Transformer Sizing Calculator

The Transformer Sizing Calculator picks a distribution transformer for a load. You describe the load in whichever form you have it: secondary current, real power with a power factor, or apparent power directly. Apply growth and diversity factors, and it returns the design load in kVA, the smallest standard rating that covers it, how hard that rating will work, and the full-load currents on both sides.

Power is in kilovolt-amperes, currents in amperes and voltages in volts.

When the design load exceeds the largest rating in the table, the tool says so and recommends nothing. It does not extrapolate, and it does not fall back to a small rating. That refusal is a deliberate product decision, and it is described below because it is the behaviour most worth knowing about.

Purpose and model boundary

This model converts a connected load to kilovolt-amperes, applies growth and demand allowances, and selects the first rating in the workbook’s ordered standard-rating ladder that is at least as large as the design load. It also reports utilization, spare capacity, full-load currents, and voltage ratio.

It is a rating-screening model. The rating ladder is illustrative rather than a manufacturer catalogue, and the model does not perform protection, fault, thermal, harmonic, efficiency, or voltage-regulation design.

Inputs and units

• Phase selects single phase or three phase.
• Primary voltage and secondary voltage are positive line voltages in volts.
• Load input method chooses one of three mutually exclusive load bases.
• Secondary current in amperes is used only by the current method.
• Real power in kW and power factor in (0, 1] are used only by the real-power method.
• Apparent power in kVA is used only by the apparent-power method.
• Growth allowance is a multiplier from 1 to 2; 1.25 means 25% headroom, not 1.25%.
• Diversity factor is the workbook’s multiply-down demand/coincidence factor in (0, 1]; it is not the reciprocal convention sometimes used for the term diversity factor.

The contract carries visibility controls for method-specific fields. Hidden fields remain part of the fixed contract but are ignored by the selected workbook branch.

Governing relationships

Let m = 1 for single phase and m = sqrt(3) for three phase. Connected apparent load S_connected is calculated by the selected method:

S_connected = m x V_secondary x I_secondary / 1000

or

S_connected = P_real / PF

or

S_connected = entered kVA

The design load is:

S_design = S_connected x Growth allowance x Diversity factor

The selected rating S_rating is the first value in the workbook’s ascending rating ladder satisfying:

S_rating >= S_design

When the design load is non-positive, the growth allowance is out of range, or the design load exceeds the largest table value, the rating result is explanatory text rather than a numeric rating.

For a valid numeric rating:

Spare capacity = S_rating / S_design - 1

Utilization = S_design / S_rating

Primary full-load current = S_rating x 1000 / (m x V_primary)

Secondary full-load current = S_rating x 1000 / (m x V_secondary)

Turns ratio = V_primary / V_secondary

Full-load currents are calculated at the selected transformer rating, not at the connected or design load.

Calculation sequence

1. Select the phase multiplier and load-input branch.
2. Convert the active load inputs to connected kVA.
3. Multiply connected kVA by growth allowance and the multiply-down diversity factor.
4. Compare the design load with the workbook’s largest rating and validate the growth range.
5. Select the first rating at or above the design load, or return the workbook’s refusal text.
6. For a valid rating, calculate spare capacity, utilization, primary and secondary full-load current, and turns ratio.

Outputs and interpretation

Selected standard rating is the primary recommendation and is limited to the workbook ladder. Design load with growth and diversity is the kVA against which that recommendation is made; connected load is the pre-adjustment value.

Utilization shows design load divided by selected rating. Spare capacity shows rating headroom relative to design load. A large gap is not automatically wrong because rating ladders are discrete, but the workbook flags more than 50% spare capacity for review. Primary and secondary full-load current support downstream analysis but are not protection settings. Turns ratio is the entered voltage ratio.

Validation and status logic

The workbook evaluates status in this order; the first matching row is returned.

In the real-power branch, a zero power factor causes the calculated load to collapse to the workbook’s guarded zero value, so the first status row is the exact returned message. The proxy may reject other out-of-contract values before this formula is reached.

Assumptions and limitations

The model assumes balanced steady-state loading and ideal conversion between the selected load basis and kVA. Growth and diversity are user judgments applied as simple multipliers. The rating ladder is only the table delivered in the workbook; market availability, manufacturer product ranges, parallel units, redundancy, and procurement constraints are not evaluated.

Transformer impedance, available fault current, voltage regulation, energization inrush, losses, efficiency, temperature rise, insulation class, enclosure, cooling, altitude, ambient temperature, harmonics/K-factor, motor starting, sound, grounding, and primary/secondary protection are outside the model. These factors can govern a real selection even when the kVA result is arithmetically sufficient.

Restrictions and non-computing states

Only the input fields belonging to the selected load method affect connected load. Connected load and both voltages must be positive. Growth allowance must be between 1 and 2, and diversity factor must be greater than zero and no greater than one. A design load above the table receives no rating; the model intentionally refuses rather than returning the largest or wrapping to a smaller value.

Errors and warnings

The LogicCommons proxy can reject enum, type, or bound violations before SpreadsheetWeb runs. Workbook NOT VALID messages mark unusable sizing states. Workbook CHECK messages indicate a completed calculation that needs engineering review or, above the ladder, an explicit refusal to recommend. A proxy, network, or SpreadsheetWeb service failure is not a transformer-sizing status.

The workbook derives its relations rather than reproducing any manufacturer catalogue or jurisdiction table. The kVA conversions are the standard single- and three-phase relations, and the rating ladder is an illustrative set of conventional sizes.

• Schneider Electric. Transformer selection formulas, a manufacturer reference for the sizing relations. https://download.schneider-electric.com/files?p_Doc_Ref=39000-323-01&p_EnDocType=Instruction+sheet&p_File_Name=39000-323-01.pdf
• Schneider Electric. Demand and diversity definitions, for how growth and diversity factors are conventionally applied. https://productinfo.se.com/0100db2301_elecdistfunddesignguide/viewer?docidentity=LoadPlanning-A48A0BDD&extension=xml&lang=en&manualidentity=DocumentTypeProductCategoryRangeNam-A482E1D1
• Wikipedia. Transformer. https://en.wikipedia.org/wiki/Transformer

The standard rating ladder shipped with the workbook is illustrative and carries no authority; confirm the available range and lead times with the manufacturer. Overcurrent protection sizing for transformers is a code rule not addressed here. No trademark or organisation name appearing in this tool implies endorsement by its owner.

Additional source notes migrated from Methodology

The workbook uses standard single- and three-phase kVA/current relationships and an illustrative internal rating ladder. See the page’s References section for the manufacturer formula and demand/diversity sources. Confirm the available rating, protection, installation, and derating requirements with the manufacturer and governing code.