and the site derating, then take the next drive rating up.
- Leave FLA blank in the calculator above and it uses this table for you.
Three-phase AC induction motors · full-load amps
NEC Table 430.250 · amps
| HP | kW | 208 V | 230 V | 460 V | 575 V |
|---|---|---|---|---|---|
| ½ | 0.37 | 2.4 | 2.2 | 1.1 | 0.9 |
| ¾ | 0.55 | 3.5 | 3.2 | 1.6 | 1.3 |
| 1 | 0.75 | 4.6 | 4.2 | 2.1 | 1.7 |
| 1½ | 1.1 | 6.6 | 6.0 | 3.0 | 2.4 |
| 2 | 1.5 | 7.5 | 6.8 | 3.4 | 2.7 |
| 3 | 2.2 | 10.6 | 9.6 | 4.8 | 3.9 |
| 5 | 3.7 | 16.7 | 15.2 | 7.6 | 6.1 |
| 7½ | 5.5 | 24.2 | 22 | 11 | 9 |
| 10 | 7.5 | 30.8 | 28 | 14 | 11 |
| 15 | 11 | 46.2 | 42 | 21 | 17 |
| 20 | 15 | 59.4 | 54 | 27 | 22 |
| 25 | 18.5 | 74.8 | 68 | 34 | 27 |
| 30 | 22 | 88 | 80 | 40 | 32 |
| 40 | 30 | 114 | 104 | 52 | 41 |
| 50 | 37 | 143 | 130 | 65 | 52 |
| 60 | 45 | 169 | 154 | 77 | 62 |
| 75 | 55 | 211 | 192 | 96 | 77 |
| 100 | 75 | 273 | 248 | 124 | 99 |
| 125 | 90 | 343 | 312 | 156 | 125 |
| 150 | 110 | 396 | 360 | 180 | 144 |
| 200 | 150 | 528 | 480 | 240 | 192 |
| 250 | 185 | — | 604 | 302 | 242 |
| 300 | 220 | — | 722 | 361 | 289 |
| 350 | 260 | — | 828 | 414 | 336 |
| 400 | 300 | — | 954 | 477 | 382 |
| 450 | 335 | — | 1,030 | 515 | 412 |
| 500 | 375 | — | 1,180 | 590 | 472 |
Code values for sizing conductors and protection, for squirrel-cage and wound-rotor motors at usual speeds. NEC lists 208 V values up to 200 HP. kW values are approximate.
VFD sizing questions, answered
Call (877) 727-8757 and an engineer will check the nameplate and overload duty with you.
Can I size a drive by horsepower alone?
Not safely. Drives are rated in continuous amps. Two 20 HP motors can differ by several amps, and the wrong one trips on overcurrent or runs the drive at its limit.
Why does a correctly sized drive still trip on start?
Usually one of four things: the load needs the Heavy Duty rating but the drive was picked on Normal Duty; the drive’s ambient is higher than expected because it sits in a sealed enclosure; the carrier frequency was raised without derating; or the motor draws more amps than its horsepower suggests.
Should I use measured amps or nameplate FLA?
Measure if you can. The nameplate tells you what the motor can draw; a clamp meter on the heaviest normal cycle tells you what the process actually demands. If the reading is at or above nameplate, size on the reading.
Does single-phase input change the size?
Yes. A three-phase drive fed from a single-phase supply delivers only about 60% of its rated current, so the calculator multiplies the required amps by 1.70. Many families also need a line reactor. If the supply is permanently single-phase, a drive rated for single-phase input is the cleaner choice.
Can one drive run several motors?
Yes, sized for the sum of the motor FLAs plus about 25%. Each motor still needs its own overload relay, the motors share one speed, and the drive has to run in V/Hz mode rather than sensorless vector.
Do I need a braking resistor?
Only if the load has to stop faster than it coasts, or it overhauls the motor, as on a hoist lowering or a high-inertia fan. The resistor must not fall below the drive’s minimum ohms, and its wattage follows the braking duty cycle.
Not sure about the duty?
Send the nameplate and a line about the load. An IAC engineer confirms the overload class, the derating and the drive, then quotes it, free of charge.
(877) 727-8757 · sales@iac.us.com
What to send
- A nameplate photo, or HP, voltage, FLA and service factor
- What the motor drives and how often it starts
- Where the drive will live: ambient, altitude, enclosure
Related tools and guides
Keep going from the sizing: estimate the payback, diagnose a drive that will not start, or compare new and refurbished options.
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VFD energy savings calculator
Annual savings and payback from the load profile and tariff.
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VFD will not start: troubleshootingSymptom-to-cause table for drives that trip or stay off.
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New vs refurbished vs repaired drivesWhat the 24-month IAC warranty covers on each.
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Brand playbooks: Schneider, LSModel families, replacements and settings by brand.
Method references: NEC Table 430.250, NEMA MG 1 Part 31, IEC 61800-9-2, IEEE 519 and the drive manufacturers’ derating tables.