Disclaimer: RacewayIQ is an informational engineering tool only. All calculations must be independently verified by the user. Do not rely on these results as the sole basis for design, procurement, installation, construction, permitting, or code compliance decisions.

Transformer Calculator

Calculate transformer primary and secondary full-load current, connected-load current, transformer loading, remaining capacity, and voltage ratio from transformer kVA, voltage, phase, and load inputs.

Job Information

Project nameLocationPrepared byDateNotes

Main Inputs

Transformer calculation modeCheck an existing transformer, recommend the next standard size from target loading, or calculate currents only.PhaseSelect the transformer system so current uses the correct phase formula.Transformer nameplate rating (kVA)

Use the transformer nameplate kVA rating or proposed standard transformer size.Primary voltage, line-to-line (V)Use the applicable primary voltage for the transformer.Secondary voltage, line-to-line (V)Use the applicable secondary voltage for the transformer.Connected or demand load (kVA)Use connected load, calculated load, or demand load basis being checked.Target maximum transformer loading (%)

Planning value used to reserve spare transformer capacity. Final transformer loading depends on project requirements, load type, heat, ventilation, and manufacturer data.

Advanced Inputs

Use these when the job has project-specific assumptions, manufacturer data, or review conditions.

Efficiency (%)Efficiency is informational unless load-side/input-side current adjustment is enabled.Transformer typeInstallation environmentCooling / ventilation reviewTransformer impedance (%), optionalIf entered, a simplified transformer secondary fault-current estimate is shown.Include source contributionDocumentation field only unless a verified source contribution module is implemented.Fault current notesPrimary OCPD rating, optionalSecondary OCPD rating, optionalProtection reviewThis calculator does not select transformer overcurrent protection unless a verified protection module is implemented.

Result

Pass

Transformer nameplate rating

75 kVA

Connected/demand load

60 kVA

Transformer loading

80%

Remaining capacity

15 kVA

Target loading

80%

Primary full-load current

90.2 A

Secondary full-load current

208.2 A

Primary connected-load current

72.2 A

Secondary connected-load current

166.5 A

Voltage ratio

2.308:1

Status

Pass - Verify

Reason

Connected load is within the selected transformer loading target.

Practical Next Steps

Use the result as a design check, then verify the assumptions shown in the warnings and references.
Save or export the result with the project record if the inputs match the current design.
Recalculate when field routing, equipment selections, loads, or owner criteria change.

This calculation does not select transformer overcurrent protection or account for nonlinear loads, harmonics, inrush, impedance, temperature rise, or ventilation unless those inputs are specifically provided.

Primary and secondary full-load currents are based on the transformer nameplate kVA rating. Connected-load current may be lower when the connected load is less than the transformer rating.

Efficiency is informational unless an input-power or loss calculation mode is enabled.

Transformer impedance was not entered. Fault current was not estimated.

Verify transformer enclosure, ventilation, working clearance, ambient temperature, noise, grounding, protection, and manufacturer installation requirements.

This calculator does not select transformer overcurrent protection unless a verified protection module is implemented.

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Calculation Breakdown

Transformer Summary

Transformer calculation mode: Check existing transformer
Transformer type: Dry-type
Installation environment: Electrical room
Phase: Three-phase
Transformer nameplate rating: 75 kVA
Primary voltage: 480 V
Secondary voltage: 208 V
Connected/demand load: 60 kVA

Full-Load Current

Phase factor: sqrt(3) = 1.732
Primary full-load current: 75,000 VA / (1.732 x 480 V) = 90.2 A
Secondary full-load current: 75,000 VA / (1.732 x 208 V) = 208.2 A

Connected-Load Current

Primary connected-load current: 60,000 VA / (1.732 x 480 V) = 72.2 A
Secondary connected-load current: 60,000 VA / (1.732 x 208 V) = 166.5 A

Loading

Transformer loading: 60 kVA / 75 kVA = 80%
Remaining capacity: 15 kVA
Target loading: 80%
Result: Pass - Verify
Reason: Connected load is within the selected transformer loading target.

Voltage Ratio

Voltage ratio: 480 V / 208 V = 2.308:1

Fault Current Estimate

Transformer impedance: Not entered
Estimated transformer secondary fault current: Not estimated
Source contribution: No

Protection Review

Primary OCPD rating: Not entered
Secondary OCPD rating: Not entered
Protection review: Not reviewed

Code and Design References

Transformer Nameplate and Manufacturer Data

Use transformer nameplate and manufacturer data for final transformer rating, impedance, efficiency, temperature rise, enclosure, and installation requirements.

Nameplate and manufacturer values govern final equipment selection.

Full-Load Current

Transformer full-load current is calculated from transformer kVA, voltage, and phase.

Connected-Load Review

Connected-load current may be lower than nameplate full-load current when the connected load is less than transformer rating.

Protection Review

Transformer overcurrent protection, conductor sizing, grounding, and disconnecting means require separate review.

Fault Current

Transformer impedance can be used for a simplified secondary fault-current estimate, but final fault current should be verified through a proper short-circuit study where required.