Introduction #

This guide is for electrical engineers, facility managers, and designers who need to size transformers for data center loads. It solves the problem of accounting for IT load, UPS and distribution overhead, cooling, redundancy (N+1, 2N), and harmonics when selecting transformer kVA. Use this knowledge when specifying data center power distribution, planning N+1 or 2N transformer schemes, or coordinating transformers with UPS and non-linear load.

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

Data Center Electrical Load Characteristics #

Data center loads differ from typical commercial or industrial loads: IT equipment (servers, storage, network) is the core load, with significant support loads (cooling, lighting, auxiliary). IT load is often highly non-linear (harmonic-rich) and is frequently backed by UPS, which adds rectifier input and further harmonics. Load may be relatively constant (always-on) with limited diversity. Redundancy (N+1, 2N) is common, so transformer sizing must account for both the total load per path and the redundancy topology. For the base sizing method, see the Transformer Sizing Guide. This section focuses on data-center-specific factors: UPS and non-linear load, redundancy, and the combination of harmonics and derating.

UPS and Non-Linear Load Considerations #

UPS systems supply the critical IT load. Double-conversion UPS with 6-pulse rectifier input draw non-sinusoidal current and produce high total harmonic distortion (THD) on the transformer. 12-pulse or active front-end units reduce harmonics but do not eliminate them. The transformer supplying the UPS (and any other non-linear load) must be sized or derated for harmonic heating: either use a K-factor rated transformer (e.g. K-13 or K-20 for data centers) or apply harmonic derating (e.g. 0.85–0.90) to a standard transformer. In addition, transformer kVA must account for UPS input power (load ÷ UPS efficiency) and any distribution losses. Typical overhead above IT load for UPS and distribution is on the order of 5–15%; add this to the IT kVA when sizing the transformer. For detailed harmonic sizing, see Transformer Sizing for Harmonic Loads.

Redundancy (N, N+1, 2N) and Transformer Sizing #

Data center power often uses redundant paths. Sizing depends on the topology:

Configuration Meaning Transformer sizing per path
N Single path, no redundancy Size for full load (IT + cooling + overhead + margin).
N+1 One extra unit; any one can carry full load Each transformer sized to carry full critical load so that one unit can be out of service. Same kVA per transformer as for N, but with redundancy.
2N Fully redundant dual path Each path sized for full load. Each transformer in a path sized for full load of that path.

For N+1, the total load does not change, but each transformer must be able to supply the entire load when the other is off. So: calculate total required kVA (IT + cooling + overhead + margin), then select transformer nameplate ≥ that kVA for each unit in the N+1 pair. For 2N, each path is independent; size each path’s transformer(s) for the full load assigned to that path. Do not divide the load by the number of units when each unit must be capable of full load.

Formula (single path, before redundancy):

Total kVA = (IT_kW ÷ PF) × (1 + UPS/distribution overhead) + Cooling_kVA + Auxiliary_kVA
Required kVA = Total kVA × (1 + Safety margin)

For N+1: Per-transformer nameplate kVA ≥ Required kVA (each unit must be able to supply the full required kVA when the other is out). Then apply harmonic derating or K-factor as below.

Harmonics and Derating in Data Centers #

Data centers combine high non-linear load (UPS, IT power supplies) with possible high ambient (server rooms, confined spaces). Both affect transformer capacity.

  • Harmonics: Apply harmonic derating (e.g. 0.85–0.90) to the required kVA when using a standard transformer, or select a K-factor transformer (K-13 or K-20) sized for the required kVA so that no additional harmonic derating is applied. See Transformer Sizing for Harmonic Loads.
  • Ambient / altitude: If ambient exceeds 40°C or altitude exceeds 1000 m, apply temperature and altitude derating. Combined: Minimum nameplate kVA = Required kVA ÷ (k_temp × k_altitude × k_harmonic).

Use both when applicable: first calculate required kVA (load + margin), then apply derating factors for harmonics and environment, then round up to standard size. For N+1, the result is the minimum nameplate per transformer.

Example: Data Center Transformer Sizing #

Given: Critical IT load 500 kW at 0.95 PF. UPS efficiency 94%; distribution and UPS overhead 10% of IT load. Cooling 30% of IT load (150 kW at 0.85 PF). Auxiliary 20 kW at 0.9 PF. N+1 transformer configuration. Safety margin 25%. Harmonic derating 0.90 (no K-factor). Ambient and altitude standard.

