How to Determine the Burden of a Current Transformer

Introduction

Current transformers (CT) are core components for current measurement, energy metering, and relay protection in power systems. “Burden” is one of the most critical parameters for a current transformer. Incorrect burden matching will cause waveform distortion, large measurement errors, inaccurate energy-meter reading, or even failure of protection relays. Understanding how to calculate and select a proper CT burden is essential for stable power‑system operation. Here we explain CT burden fundamentals, calculation steps, common pitfalls and practical selection guidelines for engineers.

 1. What is Current Transformer Burden?

CT burden refers to the total apparent power (unit: VA, Volt‑Ampere) consumed by all secondary‑side connected components at rated secondary current. It represents the total load on the CT secondary winding.

The secondary‑side load includes:

• Internal resistance of CT secondary winding
• Resistance of connecting wires / cables
• Impedance of connected devices: ammeters, energy meters, protection relays, sampling circuit boards

Formula:

Burden can also be expressed in ohms(Ω). Conversion between VA and ohms is frequently used in engineering practice.

2. Why Burden Matching Matters

Too small actual burden

The CT works under light load. For measuring‑class CTs, accuracy can stay within specification. However, for protection‑class CTs, excessively low burden may alter saturation characteristics under fault conditions.

Too large actual burden (over‑burden)

This is the most‑common field issue:

1. Secondary voltage rises, magnetic core saturates

2. Output current waveform distorts

3. Measurement error increases, metering data deviates

4. Protection CT fails to correctly reflect fault current, leading to protection mal‑operation or refusal‑to‑operate

Rule: The calculated actual circuit burden must be less than or equal to the rated burden marked on the CT nameplate.

3. Step‑by‑Step: Calculate Actual Secondary Burden

Step 1: Confirm CT rated secondary current

Two mainstream standards:

• 5A secondary output (widely used in traditional power cabinets)
• 1A secondary output (long‑distance wiring scenarios, reduces wire power consumption)

Step 2: Sum impedance of all secondary‑side devices

Check datasheet for each connected instrument / PCB sampling unit for its power consumption or impedance value.

Example:

• Energy meter burden consumption: 1.5 VA
• Protection relay: 2.0 VA

        Sum all individual device VA values.

Step3: Calculate wire burden

Copper connecting cables contribute significant burden, especially for long wiring distance.

Wire resistance formula:

- ρ: Resistivity of copper conductor

- L: One‑way cable length between CT and load device

- A: Cross‑sectional area of copper wire

Note: CT secondary forms a closed loop, so round‑trip length (2L) shall be adopted. Wire burden:

Step4: Get total actual burden

Total Actual Burden = Sum of instrument burden + Wire burden

Step5: Compare with CT rated burden

Requirement: Actual total burden ≤ CT rated burden on nameplate.

Keep reasonable margin; it is recommended actual burden ≤75% of rated burden for reliability margin.

4. Practical Engineering Examples

Case parameters

• CT rated secondary current: 5A
• Rated burden on nameplate: 10 VA
• Connected loads: Energy meter 2 VA, current indicator 1 VA
• Copper wire: 1.5 mm², one‑way length L=20 m

1. Sum instrument burden: 2 VA +1 VA = 3 VA

2. Compute round‑trip wire resistance and wire burden

3. Total actual burden = instrument burden + wire burden

4. Compare against CT rated 10 VA, verify margin.

If calculated actual burden exceeds rated burden, feasible solutions:

1. Increase copper wire cross‑section, reduce wire resistance

2. Select CT with higher rated burden

3. Change to 1A secondary‑current CT to cut wire loss for long‑distance transmission

5. Common Misunderstandings

1. Ignore cable burden: Engineers only consider instrument VA, neglect long‑cable impedance, resulting in hidden over‑burden.

2. Confuse burden class and accuracy class: 0.5, 0.2S represent accuracy; burden(VA) represents load capacity. A high‑accuracy CT still fails when over‑burdened.

3. Multiple devices series connection: Each additional device adds burden; do not cascade unlimited instruments on one CT secondary loop.

4. Secondary open‑circuit risk: While calculating burden, remember CT secondary circuit must never be open‑circuited. Open circuit produces extremely high dangerous voltage and damages the magnetic core.

6. Quick Selection Checklist for CT Burden

• Record rated secondary current (1A /5A)
• Collect power consumption of every secondary‑connected device
• Measure actual wiring length, calculate copper wire burden
• Sum to get total actual burden
• Actual burden ≤ CT rated burden, retain 20‑30% design margin
• Re‑check when modifying cabinet wiring or adding secondary devices

Conclusion

Determining CT burden is not merely reading nameplate parameters; it requires full calculation of instruments plus wiring impedance. Reasonable burden matching guarantees measurement precision and reliable protection action for your power‑monitoring hardware. When designing current‑sampling hardware including zero‑sequence CT and ordinary current transformers, always run burden calculation at the design phase, rather than adjusting after on‑site faults

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