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Calcimator

Transformer Sizing Calculator

Size single-phase and three-phase transformers: calculate required KVA, full load current, and select next standard size.

Inputs

V
A

IEEE Std 141: motors 0.80–0.90; commercial mixed 0.85–0.90; resistive loads 1.0; HVAC chiller 0.85

NEC Art. 220 / IEEE Std 141: lighting 0.75–0.90; industrial 0.60–0.80; motor loads apply NEC 430.24

phase

Results

Required KVA

156.5 kVA

Next Standard Size

167 kVA

Full Load Current188.24 A
Percent Loading85%
How to Use This Calculator
  1. Enter the System Voltage in volts — this is the transformer's secondary (output) voltage.
  2. Enter the Total Load Current in amperes — sum all connected loads on the secondary.
  3. Set the Power Factor for the load mix: 0.80–0.95 is typical for industrial and commercial applications.
  4. Set the Demand Factor — the ratio of maximum expected demand to total connected load (0.6–0.9 for most applications).
  5. Select the Number of Phases (1 for single-phase, 3 for three-phase).
  6. Read the Required KVA and the Next Standard Size to select the correct transformer from manufacturer catalogs, then review the Percent Loading to ensure the transformer is not oversized.

How the result changes with Power Factor

Power FactorRequired KVANext Standard Size
0.55241.86 kVA300 kVA
0.68197.07 kVA200 kVA
0.83161.24 kVA167 kVA
0.95140.02 kVA150 kVA

What each input means

System Voltage
Secondary voltage of the transformer (e.g., 208, 240, 480V).
Total Load Current
Total connected load current in amperes.
Power Factor
Load power factor. Per NEC Article 220 and IEEE Std 141 (Red Book). Typical ranges: resistive loads (heaters, incandescent) PF=1.0; fluorescent/LED with ballast 0.90–0.95; induction motors 0.80–0.90; mixed commercial 0.85–0.90.
Demand Factor
Ratio of maximum demand to total connected load per NEC Article 220 and IEEE Std 141. Commercial lighting: 0.75–0.90; motors (NEC 430.24): 125% of largest motor + 100% of others; general commercial 0.70–0.85; industrial 0.60–0.80.
Number of Phases
Select 1 for single-phase or 3 for three-phase system.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    System Voltage = 480, Total Load Current = 200, Power Factor = 0.85, Demand Factor = 0.8 = 5 input(s) provided
  2. Calculate Required KVA
    Required KVA
    156.5 = 156.5
  3. Calculate Next Standard Size
    Next Standard Size
    167 = 167
  4. Calculate Full Load Current
    Full Load Current
    188.24 = 188.24
  5. Calculate Percent Loading
    Percent Loading = Math
    85 = 85

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