VFD Derating Calculation Guide: From Site Conditions to Drive Selection

A variable frequency drive that worked correctly in one factory may need to be checked again after moving to a hotter workshop or a high altitude installation. Its nameplate rated output current has not changed, but its permitted continuous output current may be lower under the new conditions. This is why VFD derating matters to drive sizing, even when the motor power in kW or HP remains unchanged.

The practical question is not simply whether a VFD needs a larger power rating. It is whether the selected drive can deliver the required current continuously, meet the load's overload demands, and operate within the manufacturer's installation limits. This guide explains how to collect site data, read a VFD derating curve, calculate usable current, and decide whether to keep, cool, or replace a drive.

Quick Answer: Identify the exact VFD model and operating conditions, then obtain the applicable manufacturer rating and derating rule. Where one output current factor applies, calculate usable current = rated continuous current × derating factor. Compare the result with required motor current and check overload and installation limits separately. 

Does Your VFD Actually Need Derating?

What VFD Derating Changes and What It Does Not

VFD derating means limiting a specified drive rating under particular operating conditions. Here, the main concern is available continuous output current, not a change to the motor's nameplate power.

Passing the continuous current check does not establish adequate overload capacity.

Five Warning Signs That a Derating Check Is Needed

Check the exact manual when any of these conditions changes:

• Higher ambient temperature: The air surrounding the drive may exceed its permitted rating without derating.
• Higher installation altitude: Reduced air density may affect cooling or other electrical limits.
• Higher carrier frequency: A quieter switching setting may increase internal losses.
• Different enclosure or mounting: Cabinet ventilation, mounting direction, spacing, and accessories may change the applicable rating.
• Different input supply: Voltage, phase configuration, or grounding arrangements may require a different assessment.

If none applies, verify that the original rating conditions still match the site. For introductory motor and VFD selection, see SZCT's guide to choosing a variable frequency drive.

Collect the Right Operating Data Before You Calculate

Measure the Temperature Where the Drive Operates

Room temperature may differ from the drive's surroundings. For example, a 35°C workshop may contain a control cabinet at 45°C. This is illustrative, not a universal temperature difference.

Follow the manual's temperature definition and measure during demanding production. Consider nearby heat sources, blocked filters, sunlight, and cabinet ventilation.

Record Altitude, Enclosure Type, and Mounting Conditions

Record altitude, drive enclosure rating, cabinet arrangement, mounting orientation, spacing, and cooling accessories. An IP54 cabinet does not make an enclosed IP20 drive an IP54 drive, nor does it establish the same thermal conditions.

Confirm the Motor and Electrical Supply Data

Record motor nameplate voltage, frequency, power, and full load current. Include supply details, application, duty, and overload requirements. Never estimate motor current from kW alone.

Illustrative entries only. Mark missing measurements to be confirmed.

Find the Correct Manufacturer Rating and Derating Curve

Match the Exact Model Number and Manual Revision

A series name such as Siemens SINAMICS G120 or PowerFlex 525 is insufficient. Variants differ in modules, frames, enclosures, and cooling arrangements.

Copy the complete catalog number, identify voltage and options, and locate the corresponding manufacturer technical document. Record its number, revision, date, and applicable pages. On modular drives, confirm the power module.

SZCT lists a SINAMICS G120 power module identified as 6SL3210-1PE23-8UL0. Use this product page for identification, not as a substitute for Siemens derating data.

Check Continuous Current and Overload Duty Separately

Drives may publish different continuous ratings for Normal Duty, Heavy Duty, or other duty classes. Select the rating appropriate to the load, rather than the largest current shown on a listing.

Review short term overload magnitude, duration, recovery conditions, and the demands of loaded starts or acceleration independently. A conveyor and a centrifugal fan with the same motor kW can require different overload capabilities.

Read the Applicable Derating Curve

Read a VFD derating curve in this order:

1. Match the voltage class, enclosure, mounting, accessories, and duty category.
2. Locate the applicable ambient temperature or altitude on the horizontal axis.
3. Follow the correct curve to the percentage of rated current on the vertical axis.
4. Convert that percentage to a decimal factor, such as 80% to 0.80.
5. Check notes, absolute operating limits, and any additional restrictions.

For an actual manufacturer example, Rockwell PowerFlex 520 technical data, 520-TD001I-EN-E (September 2025), page 17, shows different curves for mounting and enclosure arrangements. Page 18 permits two alternative high altitude treatments above 1,000 m: lower maximum ambient temperature by 5°C per additional 1,000 m or reduce output current by 10% per additional 1,000 m, subject to voltage and grounding limits. These are PowerFlex 520 rules, not universal VFD percentages.

