Sizing Results
Protection Checks
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Formula Used
Single-Phase Full-Load Current
Three-Phase Full-Load Current
Breaker Planning Current
Transformer Terminal Fault Current
Voltage-Drop Estimate
How to Use This Calculator
- Enter the transformer nameplate capacity and both voltages.
- Select single-phase or three-phase operation.
- Choose the protection arrangement and code framework.
- Enter permitted primary and secondary sizing multipliers.
- Select breaker technology and trip settings.
- Add transformer inrush information when available.
- Enter fault-current and interrupting-capacity data.
- Review conductor ampacity and voltage-drop assumptions.
- Check every warning before selecting equipment.
- Export the report for professional verification.
Example Data Table
| Capacity | Phase | Primary | Secondary | Primary current | Secondary current |
|---|---|---|---|---|---|
| 15 kVA | Single | 240 V | 120 V | 62.50 A | 125.00 A |
| 45 kVA | Three | 480 V | 208 V | 54.13 A | 124.91 A |
| 75 kVA | Three | 480 V | 208 V | 90.21 A | 208.18 A |
| 150 kVA | Three | 480 V | 208 V | 180.42 A | 416.36 A |
| 300 kVA | Three | 13.8 kV | 480 V | 12.55 A | 360.84 A |
Understanding Transformer Breaker Selection
Transformer protection requires several coordinated checks. Current alone never completes the study. The primary breaker must carry normal load. It must also survive transformer energization. The secondary breaker must protect connected conductors. It must respect the selected protection arrangement.
Nameplate capacity establishes full-load current. Single-phase calculations divide volt-amperes by voltage. Three-phase calculations also use the square-root-of-three factor. Efficiency may slightly increase primary current. Multiple units can increase feeder current considerably.
Protection Multipliers Need Local Verification
Code tables contain conditions, limits, and exceptions. Transformer voltage affects permitted protection. Primary-only protection differs from dual protection. Supervised installations may use different provisions. Local amendments can change final requirements. This calculator leaves multipliers configurable for that reason.
Rounding also needs careful review. The next standard breaker size may be allowed. Another installation may prohibit upward rounding. The calculated value and selected size remain visible. This helps reviewers identify each assumption.
Inrush Can Cause Unexpected Trips
Transformer energization can create high magnetizing current. Its magnitude depends on core design. Switching angle also affects the peak. Residual flux can increase asymmetry. Step-up operation may worsen nuisance tripping. Simultaneous energization can create large feeder inrush.
Breaker instantaneous pickup should exceed expected inrush. Adequate margin is still necessary. Excessive pickup can reduce fault protection. Manufacturer curves provide the best comparison. Coordination software offers a stronger final study.
Interrupting Rating Protects the Installation
A breaker must interrupt available fault current. Ampere rating does not prove this capability. Transformer impedance limits secondary terminal current. Motors can add fault contribution. Generators can also increase available duty. Cable impedance usually reduces downstream current.
The breaker interrupting rating must exceed calculated duty. Series ratings require documented combinations. Current-limiting devices need verified let-through data. IEC devices may show several breaking values. Always use the correct application rating.
Conductors Must Match Protection
Breaker selection must coordinate with conductors. Ambient temperature can reduce ampacity. Grouping can require additional derating. Terminal ratings can limit insulation benefits. Parallel conductors must share current properly. Harmonics may increase neutral heating.
Voltage drop affects performance and starting behavior. Long secondary runs deserve special attention. Larger conductors can reduce drop. They can also reduce losses. Final conductor sizes require adopted ampacity tables.
Coordination Improves System Reliability
Selective coordination keeps upstream service available. Downstream breakers should clear local faults first. Transformer damage curves establish thermal limits. Inrush curves establish temporary current tolerance. Breaker curves should fit between both boundaries.
This calculator provides a planning overview. It highlights weak margins quickly. It also documents important assumptions. Final settings require approved manufacturer data. Qualified professionals must review installation conditions. Careful verification supports safer transformer protection decisions every time.
Frequently Asked Questions
Does one breaker percentage fit every transformer?
No. Applicable percentages depend on voltage, protection placement, device type, and adopted rules.
Why can the recommended breaker exceed full-load current?
Transformer protection often needs allowance for normal loading and temporary magnetizing inrush.
What is transformer inrush current?
It is a brief energization current caused by core magnetization and switching conditions.
Should I use primary and secondary breakers?
The correct arrangement depends on local rules, conductor protection, equipment layout, and design goals.
What breaker curve works with transformers?
The curve must carry inrush while clearing faults. Manufacturer curves should decide the final choice.
Why is interrupting capacity important?
The breaker must safely clear the maximum available fault current at its installation point.
Can transformer impedance estimate fault current?
Yes. It provides a useful terminal estimate. Source and conductor impedance still matter.
Does this calculator size conductors?
It provides a planning comparison only. Final sizing needs adopted ampacity and installation tables.
Can several transformers share one feeder breaker?
Yes, in some designs. Demand, diversity, inrush, and individual protection need review.
What changes for step-up transformers?
Current locations reverse, and energization behavior may require special manufacturer guidance.
Is the result ready for installation?
No. Treat it as preliminary engineering guidance requiring professional and authority review.