Introduction #

This guide is for electrical engineers, facility managers, and designers who need to apply transformer derating for temperature, altitude, and harmonics. It solves the problem of sizing transformers correctly when site conditions exceed standard rating basis (40°C ambient, 1000 m altitude, sinusoidal load). Use this knowledge when specifying transformers for hot or high-altitude locations, for harmonic-rich loads, or when reviewing existing installations for derating compliance.

For the overall sizing process and formulas, see the Transformer Sizing Guide.

What Is Transformer Derating and Why It Matters #

Transformer derating is the practice of limiting the usable kVA of a transformer below its nameplate rating when operating conditions are worse than the standard reference conditions (typically 40°C ambient, 1000 m altitude, sinusoidal load). Derating is not a safety margin: it corrects the nameplate rating for actual environment and load so that the transformer does not overheat. A safety margin is applied on top of the corrected load requirement when you size the transformer. Ignoring derating in high ambient, high altitude, or harmonic-rich applications leads to overheating, insulation degradation, and shortened life. Applying the correct derating factors ensures the selected transformer can carry the design load within temperature limits under real site conditions.

For the overall sizing process, see the Transformer Sizing Guide. After applying derating, you can check the resulting kVA with the Transformer Size Calculator.

Main Transformer Derating Factors #

Ambient Temperature Derating #

Transformers are rated for a maximum ambient temperature (often 40°C for dry-type). Above this, cooling is reduced and the same load produces higher winding and core temperatures. Manufacturer data sheets provide derating curves; a typical rule for dry-type transformers is to derate by about 1.5% per degree C above 40°C. At 50°C ambient, a 100 kVA unit may be limited to approximately 85 kVA continuous. Liquid-filled units may use different base temperatures and curves. Always use the curve for the specific product and enclosure.

Altitude Derating #

At higher altitude, air density decreases and convective cooling is reduced. Above 1000 m (3300 ft), derating is required. A common rule is 0.5% derate per 100 m above 1000 m. At 2000 m a transformer might be derated to about 95% of nameplate; at 3000 m, to about 90%. IEEE and manufacturer literature give altitude correction factors; use them for the actual installation elevation.

Harmonic Load Derating #

Non-sinusoidal (harmonic) currents increase winding eddy and stray losses and can raise hotspot temperature. Standard transformers are not designed for significant harmonic content. When the load has high total harmonic distortion (THD), either derate the transformer or use a K-factor rated unit. Typical derating for moderate harmonic loads is 10–20% depending on THD and harmonic spectrum; for heavy non-linear loads (e.g. large VFD or UPS concentration), 20% or more may be required. K-factor transformers are designed for harmonic heating and may not need the same derating when selected per the application K-factor.

Ventilation and Cooling Conditions #

Enclosed installations or restricted airflow reduce heat dissipation. NEMA enclosure type (e.g. NEMA 1 vs NEMA 12) and clearances affect cooling. Follow manufacturer derating for the actual enclosure and installation (indoor/outdoor, ducting, etc.). NEC and IEEE guidelines on clearances and ventilation must be met. Poor ventilation can compound temperature derating: a hot room plus an enclosed transformer may require a larger unit or improved cooling.

Typical Transformer Derating Table #

The following table gives indicative derating multipliers for ambient temperature and altitude. Use manufacturer data for final design.

Condition Derating multiplier (typical) Notes
40°C ambient 1.00 Standard rating base
45°C ambient 0.92–0.95 Dry-type; check manufacturer
50°C ambient 0.85–0.90 Dry-type
55°C ambient 0.78–0.85 Dry-type
≤1000 m altitude 1.00 Standard rating base
1500 m altitude 0.975 ~0.5% per 100 m above 1000 m
2000 m altitude 0.95
2500 m altitude 0.925
3000 m altitude 0.90
Combined (e.g. 45°C + 2000 m) Multiply: k_temp × k_altitude Apply both when applicable

Harmonic derating is not in this table because it depends on THD and spectrum; use 0.85–0.90 for moderate harmonic load and 0.80 or lower for heavy non-linear load unless a K-factor transformer is used.

How to Apply Derating in Transformer Sizing Calculations #

Derating adjusts the available capacity of a given transformer; it does not replace the need for a safety margin. Steps:

  1. Calculate the required kVA from load (kW, power factor, diversity) per the Transformer Sizing Guide.
  2. Apply a safety margin (e.g. 1.25) for growth and contingencies → Required kVA.
  3. Determine derating factors for the site: ambient temperature, altitude, harmonic content (or K-factor), and ventilation.
  4. Size the transformer so that its derated capacity is at least equal to Required kVA.

