CT Ratio Inputs
Fill any method. Provide either currents or turns. Nameplate and burden fields are optional, but improve the analysis.
Formula Used
- Current ratio: CT Ratio = Ip / Is
- Turns relationship (ideal): Ip / Is ≈ Ns / Np
- Nameplate ratio: Rn = Inp / Ins
- Ratio error vs nameplate: Error% = ((Rcalc − Rn) / Rn) × 100
- Burden: VA = Vsec × Is and Vsec ≈ VA / Is
These equations support quick checks. CT behavior can vary with burden, saturation, wiring, and waveform.
How to Use This Calculator
- Enter either measured currents (Ip and Is) or turns (Np and Ns).
- Optionally add nameplate ratings to estimate ratio error.
- Add burden VA or secondary voltage to evaluate loading.
- Press Submit to view results above the form.
- Use the export buttons to download CSV or PDF.
Example Data Table
| Primary (A) | Secondary (A) | Calculated Ratio | Nameplate | Error (%) | Burden (VA) |
|---|---|---|---|---|---|
| 200 | 5 | 40 | 200/5 | 0 | 10 |
| 600 | 5 | 120 | 600/5 | 0 | 15 |
| 300 | 1 | 300 | 300/1 | 0 | 5 |
Example rows are illustrative. Use your site measurements for engineering decisions.
FAQs
1) What is a current transformer ratio?
The ratio compares primary current to secondary current. A 200/5 CT produces 5 A secondary when 200 A flows in the primary, ideally.
2) How do I calculate ratio from measurements?
Measure primary current and secondary current at the same time. Divide Ip by Is to estimate the operating ratio at that load and burden.
3) Why can the calculated ratio differ from nameplate?
Burden, wiring resistance, temperature, waveform, and core saturation can shift performance. Metering CTs are optimized for accuracy within a rated burden range.
4) What does burden mean in CT work?
Burden is the load on the CT secondary, typically expressed in VA. Higher burden demands higher secondary voltage and can increase error or cause saturation.
5) Can I use turns to estimate ratio?
Yes. Ideally, Ip/Is is approximately Ns/Np. Turns are useful when currents are unknown, but real devices still depend on burden and core conditions.
6) How is secondary voltage related to burden?
For a given secondary current, VA equals Vsec times Is. If you know VA and Is, Vsec is roughly VA divided by Is.
7) What ratio should I use for test conversions?
The calculator uses measured ratio first, then turns ratio, then nameplate ratio. For protection studies, confirm behavior near high currents and rated burden.
8) Are the exported files safe to share?
Exports include only the values you entered and computed results. Avoid sharing sensitive site identifiers, and confirm readings before using them in compliance reports.
Tip: Never open-circuit a CT secondary under load. Follow your site safety procedures.