Select the Correct Derating Method for Your Conditions

High Ambient Temperature

For VFD temperature derating, identify the permitted temperature without derating. If the measured value is higher, apply the manufacturer's method and confirm the absolute operating limit. A reduced current cannot authorize operation beyond that limit.

High Installation Altitude

For altitude derating, check the permitted range, voltage and grounding restrictions, and required method. As the Rockwell example shows, ambient temperature limits and current reduction may be alternatives. Do not automatically stack them.

Higher Carrier or Switching Frequency

A higher carrier frequency can reduce noise while increasing drive losses. Check whether the selected setting reduces rated current. Consider motor insulation and cable requirements too.

Single-Phase Input and Special Supply Conditions

Use published ratings for dedicated single phase drives. A three phase input model may use single phase only if explicitly permitted. Verify input current, voltage, protection, and revised output ratings. No universal 50% factor makes an unsupported connection acceptable.

Once the manufacturer's method is clear, current calculation becomes straightforward.

Calculate the Usable VFD Output Current

Basic Formula for a Single Applicable Derating Factor

Use the following VFD derating calculation only when the manufacturer identifies one applicable output current factor for the selected drive and operating configuration:

I_usable = I_rated × k

Here, I_usable is permitted continuous output current in amperes, I_rated is the correct manufacturer rated continuous output current in amperes, and k is the applicable decimal derating factor.

For an illustrative 30 A drive with an assumed factor of 0.80:

30 A × 0.80 = 24 A usable continuous current

This is a mathematical example, not a verified factor for any particular model. For preliminary VFD sizing after derating, rearrange the formula:

I_rated,required ≥ I_required ÷ k

This reverse calculation identifies the minimum nominal continuous current to investigate. It does not establish a compatible order number or eliminate the need to check the candidate's own ratings.

How to Handle Multiple Derating Factors

Temperature and altitude can occur together, perhaps with a higher carrier frequency. Do not automatically multiply three independently found factors. Determine which of the following instructions the manufacturer actually provides.

Method A: Combined curve or table. Read the allowable current for the specified combination directly. Do not split the result into separate invented factors.

Method B: Explicit multiplication. Multiply only the factors the manufacturer instructs you to combine, using the stated conditions and calculation order.

Method C: Another method or unclear guidance. Follow prescribed alternate limits, use the relevant engineering selection procedure, or obtain manufacturer clarification. Do not substitute addition, multiplication, or an average.

A conservative looking calculation can still be technically incorrect if it ignores an absolute temperature, altitude, or insulation limit.

When a Formula Is Not Enough

A result above the required motor current is only a preliminary pass. Confirm supply compatibility, allowed ambient temperature, altitude, mounting, cooling accessories, and applicable duty class. Separately check overload capacity and motor protection. Calculated current is not the same as an approved operating condition.

Worked Example: Should You Keep or Replace Your Existing VFD?

Define the Motor, Drive, and Derating Assumptions

Consider this hypothetical engineering example, created to show how a current calculation affects the replacement decision. Neither drive represents a verified SZCT product specification.

For illustration, both drives use the same assumed factor. In practice, each complete model may have different ratings and derating rules. Overload, supply, cooling, and control compatibility remain unverified.

Calculate the Available Current

Existing drive: 28 A × 0.80 = 22.4 A

Candidate drive: 32 A × 0.80 = 25.6 A

Although 28 A exceeds 24 A on paper, derating leaves the existing drive 1.6 A short.

The candidate exceeds the requirement by 1.6 A. The reverse calculation gives 24 A ÷ 0.80 = 30 A as a preliminary minimum nominal rating under these assumptions.

Check What the Calculation Does Not Prove

This result does not verify starting overload, supply, protective devices, braking, cabinet space, or PLC communication. Confirm the candidate's real derating factor from its manual.

Decide Whether Upsizing Is Necessary

The 28 A drive fails this assumed current check; the 32 A candidate passes it. Evaluate cooling improvements before selecting a verified replacement. This is not approval of the candidate.

If your own calculation shows a current shortfall, send SZCT the existing drive model and operating conditions for a model sourcing enquiry.

Upsize, Improve Cooling, or Choose a Different Drive?

Improve the Installation Environment

If temperature is the main constraint, inspect blocked filters, failed fans, congested air paths, and nearby heat sources before replacing equipment. Correct the cooling arrangement within manufacturer requirements, then measure the drive's actual operating ambient again. SZCT's Siemens VFD thermal management guide provides a broader introduction to cooling and enclosure considerations.

