Transformer sizing calculator
Free transformer sizing calculator: get load apparent power from secondary (load) voltage, current, primary voltage, growth allowance and design loading target.
Project inputs
Updates as inputs change. Unit changes preserve the same physical values.
Method & assumptions
US (NEC) only. Ratings come from separate single-phase and three-phase lists of common ANSI/IEEE C57 sizes; confirm availability with the manufacturer. The 80% design loading target is common design practice for headroom, not an NEC limit; Article 450.3 sets overcurrent protection from rated current. Does not check inrush, harmonics (K-factor), altitude or ambient derating. Engineering arithmetic, not installation approval.
Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.
Sources and validation →Worked example
Secondary (load) voltage: 240 V · Current: 20 A · Transformer: Single phase · Primary voltage: 480 V · Growth allowance: 0 % · Design loading target: 80 %
Load apparent power: 4.8 kVA · Required rating at design loading: 6 kVA · Next standard single-phase rating: 7.5 kVA · Load as share of selected rating: 64 % · Primary full-load current (selected rating): 15.625 A · Secondary full-load current (selected rating): 31.25 A
This example uses the stated inputs; results for the entered values appear above.Estimating and invoicing tool
Ampxa is developing an estimating and invoicing application for electrical contractors. The list below receives notice when it opens.