Step 1 – IT and UPS/distribution:
IT kVA = 500 ÷ 0.95 ≈ 526.3 kVA.
Overhead 10%: 526.3 × 0.10 ≈ 52.6 kVA.
Subtotal: 526.3 + 52.6 = 578.9 kVA.

Step 2 – Cooling and auxiliary:
Cooling kVA = 150 ÷ 0.85 ≈ 176.5 kVA.
Auxiliary kVA = 20 ÷ 0.9 ≈ 22.2 kVA.
Total kVA = 578.9 + 176.5 + 22.2 ≈ 777.6 kVA.

Step 3 – Safety margin:
Required kVA = 777.6 × 1.25 ≈ 972 kVA.

Step 4 – Harmonic derating:
Minimum nameplate = 972 ÷ 0.90 ≈ 1080 kVA.

Step 5 – N+1: Each transformer must be at least 1080 kVA. Select 1000 kVA (if acceptable per design) or 1250 kVA (next standard). With 1000 kVA: usable per unit = 1000 × 0.90 = 900 kVA, which is below 972; so 1250 kVA per transformer is the correct choice. Each unit can then supply the full required load with harmonic derating applied.

Verify with the Transformer Size Calculator using the load kVA (before redundancy) to confirm base numbers; then apply harmonic derating and N+1 as above.

Common Data Center Transformer Sizing Mistakes #

Mistake 1: Dividing Load by Two for N+1 #

Error: Sizing each transformer for half the total load in an N+1 configuration.

Correct approach: For N+1, each transformer must be able to supply the full required load when the other is out of service. Calculate total required kVA (IT + cooling + overhead + margin), then apply harmonic derating; the result is the minimum nameplate kVA per transformer. Do not divide by the number of units.

Mistake 2: Ignoring Harmonics or Derating #

Error: Sizing on IT and cooling kVA only, without harmonic derating or K-factor, or without ambient/altitude derating when applicable.

Correct approach: Data center load is highly non-linear (UPS, IT). Apply harmonic derating (e.g. 0.85-0.90) to required kVA or select a K-factor transformer (K-13 or K-20). If ambient exceeds 40°C or altitude exceeds 1000 m, apply temperature and altitude derating. Combine factors: minimum nameplate = Required kVA / (k_temp x k_altitude x k_harmonic).

Best Practices for Data Center Transformer Selection #

  • Document assumptions: Record IT load, PF, overhead %, cooling and auxiliary, margin, redundancy (N+1/2N), and derating factors (harmonic, ambient, altitude). This supports future capacity reviews and avoids undersizing when load grows.
  • Match redundancy to topology: For N+1, size each transformer for full load; do not divide load by two. For 2N, size each path for the load on that path.
  • Combine harmonics and derating: Apply harmonic derating (or K-factor) and, if needed, temperature/altitude derating together. Use manufacturer curves for the actual product.
  • Avoid over-margin: 20-25% margin is typical. Do not add 50% or more for data center; it leads to oversizing, higher cost, and possibly worse part-load efficiency.
  • Coordinate with UPS and distribution: Ensure transformer secondary voltage and kVA match UPS input requirements and that upstream protection and cables are sized for the derated capacity and redundancy scheme.

Frequently Asked Questions #

Q1: How do I size transformers for N+1 data center power? #

A: Calculate total required kVA (IT + cooling + overhead + margin), then apply harmonic derating (or K-factor). Each transformer in the N+1 pair must have nameplate kVA at least equal to that result, so either unit can carry the full load when the other is out. Do not divide the load by two.

Q2: Why do data center transformers need harmonic derating or K-factor? #

A: Data center load is predominantly non-linear (UPS rectifier input, IT power supplies). Harmonic currents increase transformer heating. Either derate a standard transformer (e.g. 0.85-0.90) or use a K-factor transformer (K-13 or K-20) sized for the required kVA so that no additional harmonic derating is applied.

Conclusion #

Data center transformer sizing must include IT load, UPS and distribution overhead, cooling and auxiliary, safety margin, and redundancy topology. For N+1, each transformer is sized for full load. Apply harmonic derating or K-factor, and temperature/altitude derating when conditions exceed standard. Document assumptions and verify with the calculator.

About the Author: David Wang, P.E. is a power systems engineer with 10+ years of experience in critical power systems and data center infrastructure. Specializes in UPS sizing, battery systems, and backup power design for mission-critical applications. Has designed transformer and distribution systems for data centers and manufacturing facilities. All content in this guide has been reviewed and validated by licensed engineers.