Transformers and Power Supplies
Current Transformer Ratio Calculator
Use Current Transformer Ratio when primary current is the quantity you need. Enter secondary current and CT ratio, then compare the answer with the equipment or operating limits that apply to your case.
Run the Current Transformer Ratio scenario
Check the source and units before calculating.
Input notes
Record waveform, frequency, load level, and winding or device temperature. Write down whether each entry is measured, rated, assumed, or calculated.
A prefix error can move the result by several orders of magnitude. For the companion primary current calculation, open Current Transformer Reading Calculator.
- Secondary current
- Default example: 5 A. Enter secondary current in A.
- CT ratio
- Default example: 200. Enter CT ratio.
Formula used on this page
The arithmetic is based on Ip = Is × ratio. The participating entries are Secondary current and CT ratio.
The defaults produce 1,000.00 A. Replace them with measurements from one operating state.
Calculate primary current from secondary current and CT ratio. For a downstream secondary current value, open Transformer Current Calculator.
Transferring the result
Archive secondary current and CT ratio with primary current. Identify the instrument, rating, or assumption behind each entry.
Pass primary current before applying any display rounding. Keep a separate displayed value if reporting precision differs.
Applying the result
The calculated Primary current is meaningful only with its input conditions attached. Compare it with voltage stress, current stress, ripple, loss, regulation, and temperature.
Save the raw entries before testing another scenario.
Common entry mistakes
Values recorded at different operating points should not share one case.
Trace an unexpected result back through the raw entries first.
Assumptions and limitations
Never open-circuit a loaded current-transformer secondary.
Depending on the application, account for isolation, creepage, waveform shape, and manufacturer operating limits. Test an additional scenario when an omitted effect has a plausible range.
Record waveform, frequency, load level, and winding or device temperature.
Trace the result back to its source
Begin by confirming CT ratio at the operating point represented by the other entries. Record whether it came from a meter, nameplate, data sheet, or design assumption.
The displayed primary current should be checked against a no-load, nominal-load, and limiting-load case. Investigate an unexpected magnitude before changing the model or adding margin.
Keep a baseline and vary the least certain measurement first. A separate analysis may be needed for leakage inductance, parasitic capacitance, saturation, switching transients, and cooling.
Checking the measurement basis
The calculator rejects values outside its stated field limits, but a numerically valid entry can still describe the wrong condition. Confirm secondary current and CT ratio against the original source before saving the result.
Use an additional case for diode loss, ESR, regulation, ripple current, and thermal resistance when that behavior is not negligible. Keep the baseline intact so the effect of each changed assumption remains visible.