Introduction #

This guide is for electrical engineers, facility managers, and designers who need to size transformers for harmonic and non-linear loads (VFDs, UPS, rectifiers, IT). It solves the problem of selecting transformer kVA and K-factor when load current is non-sinusoidal and THD is significant. Use this knowledge when specifying transformers for data centers, industrial plants with drives, or any installation where harmonic content affects heating and capacity.

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

What Are Harmonic Loads in Electrical Systems #

Harmonic loads are non-linear loads that draw current in a non-sinusoidal waveform. The current contains integer multiples of the fundamental frequency (5th, 7th, 11th, etc.), which increase RMS current and cause additional heating in transformers, cables, and protective devices. Common sources include variable frequency drives (VFDs), uninterruptible power supplies (UPS) with rectifier input, rectifiers and power electronic loads, and IT/server power supplies. In industrial and data center applications, harmonic content is often significant and must be accounted for in transformer sizing. A transformer sized only for fundamental kVA may overheat when supplying harmonic-rich load.

Why Harmonics Affect Transformer Sizing #

Harmonics increase transformer losses in two main ways: higher RMS current (copper loss ∝ I²) and additional eddy and stray losses in windings and core due to harmonic frequencies. The result is higher operating temperature for the same fundamental kVA. Standard distribution transformers are designed and tested with sinusoidal load; they are not rated for the extra heating from harmonics. Therefore either the transformer must be derated (reducing usable kVA) or a transformer designed for harmonic load—typically K-factor rated—must be used. Sizing without considering harmonics risks overheating and shortened life. For the base sizing method, see the Transformer Sizing Guide; for quick kVA checks use the Transformer Size Calculator.

K-Factor Explained for Transformers #

K-factor is a weighting of harmonic currents used to rate transformers for non-linear load. It is defined so that a transformer with a given K-factor rating can supply the associated harmonic current spectrum without exceeding design temperature rise. Higher K-factor means the transformer is designed for more harmonic content.

K-Factor Typical application Harmonic content
K-4 General mixed load, some non-linear Low to moderate THD
K-13 Significant VFD, UPS, or rectifier load Moderate to high THD
K-20 Heavy non-linear load (e.g. data center, large VFD bank) High THD

K-4 is often used where non-linear load is a small fraction of total load. K-13 is common for industrial plants with multiple VFDs or UPS-backed loads. K-20 is used where the majority of load is non-linear (e.g. IT and UPS in data centers). Selection should be based on measured or estimated harmonic spectrum; oversizing K-factor adds cost, undersizing risks overheating. When a K-factor transformer is correctly selected, additional harmonic derating of nameplate is usually not applied; the nameplate already reflects the design for that K-factor.

How to Size a Transformer for Harmonic Loads #

Two approaches are used in practice.

1. Standard transformer with harmonic derating
Calculate required kVA from load (kW, power factor) and safety margin. Apply a derating factor for harmonics (e.g. 0.85–0.90 for moderate THD, 0.80 or lower for heavy non-linear load). Minimum nameplate kVA = Required kVA ÷ k_harmonic. Round up to standard size. This is equivalent to oversizing: you buy a larger standard unit so that after derating it still delivers the needed kVA.

2. K-factor transformer
Determine the application K-factor (from measurement or typical values for the load type). Size the transformer so that its K-factor rating is at least equal to the application K-factor and its nameplate kVA is at least equal to the required fundamental kVA (with normal safety margin). No extra harmonic derating is applied to the nameplate.

Formula (standard transformer with harmonic derating):

Transformer nameplate kVA (minimum) = (Load kVA × Safety margin) ÷ k_harmonic

Where Load kVA = kW ÷ power factor (or diversified kVA), and k_harmonic is the derating factor (e.g. 0.85–0.90 for moderate, 0.80 for heavy non-linear load). For K-factor units, use the same formula with k_harmonic = 1.0 and ensure K-factor rating ≥ application K-factor.

