Enter Circuit and Transformer Data
Use a single-phase equivalent. Leave the turns ratio blank to derive it from source and load voltage. Leave both load impedance fields blank for an estimated value.
Example Data Table
| Input | Example Value | Purpose |
|---|---|---|
| Source voltage | 480 V | Primary supply voltage |
| Load voltage | 120 V | Secondary circuit voltage |
| Load current | 20 A | Connected secondary demand |
| Turns ratio | 4.00 | Primary turns divided by secondary turns |
| Load power factor | 0.90 | Real-power share of load demand |
| Efficiency | 95% | Transformer conversion allowance |
| Source impedance | 0.15 + j0.10 Ω | Supply and feeder equivalent |
Formula Used
Turns ratio: a = Np / Ns
Reflected impedance: Zref = a2 × Zload
Total source impedance: Ztotal = Zsource + Zref
Impedance-model current: Isource = Vsource / |Ztotal|
Load real power: Pload = Vload × Iload × PFload
Power-model source current: Isource = Pload / (Vsource × η × PFsource)
These formulas use RMS values and a single-phase equivalent. Treat reactance as positive for inductive loads and negative for capacitive loads.
How to Use This Calculator
- Enter the source voltage and the load voltage.
- Enter the load current from equipment data or measurements.
- Enter the transformer turns ratio, or leave it blank for automatic voltage-ratio calculation.
- Enter realistic power factors and transformer efficiency.
- Add source resistance and reactance when feeder or supply data is available.
- Enter both load impedance values only when measured or specified values exist.
- Select Calculate Source Current and compare both current estimates.
- Review notes before using the result for engineering decisions.
Source Current and Load Reflection Explained
Source current is the current drawn from the upstream supply. Load reflection describes how a transformer changes a secondary impedance when viewed from its primary side. These values matter when selecting feeders, protective devices, transformers, and distribution equipment. They also help reveal whether a proposed circuit can support its connected demand.
A transformer reflects impedance by the square of its turns ratio. A large primary-to-secondary ratio increases the apparent impedance seen by the source. This reduces primary current for the same secondary load. A smaller ratio has the opposite effect. The effect is fundamental in voltage conversion and electrical isolation.
Real installations include losses and phase angle. Load power factor separates useful real power from apparent power. Efficiency accounts for transformer heating, core losses, and winding losses. The source power factor affects the apparent input demand. Ignoring these factors can make an input-current estimate too low.
The impedance model gives another useful view. The calculator combines reflected load resistance and reactance with source resistance and reactance. It then finds total impedance magnitude. Dividing source voltage by this magnitude estimates source current. This model is useful for steady-state checks. It assumes sinusoidal conditions and a balanced, single-phase equivalent circuit.
The power model uses load voltage, load current, power factor, efficiency, and source power factor. It converts load demand into source apparent power. It then divides by source voltage. Compare this estimate with the impedance-model result. Large differences may show inconsistent inputs, omitted cable impedance, nonlinear loads, or an unsuitable equivalent circuit.
Use the result as a planning aid. Confirm all ratings against project drawings, equipment labels, local codes, and manufacturer data. Include conductor temperature limits, fault duty, overcurrent protection, and harmonic effects where applicable. Three-phase systems require line-to-line conventions and appropriate formulas. Complex networks may need a full power-flow or short-circuit study.
Good inputs produce better decisions. Enter measured resistance and reactance when they are known. Otherwise, the calculator derives an estimated load impedance from voltage, current, and load power factor. Review the warnings before accepting a result. Use conservative engineering judgment for final design selections.
Document assumptions with the calculation record. Recheck values after equipment substitutions. Keep measured data current during commissioning. Small input changes can create meaningful source-side differences. Clear records make future troubleshooting faster and much more reliable.
Frequently Asked Questions
1. What does load reflection mean?
Load reflection is the impedance a source sees through a transformer. The secondary load impedance is multiplied by the square of the primary-to-secondary turns ratio.
2. Why does the turns ratio get squared?
Voltage changes by the turns ratio and current changes inversely. Since impedance equals voltage divided by current, the combined effect produces the turns ratio squared.
3. Which source current result should I use?
Use the power-model result for demand planning. Use the impedance-model result for a circuit-equivalent check. Investigate large differences before selecting equipment.
4. Can I leave the turns ratio blank?
Yes. The calculator divides source voltage by load voltage. Enter a stated ratio when the transformer has a nonstandard design or when voltage regulation is significant.
5. What happens when load impedance fields are blank?
The calculator estimates resistance and inductive reactance from load voltage, load current, and load power factor. Measured impedance values are better for detailed work.
6. Can reactance be negative?
Yes. Negative reactance represents a capacitive condition. Positive reactance represents an inductive condition. Enter values using the sign convention used in your electrical study.
7. Does this calculator support three-phase circuits?
It uses a single-phase equivalent. For three-phase work, use the correct line-to-line or phase values and apply the appropriate three-phase power relationships.
8. Why include source resistance and reactance?
They represent supply, transformer, and feeder impedance. Including them estimates voltage drop and provides a more realistic impedance-model source current.
9. Is transformer efficiency always constant?
No. Efficiency changes with loading, temperature, and transformer design. Use manufacturer data or a conservative project assumption for preliminary calculations.
10. Can I use this result for breaker selection?
Use it as one input only. Breaker selection also requires fault current, conductor ampacity, coordination, local code rules, and equipment ratings.
11. Why do the calculator warnings matter?
Warnings identify assumptions that may distort the result. They help you catch mismatched transformer ratios, missing impedance data, or inconsistent power assumptions.