CT Burden Input Form
Enter the CT secondary values, wiring details, connected devices, and accuracy limits. The result appears above this form after submission.
Example Data Table
| Case | Secondary | Lead Length | Wire | Device VA | Rated VA | Expected Review |
|---|---|---|---|---|---|---|
| Feeder relay cabinet | 5 A | 50 ft | 12 AWG copper | 7.5 VA | 25 VA | Normally acceptable with good margin. |
| Long control cable | 5 A | 180 ft | 14 AWG copper | 8 VA | 25 VA | Lead loss may dominate the burden. |
| Low burden relay | 1 A | 120 ft | 16 AWG copper | 2 VA | 10 VA | Usually strong because current is lower. |
| High accuracy check | 5 A | 80 ft | 10 AWG copper | 12 VA | 25 VA | Check margin and class voltage carefully. |
Formula Used
CT ratio: Ratio = Primary current / Secondary current
Temperature adjusted wire resistance: RT = R20 × [1 + α × (T - 20)]
Loop lead resistance: Rloop = RT × length × paths / 1000
Lead burden: VAlead = Isecondary2 × Rloop
Non-lead impedance: Zload = VAdevices / Isecondary2
Total impedance: Ztotal = √(Rtotal2 + Xtotal2)
Effective burden: VAtotal = Isecondary2 × Ztotal
Class VA limit: VAclass = Vclass × Isecondary / ALF
This page provides an engineering screening estimate. Always confirm final values with project standards, relay manuals, CT nameplates, and manufacturer documentation.
How to Use This Calculator
- Enter the CT primary and secondary current ratings.
- Add meter, relay, test switch, and internal wiring burden values.
- Enter the one-way lead length, wire size, material, and temperature.
- Set rated burden, class voltage, accuracy limit factor, and design margin.
- Press the calculate button.
- Review the result above the form.
- Download the CSV or PDF report for records.
Article: CT Burden Review for Solid Dielectric Internal Units
Why CT Burden Matters
CT burden is the electrical load connected to a current transformer secondary. It includes relays, meters, switches, terminals, internal wiring, and field leads. A high burden can force the CT to develop more secondary voltage. If the voltage demand becomes too high, accuracy can suffer. Protection performance can also become less dependable.
Solid Dielectric Internal Applications
Solid dielectric equipment often has compact internal layouts. The CT may feed internal terminal points before reaching external relays. These internal paths are usually short, but they still add burden. The calculator lets the user include that internal portion separately. This improves review quality during design and commissioning.
Lead Loss Is Important
Lead resistance is often the largest hidden burden. The loss increases with the square of secondary current. A five ampere CT can create much higher lead burden than a one ampere CT. Long cable routes, small conductors, and high temperatures increase resistance. The tool adjusts conductor resistance for temperature.
Accuracy Margin
A CT should not be selected only by matching total burden to rated burden. A margin should be kept for field changes, temperature variation, and future devices. This calculator compares the effective burden with the selected limit. It also checks a required margin percentage. The result is shown as pass, caution, or over limit.
Class Voltage Check
Protection CT classes often relate to voltage capability at a fault multiple. The class voltage, secondary current, and accuracy limit factor can define a practical burden ceiling. The calculator estimates this ceiling. It then compares it with the rated burden value. The lower value becomes the limiting burden.
Using the Result
Use this result as an engineering guide. Compare the output with relay data sheets and CT test reports. Review cable schedules before final approval. If the margin is low, reduce lead length, increase wire size, use lower burden devices, or select a stronger CT. Final settings should be approved by qualified electrical personnel.
FAQs
1. What is CT burden?
CT burden is the load connected to the CT secondary circuit. It includes meters, relays, test switches, leads, and internal wiring.
2. Why does lead length affect CT burden?
Longer leads have more resistance. CT lead burden equals secondary current squared multiplied by loop resistance, so long runs can add major VA.
3. Why are 1 A CT circuits often easier to burden?
Lead burden depends on current squared. A 1 A secondary produces much lower lead loss than a 5 A secondary using the same wire route.
4. What does internal wiring burden mean?
It is the estimated burden from wiring or components inside the equipment before the external secondary circuit reaches connected devices.
5. What is a good CT burden margin?
Many designers keep 20 percent or more. The correct margin depends on standards, accuracy needs, protection duty, and future expansion.
6. What happens if burden is too high?
The CT may lose accuracy, saturate earlier, or fail to supply the required secondary voltage during high current conditions.
7. Should I use effective or arithmetic burden?
Effective burden includes impedance and power factor effects. Arithmetic burden is simpler, but it can be less accurate for mixed loads.
8. Can this replace manufacturer data?
No. Use it for screening and documentation. Always confirm final CT suitability with nameplates, manuals, project standards, and test reports.