Transformer Sizing Calculator

Plan electrical capacity with clear load inputs. Apply demand, diversity, efficiency, and power factor values. Get a standard size recommendation for safer installations today.

Calculator Inputs

Sum of nameplate or estimated operating kW.
Expected simultaneity of connected loads.
Lower value increases required capacity.
Use measured PF when available.
Accounts for conversion losses.
Future capacity margin for additions.
For VFDs, UPS, and non-linear loads.
Site ambient and enclosure conditions.
Example: 415, 400, 480, 230.
Used for current estimation and checks.
Helps review starting and transient conditions.
Reset

Formula Used

Use consistent units and realistic factors for your project conditions.

How to Use This Calculator

  1. Enter the total connected kW from equipment schedules and panel estimates.
  2. Set a demand factor reflecting expected simultaneous operation.
  3. Apply a diversity factor for grouped loads sharing spare capacity.
  4. Use measured power factor and efficiency where possible.
  5. Add growth and derating allowances for expansion and harsh conditions.
  6. Click calculate, then select the recommended standard transformer size.
  7. Validate against motor starting, protection, and local requirements.

Why accurate kVA sizing reduces site risk

Temporary and permanent construction power often changes weekly. Undersized transformers can overheat, trip protection, and interrupt critical works such as concrete placement, dewatering, and commissioning. Oversizing increases capex, transport weight, and no-load losses. This calculator balances realistic demand with practical safety margins using demand, diversity, growth, and derating inputs.

Demand and diversity for mixed construction loads

Connected load totals from panel schedules rarely operate at once. Demand factor reflects simultaneous usage, while diversity accounts for how grouped loads share spare capacity. For example, welfare facilities and lighting may be steady, while welders and small tools are intermittent. Applying both factors produces a more defendable demand kW for sizing.

Power factor and efficiency effects on capacity

Transformers are rated in kVA, not kW. Low power factor increases kVA needed for the same real power. Efficiency accounts for conversion losses; using realistic values prevents optimistic sizing. Where possible, use measured PF from meters or manufacturer data for VFD-driven and UPS-supported systems.

Derating for harmonics and temperature conditions

Non-linear loads (VFDs, rectifiers, UPS) raise heating through harmonics, and high ambient temperatures reduce thermal headroom. A combined derating allowance converts base kVA into a required kVA that better matches site conditions. Always cross-check enclosure ventilation, clearance, and duty cycle.

Example input set for a typical site package

Example data: Connected load 120 kW, demand 0.80, diversity 0.90, PF 0.90, efficiency 0.98, growth 20%, harmonic derating 5%, temperature derating 5%, voltage 415 V, three-phase. The calculator returns a derated requirement near 185 kVA and selects a 200 kVA standard rating.

FAQs

1) Should I size using kW or kVA?

Use kVA for transformer selection. Convert from kW using power factor and efficiency because low power factor increases apparent power demand and current.

2) What demand factor is typical on construction sites?

Many mixed sites use 0.70–0.90 depending on simultaneity. Base it on work sequencing, shift patterns, and whether heavy equipment runs together.

3) Why does diversity factor increase required size?

In this calculator, a lower diversity factor represents less sharing of spare capacity across groups, so the adjusted demand kW increases to avoid undersizing.

4) How do harmonics affect transformer sizing?

Harmonics add heating and can reduce usable capacity. Apply harmonic derating for VFDs, UPS systems, and large rectifier loads, then verify with equipment specifications.

5) Does this replace motor starting and short-circuit studies?

No. It estimates steady-state capacity. Motor starting currents, protection coordination, fault levels, and voltage drop should be checked separately for compliance and reliability.

6) What growth allowance should I apply?

Common allowances range from 10% to 30% for evolving sites. Use higher values when future trades, temporary plants, or additional cabins are expected.

7) Which standard kVA size should I select after calculating?

Choose the next standard rating at or above the derated requirement, then confirm physical constraints, voltage class, tap settings, and utility or generator compatibility.

Example Data Table

Scenario Connected kW Demand Diversity PF Eff Growth Derating Voltage Phase Recommended Size
Site offices + lighting 55 0.85 0.95 0.92 0.98 15% 6% 230 Single 75 kVA
Mixed temporary works 120 0.80 0.90 0.90 0.98 20% 10% 415 Three 200 kVA
Motor-heavy pumping 180 0.75 0.85 0.88 0.97 25% 12% 480 Three 400 kVA
Examples are illustrative and should be validated with project data.

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Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.