How to Choose the Right Transformer Size and Capacity

Selecting the correct transformer size is an engineering decision that balances load demands, reliability, cost and future growth. This guide walks you through plain-language concepts, exact formulas, a worked example, and an actionable checklist so you can confidently specify the transformer kVA and rating best suited for your installation.

Why transformer sizing matters

A transformer rated too small will overheat, trip protection, or fail early. Oversizing wastes capital and reduces operating efficiency. Proper sizing ensures safe operation, predictable short-circuit behavior, and predictable voltage regulation under load.

Core concepts

• Transformer rating is in kVA, not kW. Transformers carry apparent power; real power depends on the power factor (PF).

  
  

• Three-phase current formula:

  
  

• Continuous vs intermittent loads: continuous loads (operating 3+ hours) may require higher sizing or code-based multipliers.

• Motor starting and non-linear loads (rectifiers, UPS) can demand large inrush or produce harmonics — both affect thermal design and protection.

Step-by-step method to pick transformer size

1. Create a complete load inventory

List every electrical load: name, rated power (kW or kVA), power factor (if known), phase type (single/three), duty (continuous/intermittent), and whether it’s a motor or a non-linear device (rectifier/UPS/charger).

2. Convert to apparent power (kVA)

For each load given in kW:

If the load is already in kVA, use it directly.

3. Apply coincidence and diversity

Not all equipment runs simultaneously. For groups of loads (lighting, receptacles, small motors), apply a conservative diversity factor based on historical data or engineering practice. For critical or unknown installations assume full coincidence unless you can justify a diversity reduction.

4. Add allowances

• Safety/future growth: commonly 10–25% (project-specific).

• Continuous load factor: local codes often require sizing continuous loads at 125% of rated current.

• Harmonic derating (K-factor): for heavy non-linear loads, select a K-factor transformer or derate the kVA.

5. Check motor starting and transient needs

Decide how to handle motor starts:

• Increase transformer kVA to absorb inrush, or

• Use soft starters, VFDs (variable-frequency drives), or stagger starts; or

• Provide a dedicated transformer for large motor loads.

6. Verify electrical limits

Use the three-phase current formula to confirm that conductors, breakers and switchgear can handle the load at specified kVA and voltage.

7. Consider environmental derating & cooling

Adjust ratings for ambient temperature, altitude and cooling class (ONAN, ONAF, OFAF, etc.). Consult manufacturer curves for precise derating — these are specific to each model.

8. Review transformer impedance and protection coordination

Transformer percent impedance affects fault levels and voltage drop. Ensure protection settings, fuse sizes, and upstream device ratings are coordinated.

9. Select the nearest standard kVA rating

Choose a standard, commercially available kVA rating equal to or greater than your calculated required kVA.

Worked example — practical arithmetic

Site data

• System: three-phase, 400 V (line-to-line)

• Loads:

• Motor bank — 80 kW, PF 0.85 (continuous)

• Lighting & receptacles — 20 kW, PF 1.00

• HVAC (compressor + fans) — 30 kW, PF 0.90

Step A — Convert each to kVA

• Motor bank: S1=80÷0.85≈94.12 kVA

• Lighting: S2=20÷1.00=20.00 kVA

• HVAC: S3=30÷0.90≈33.33 kVA

Step B — Sum apparent power
Total apparent power = 94.12 + 20.00 + 33.33 = 147.45 kVA

Step C — Allow for future growth (example +20%)
Required kVA = 147.45 × 1.20 = 176.94 kVA

Step D — Pick a standard transformer rating
Next commercial size: 200 kVA (common standard; provides conservative margin).

Step E — Check line current at 200 kVA on 400 V (approximate operating current at chosen size)

(Use the actual chosen kVA with the precise system voltage to size conductors and protective devices.)

Notes: If you kept the computed requirement (176.94 kVA) rather than the rounded 200 kVA, the corresponding current would be about 255 A; choosing 200 kVA increases available margin and changes coordination considerations.

Special considerations

Motor starting & inrush

Large motors can draw multiple times their full-load current at start. If several large motors start together, the transformer can experience a severe temporary overload. Mitigations:

• Soft starters or VFDs

• Staggered start sequences

• Dedicated transformer for heavy motor clusters

Harmonics & non-linear loads

Non-linear loads create harmonic currents that increase winding heating. Options:

• Use K-factor rated transformers

• Oversize the transformer

• Apply harmonic filters

Ambient conditions, altitude & cooling

High ambient temperature and altitude reduce cooling capacity. Manufacturer derating tables are authoritative — always verify with the vendor.

Protection & impedance

Transformer impedance limits fault currents and impacts voltage regulation. Ensure upstream switchgear and protective devices are compatible with the transformer's short-circuit characteristics.

Practical checklist before you order

• Full load list with kW/kVA and PF for each device

• Identification of motors and listed starting currents or locked-rotor currents

• Diversity/coincidence assumptions documented

• Safety and growth margin selected (10–25%)

• Harmonic assessment done for rectifiers/UPS/EV chargers

• Environmental derating considered (temperature, altitude)

• Protection and short-circuit coordination reviewed

• Physical constraints checked (weight, footprint, transport)

• Standards and local code compliance confirmed (IEC, IEEE, NEC, etc.)

• Manufacturer consulted for impedance, cooling class, and tap settings

FAQs

Q: Should I size transformers in kW or kVA?
A: Size in kVA. Convert kW to kVA using the power factor.

Q: How much margin should I add for future growth?
A: Typically 10–25% depending on business plans and risk tolerance. For continuous/process-critical loads consider higher conservatism.

Q: Are harmonics a big deal?
A: Yes — heavy non-linear loads can cause overheating even if apparent kVA looks acceptable. Use K-factor transformers or derate.

Final recommendations

1. Start with accurate load data. Good input yields good results.

2. Document assumptions (diversity, growth, motor starts). This protects you later.

3. Consult the transformer manufacturer for derating tables, impedance values and thermal limits — their data is specific to each model.

4. When in doubt, ask an electrical engineer. Complex sites (generators, paralleling transformers, large motor fleets or many non-linear loads) need coordination analysis.