🔄 Recommended VFD Rating
Derating Breakdown
Full Calculation Table
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How to Use the VFD Sizing Calculator
Use the nameplate full load amps rather than a horsepower-based estimate, since two motors with the same horsepower can draw different actual current.
Use the actual elevation of the install site and the expected temperature inside the mechanical room or enclosure, not necessarily the outdoor design temperature.
The calculator applies general altitude and temperature derating guidelines to the motor's FLA and recommends a VFD size with enough margin to cover the derated output requirement.
Sizing a VFD for HVAC Fan and Pump Motors
A variable frequency drive needs to be sized for more than just the motor's horsepower rating. Its continuous output current rating has to meet or exceed the motor's full load amps under the actual installation conditions, and both altitude and ambient temperature can reduce how much current a given drive can safely deliver before that drive even starts working.
Why Altitude Affects VFD Output
Thinner air at higher elevation carries away less heat from the drive's internal components, since air density and convective cooling both drop with altitude. Most manufacturers rate their drives for full output only up to about 1000 metres (3300 feet) above sea level. Above that elevation, a commonly used general guideline derates output current by roughly 1% for every 100 metres of additional altitude, though the exact curve varies by manufacturer and should be confirmed against the specific drive's documentation before finalizing a selection.
Why Ambient Temperature Matters Just as Much
VFDs generate heat internally from the switching losses in their power electronics, and that heat needs somewhere to go. Most drives are rated for full output up to 40°C ambient temperature. Above that, a commonly cited general guideline derates output current by roughly 2% per degree Celsius above 40°C, though again this varies by manufacturer and drive model. Mechanical rooms, especially ones with multiple pieces of running equipment, can run warmer than expected, so measuring or reasonably estimating the actual enclosure temperature gives a far more accurate result than assuming standard conditions.
Combining Both Derating Factors
When a project has both higher altitude and high ambient temperature, the two derating factors combine, which can meaningfully shrink a drive's effective output capacity compared to its catalog rating. This calculator multiplies both factors together against the motor's nameplate FLA to find the minimum continuous output current the VFD needs to deliver under your actual site conditions, then suggests a standard drive size with margin to cover that requirement.
Always Verify Against the Manufacturer's Derating Chart
The percentages used in this calculator reflect commonly published general guidelines, not a universal standard that every manufacturer follows exactly. Before finalizing a drive selection for a project with unusual altitude or temperature conditions, check the specific manufacturer's derating chart for the exact model under consideration, since some drives handle these conditions better or worse than the general guideline suggests.
Next Steps
Once you've confirmed the VFD size, check the wire size calculator for the conductors feeding the drive, and the breaker size calculator for input-side overcurrent protection. The motor starter calculator covers NEMA-style starters if the application doesn't need variable speed control.
Frequently Asked Questions
Thinner air at higher altitude carries away less heat from the drive's internal components, so most manufacturers rate VFDs for full output only up to about 1000 metres (3300 feet) above sea level. Above that elevation, a commonly used general guideline derates output current by roughly 1% for every 100 metres of additional altitude, though the exact curve varies by manufacturer and should be confirmed against the specific drive's documentation.
Use the maximum expected temperature inside the electrical room or enclosure where the VFD will be mounted, not the outdoor design temperature, unless the drive is mounted outdoors directly. Mechanical rooms can run significantly warmer than outdoor ambient, especially near other heat-generating equipment, so measuring or estimating the actual enclosure temperature gives a more accurate derating calculation than assuming standard conditions.
If the room is reliably maintained below 40°C at all times, including during a cooling system failure or peak summer load, temperature derating may not apply. However, altitude derating is based on the site's physical elevation and applies regardless of room cooling, since it affects the drive's ability to dissipate heat through its enclosure into the surrounding air, not just the air temperature itself.
Selecting a VFD with one size of extra margin beyond the calculated requirement is common practice and a reasonable way to add a safety buffer against derating uncertainty. However, skipping the derating calculation entirely on a project with significant altitude or high ambient temperature risks selecting an undersized drive that overheats or trips under real operating conditions.
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