Example: Transformer Sizing with Non-Linear Load #

Scenario: IT load 200 kW at 0.95 PF, supplied through UPS. UPS input is rectifier-type; load is predominantly non-linear. Diversity already applied; 200 kW is the demand. Safety margin 25%.

Step 1 – Base kVA:
Load kVA = 200 ÷ 0.95 ≈ 210.5 kVA.

Step 2 – With safety margin:
Required kVA = 210.5 × 1.25 ≈ 263 kVA.

Step 3 – Harmonic derating:
Assume k_harmonic = 0.85 (moderate to high non-linear).
Minimum nameplate kVA = 263 ÷ 0.85 ≈ 309 kVA.

Selection: 315 kVA standard transformer, or a 300 kVA K-13 (or K-20) transformer if the harmonic spectrum justifies that K-factor. The K-factor unit avoids derating and may be smaller and more appropriate for the load type. Document the assumption (k_harmonic or K-factor) for the project.

When to Use K-Factor Rated Transformers #

Use K-factor rated transformers when:

  • Non-linear load is a large share of total load (e.g. data centers, VFD-heavy industrial).
  • THD is high and a standard transformer would require heavy derating (e.g. >15–20%), making a K-factor unit more economical or physically smaller.
  • Specification or standard requires K-factor for the application (e.g. some data center or healthcare specs).
  • You want to avoid continuous derating and have a nameplate that directly reflects usable capacity for the harmonic load.

Use a standard transformer with derating when non-linear load is a small fraction of total load, THD is low, or K-factor units are not available or cost-effective. For data center sizing that combines UPS, redundancy, and harmonics, see the dedicated guide.

Common Harmonic Sizing Errors #

Mistake 1: Sizing on kW Only #

Error: Sizing the transformer on load kW without converting to kVA and without applying harmonic derating or K-factor.

Correct approach: Harmonic currents increase kVA and losses. Always use kVA (kW / PF) and then apply harmonic derating (e.g. 0.85-0.90) or select a K-factor transformer. Do not size on kW alone.

Mistake 2: Using K-Factor and Derating Together #

Error: Selecting a K-factor transformer sized for the application and also applying harmonic derating to nameplate kVA.

Correct approach: If the K-factor transformer is correctly selected for the application K-factor, do not apply additional harmonic derating. Double-counting leads to unnecessary oversizing. Use either standard transformer + derating or K-factor transformer (no derating).

After sizing, cross-check with the Transformer Size Calculator using the required kVA (before derating) to confirm base load and margin; then apply derating or K-factor selection as above.

Frequently Asked Questions #

Q1: When should I use a K-factor transformer instead of derating a standard transformer? #

A: Use a K-factor transformer when non-linear load is a large share of total load, THD is high (e.g. requiring more than 15-20% derating), or the specification requires K-factor. Use standard transformer with derating when non-linear load is a small fraction, THD is low, or K-factor units are not cost-effective.

Q2: What K-factor do I need for a data center or UPS load? #

A: Data centers and UPS-fed loads often justify K-13 or K-20 depending on THD and harmonic spectrum. K-4 is for mixed loads with limited non-linear content. Base selection on measured or estimated harmonic spectrum; when in doubt, use K-13 or higher for predominantly non-linear load.

Conclusion #

Size transformers for harmonic loads by using kVA (not kW alone), then either derating a standard transformer or selecting a K-factor unit. Do not apply both derating and K-factor. Document the assumption (k_harmonic or K-factor) for future changes. For data center sizing that combines UPS, redundancy, and harmonics, see the dedicated guide.

About the Author: Sarah Martinez, P.E. is a licensed electrical engineer with 13+ years of experience in power systems design and energy management. Former utility engineer specializing in power quality, power factor correction, and industrial energy optimization. Has designed transformer and distribution systems for facilities with VFD, UPS, and data center loads. All content in this guide has been reviewed and validated by licensed engineers.