Select a Drive with More Usable Current

If the verified derated current remains below the load requirement, compare candidates by usable continuous current, not simply their kW labels. Confirm each candidate's own duty rating, current curve, overload capacity, dimensions, and cooling needs. A larger drive may introduce installation changes that must be planned.

Choose a Drive Designed for the Actual Conditions

Where high altitude, dust, restricted ventilation, or unusual power supply is the main issue, another drive configuration may suit the site better than simply selecting a higher current rating.

Verify the Replacement VFD Before Ordering

Check the Manufacturer-Specific Requirements

SZCT supplies industrial automation products across several VFD brands. Its category pages are useful for exploring options, while final derating decisions must use the exact manufacturer's documentation.

For a broader comparison of automation ecosystems and drive families, see SZCT's VFD brand comparison guide.

Verify Electrical and Mechanical Compatibility

Two drives with similar usable current are not necessarily interchangeable. Check input supply, motor voltage and current, overload requirements, branch circuit protection, and any motor cable or output filter needs. Confirm physical dimensions, mounting holes, cabinet clearances, enclosure rating, cooling accessories, and heat dissipation.

Then check control mode, PLC communication protocol, analog and digital I/O, braking, feedback, and applicable safety functions. For replacements, preserve the original part number, connection records, and a parameter backup. Cross brand substitution often requires engineering changes rather than a direct component swap.

Confirm Protection Settings After Upsizing

Before commissioning, qualified personnel should verify motor rated current settings, thermal protection, overload limits, acceleration parameters, and coordination with the electrical installation. Do not assume a larger drive's default settings adequately protect the original motor. Follow the manufacturer's commissioning instructions and validate the installed system.

Explore the available variable frequency drive categories only after defining these requirements.

VFD Derating Worksheet: What to Send for Model Selection

Minimum Information for a Derating Review

A useful VFD selection request starts with the current equipment and operating conditions. Copy this worksheet into your enquiry:

Nameplate and cabinet photos may resolve missing details.

Example of a Complete VFD Selection Request

We need to review a replacement VFD for a motor requiring 24 A continuous output current. Supply is 400 V, three phase. The drive is installed in a cabinet; maximum ambient temperature, altitude, and mounting details are attached. Existing VFD part number: [insert]. Application and required overload duty: [insert]. Quantity and delivery country: [insert]. Please advise available options, specifications requiring confirmation, and quotation terms.

Illustrative request only. Fill in the missing fields.

How SZCT Can Help with Model Sourcing

SZCT offers VFD product categories and an enquiry channel across several automation brands. Submit the complete part number and operating data, then ask for candidate specifications, availability, and quotation terms. Never assume a series name guarantees compatibility or stock.

Send your VFD specifications for a quote.

Conclusion: Calculate First, Verify the Model, Then Select

A reliable VFD selection begins with actual site conditions and manufacturer data, not a universal temperature or altitude percentage. Identify the correct duty rating, apply only the authorized derating method, and compare usable continuous output current with the application's requirement. Then verify overload performance, electrical supply, cooling, protection, and control compatibility.

If the existing drive passes all relevant checks, replacement may be unnecessary. If cooling improvements are practical, validate them before changing equipment. When a different unit is needed, use the SZCT VFD catalog and request a model and availability check with your worksheet.

FAQ

 

 

What is a VFD derating factor?

It is a multiplier representing the permitted proportion of a stated drive rating under specified conditions. For example, 80% becomes 0.80. Use a factor only when it applies to the exact drive configuration and operating conditions.

How do you calculate derated VFD output current?

With one manufacturer approved output current factor, multiply the applicable rated continuous current by that factor. Confirm the duty rating and the source of the factor before comparing the result with motor current. See the calculation method.

At what temperature does a VFD need derating?

There is no universal threshold. The temperature without derating depends on the exact drive, mounting, enclosure, and accessories. Measure temperature according to the manufacturer's definition, then consult the correct ratings and current curves.

Does high altitude always require VFD derating?

Not necessarily. First check the model's altitude range without derating. Above that range, the manufacturer may require current reduction, a lower allowable ambient temperature, or other restrictions. Check maximum altitude and voltage limits too.

How do you combine temperature and altitude derating?

Use the manufacturer's combined condition table if provided. Multiply individual factors only when the documentation instructs you to do so. If the method is unclear, obtain clarification before treating the calculation as valid.

Can a three-phase VFD run on single-phase input without derating?

Only a manufacturer permitted configuration can be considered. A dedicated single phase input drive uses its specified ratings. Other drives may require separate input and output checks, and an unsupported connection cannot be justified with an estimated percentage.

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