Formula (derating applied to required load):

Minimum transformer nameplate kVA = Required kVA ÷ (k_temp × k_altitude × k_harmonic)

Where:

  • Required kVA = load-based kVA including safety margin (e.g. diversified kVA × 1.25).
  • k_temp = temperature derating factor (e.g. 0.92 at 45°C).
  • k_altitude = altitude derating factor (e.g. 0.95 at 2000 m).
  • k_harmonic = harmonic derating factor (e.g. 0.90 for moderate THD), or 1.0 if using a K-factor transformer sized for the application.

Then round up to the next standard transformer size. The result is the nameplate kVA to specify; the usable capacity at site is nameplate × k_temp × k_altitude × k_harmonic.

Example: Transformer Derating Calculation #

Given:
Load 80 kVA after diversity and power factor. Safety margin 25% → required kVA = 80 × 1.25 = 100 kVA.
Site: 45°C ambient, 2000 m altitude, moderate harmonic load (THD ~25%, no K-factor transformer).

Derating factors (from manufacturer or typical values):

  • 45°C: k_temp = 0.92
  • 2000 m: k_altitude = 0.95
  • Harmonic: k_harmonic = 0.90

Calculation:

Minimum nameplate kVA = 100 ÷ (0.92 × 0.95 × 0.90) = 100 ÷ 0.787 ≈ 127.1 kVA.

Selection: Next standard size 150 kVA. At site, usable capacity = 150 × 0.92 × 0.95 × 0.90 ≈ 118 kVA, which is above the 100 kVA required. Without derating, 100 kVA would have been selected and the unit would have been overloaded at 45°C and 2000 m with harmonic load.

Common Derating Mistakes Engineers Make #

Mistake 1: Using Derating Instead of a Safety Margin #

Error: Treating derating as the only margin and skipping a separate safety margin for load growth and uncertainty.

Correct approach: Derating corrects nameplate for environment and load; the safety margin covers growth and contingencies. Apply both: first add safety margin to get Required kVA, then apply derating to get minimum nameplate kVA. Round up to standard size after derating.

Mistake 2: Ignoring Combined Effects #

Error: Applying only temperature or only altitude derating when both (and possibly harmonics) apply.

Correct approach: Temperature, altitude, and harmonic factors multiply. A 100 kVA unit at 45°C and 2000 m with harmonic derating may deliver only ~79 kVA. Use: Minimum nameplate kVA = Required kVA ÷ (k_temp × k_altitude × k_harmonic). Document all factors for the project.

When to Upsize the Transformer #

Upsize (select a higher nameplate kVA) when:

  • Derating factors reduce usable capacity below the required kVA (as in the example above).
  • Future load growth is expected; size for future load and derating so you do not replace the unit soon.
  • Redundancy or reliability targets require headroom (e.g. N+1 with each unit sized for full load plus derating).
  • Ambient, altitude, or harmonic conditions are uncertain; use conservative derating factors.
  • Enclosure or ventilation is worse than standard; apply or reinforce ventilation derating.

Document the derating assumptions (ambient, altitude, THD, enclosure) in the project so that maintenance and future expansion use the same basis. After sizing, verify with the Transformer Size Calculator using the derated capacity as the effective limit.

Frequently Asked Questions #

Q1: Is transformer derating the same as a safety margin? #

A: No. Derating corrects the nameplate rating for actual conditions (temperature, altitude, harmonics) so the transformer does not overheat. A safety margin is applied on top of the load requirement for growth and uncertainty. Apply both: required kVA = load kVA × safety margin; then minimum nameplate = required kVA ÷ (derating factors).

Q2: When is altitude derating required? #

A: Above 1000 m (3300 ft), air density drops and cooling is reduced. Apply altitude derating per manufacturer or typical rule (e.g. 0.5% per 100 m above 1000 m). At 2000 m, derating to about 95% of nameplate is typical; at 3000 m, about 90%. Use manufacturer data for the specific transformer.

Conclusion #

Transformer derating factors adjust usable capacity for temperature, altitude, harmonics, and ventilation. Apply derating in addition to a safety margin, multiply factors when multiple conditions apply, and document assumptions for compliance and future expansion. Use the formula Minimum nameplate kVA = Required kVA ÷ (k_temp × k_altitude × k_harmonic) and round up to the next standard size.

About the Author: James Chen, P.E. is a licensed electrical engineer with 15+ years of experience in industrial power systems design. Former Schneider Electric application engineer specializing in 3-phase motor control and power distribution. Has designed transformer and distribution systems for manufacturing facilities, chemical plants, and high-ambient installations. All content in this guide has been reviewed and validated by licensed engineers.