Resource search

Search DataDrivenAEC

Type at least two characters to search.

Looking for live projects? Explore open projects or browse the feed.

Open the full search page →

Declared load basisNo equipment selection

Transformer Sizing
Calculator

Convert a completed load basis to apparent power and apply an explicitly entered planning allowance.

Who it's for: electrical engineers converting a completed load basis at construction documents, before equipment selection.

No standard transformer rating is selected
The former page embedded a rating series and called the next size a recommendation. Equipment selection also depends on manufacturer, construction, temperature, impedance, fault duty, harmonics, utility requirements, losses, noise, and installation conditions.
Apparent-load summary
entered verified values

How to use this calculator

1
Complete the load study
Establish the governing connected/demand load, diversity, special loads, voltage, phase, and power factor.
2
Choose the matching basis
Enter completed apparent power directly, convert verified kW with power factor, or convert verified line current with voltage and phase.
3
Declare any allowance
Enter a project-selected planning allowance; zero is valid and no default allowance is implied.
4
Select equipment separately
Coordinate actual ratings and performance with the engineer, utility, specification, and manufacturer.

The formula

kVA from kW = kW ÷ verified PF. Single-phase kVA from current = V × A ÷ 1,000. Three-phase kVA = √3 × V × A ÷ 1,000. Planning kVA = connected kVA × (1 + entered allowance/100).

Worked example

Example
A verified 180 kW load at PF 0.90 produces 200 kVA. With no entered allowance, the worksheet reports 200 kVA; it does not select a transformer.

When this estimate will be off

  • Does not perform an NEC load calculation or select demand/diversity factors.
  • Does not select a transformer rating, construction, impedance, taps, insulation, enclosure, cooling, losses, or acoustic performance.
  • Does not evaluate motors, inrush, harmonics, nonlinear loading, unbalance, fault duty, coordination, conductors, or protection.
  • The result is planning arithmetic, not equipment adequacy, approval, or a purchase specification.

Frequently asked questions

A common mistake: Does not perform an NEC load calculation or select demand/diversity factors.

A numerical next-size lookup omits manufacturer, utility, impedance, harmonics, fault, temperature, loss, noise, and installation requirements. The worksheet therefore stops at the entered-basis apparent load.

Sources

  • User-selected adopted electrical design method — source for the completed load basis and project conditions
  • Manufacturer and utility documentation — governs available ratings and the final equipment selection

This free Transformer Sizing Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.