Step-by-step guide to calculating current transformer burden with formulas, wire sizing tables, and practical
examples
This CT burden calculation guide provides engineers with step-by-step methods to calculate
the burden for current transformers. Proper burden calculation is essential for maintaining the
accuracy limit of your CT and ensuring reliable operation of metering and protection systems.This is part of our Complete CT Guide series.
CT burden refers to the total impedance connected to the CT secondary circuit. If
the actual burden exceeds the rated burden, the current transformer will operate beyond its accuracy
limit, resulting in measurement errors.
accuracy. A 15VA rated CT with 5A secondary current can support up to 0.6Ω total burden.
What is CT Burden?
CT burden is the total load connected to the secondary winding of a current
transformer, expressed in volt-amperes (VA) or ohms (Ω).
Burden Components
| Component | Description | Typical Values |
|---|---|---|
| CT Internal Resistance | Self-resistance of secondary winding | 0.02-0.2Ω |
| Lead Wire Resistance | Cable from CT to instruments | Depends on length/gauge |
| Instrument Burden | Meters, relays, transducers | 0.05-5VA per device |
VA vs Ohms Conversion
For 5A secondary CTs:
Burden (Ω) = Burden (VA) / I² = Burden (VA) / 25
Example: 15VA burden = 15/25 = 0.6Ω
For 1A secondary CTs:
Burden (Ω) = Burden (VA) / 1 = Burden (VA)
Example: 15VA burden = 15Ω
CT Burden Calculation Formula
Total Burden Formula
To calculate the burden for current transformers:
Total Burden (Ω) = CT Internal R + Wire R + Instrument R
Total Burden (VA) = I² × Total Burden (Ω)
Wire Resistance Calculation
Wire Burden (VA) = I² × R × L × 2
Where:
- I = Secondary current (A)
- R = Wire resistance (Ω/m)
- L = One-way cable length (m)
- 2 = Round-trip factor
Step-by-Step Calculation Example
Example: 600:5A CT with 25m Cable Run
📊 Given:
- CT Ratio: 600:5A
- CT Rated Burden: 15VA
- Cable length: 25m one-way
- Cable: 2.5mm² copper (0.00734 Ω/m)
- Connected meter burden: 5VA
- CT internal burden: 1VA
Step 1: Calculate Wire Burden
Wire Burden = I² × R × L × 2 = 5² × 0.00734 × 25 × 2 = 9.18VA
Step 2: Calculate Total Burden
Total Burden = CT Internal + Wire + Meter = 1VA + 9.18VA + 5VA = 15.18VA
Step 3: Compare with Rated Burden
Total Burden (15.18VA) > Rated Burden (15VA)
Result: The VA burden exceeds the CT’s rating! Solutions:
- Use larger cable (4mm² or 6mm²)
- Reduce cable length
- Use 1A secondary CT instead of 5A
Wire Sizing for CT Burden
Wire Resistance Table
| Wire Size | Resistance (Ω/m) @ 20°C |
|---|---|
| 1.5 mm² | 0.01210 |
| 2.5 mm² | 0.00734 |
| 4.0 mm² | 0.00461 |
| 6.0 mm² | 0.00308 |
| 10 mm² | 0.00183 |
Maximum Cable Length Table
| CT Secondary | Rated Burden | 1.5mm² Max | 2.5mm² Max | 4mm² Max |
|---|---|---|---|---|
| 5A | 5VA | 3m | 5m | 9m |
| 5A | 10VA | 7m | 11m | 17m |
| 5A | 15VA | 10m | 16m | 26m |
| 5A | 30VA | 20m | 33m | 52m |
| 1A | 5VA | 83m | 136m | 217m |
| 1A | 15VA | 250m | 409m | 650m |
the burden advantage is 25× compared to 5A CTs.
Burden for Current Transformers: Selection Guide
Standard CT Burden Ratings
| CT Class | Standard Burdens (VA) | Accuracy Limit |
|---|---|---|
| 0.2 | 2.5, 5, 10, 15 | ±0.2% ratio error |
| 0.5 | 2.5, 5, 10, 15, 30 | ±0.5% ratio error |
| 5P | 5, 10, 15, 30 | ±1% ratio error at ALF |
| 10P | 5, 10, 15, 30 | ±3% ratio error at ALF |
Typical Instrument Burdens
| Instrument | Typical VA Burden |
|---|---|
| Digital meter | 0.05 – 0.5 VA |
| Analog ammeter | 1 – 3 VA |
| Protection relay | 0.1 – 1 VA |
| Watt transducer | 0.5 – 1 VA |
| Energy meter | 0.5 – 2 VA |
1A vs 5A Secondary: Burden Comparison
When cable runs are long, consider 1A secondary CTs:
| Factor | 5A Secondary | 1A Secondary |
|---|---|---|
| Wire burden (25m, 2.5mm²) | 9.18VA | 0.37VA |
| Maximum cable (15VA) | ~25m | ~600m |
| Meter compatibility | Standard | May need verification |
| Cost | Lower | Slightly higher |
For installations where CT secondary leads exceed 30m, 1A secondary CTs significantly reduce
VA burden and maintain the accuracy limit.
🏭 Factory Experience:
- Always Calculate Before Installation: We’ve seen many field issues where burdens were
estimated, not calculated. The 5VA “digital meter” assumption often ignores cable burden entirely. - 1A vs 5A Decision: For any cable run over 20m, seriously consider 1A secondary CTs. The
burden advantage is 25×, making cable sizing much more forgiving. - Temperature Derating: Wire resistance increases ~0.4% per °C above 20°C. In hot
environments (40°C), add 8% to calculated wire burden. - Leave Margin: We recommend keeping actual burden at 80% of rated burden. This provides
margin for temperature effects and future additions.
Frequently Asked Questions
How do I calculate CT burden?
CT burden is calculated by summing the CT internal resistance, wire resistance, and instrument
impedance. Convert to VA using: Burden (VA) = I² × Total Resistance (Ω). For a 5A CT, this equals 25 × R.
What happens if CT burden is exceeded?
If the burden exceeds the CT’s rated value, the CT will saturate partially, causing ratio errors beyond the specified
accuracy limit. This leads to inaccurate measurements and potential protection relay malfunctions.
Why do 1A CTs have lower burden than 5A CTs?
Because burden for current transformers (VA) = I² × R, the wire burden for 1A CTs is only 1/25th of
5A CTs for the same resistance. This makes 1A CTs ideal for long cable runs.
What is a typical CT burden for digital meters?
Modern digital meters typically have very low burden of 0.05-0.5VA. However, the total burden calculation must also
include wire resistance, which often dominates in long runs.
How do I select CT burden rating?
Calculate the burden (CT + wires + instruments), add 20% safety margin, then select the next higher
standard burden rating (2.5, 5, 10, 15, or 30VA).
Related Articles
- Current Transformer (CT):
Complete Guide - CT Ratio Calculation &
Selection - CT Accuracy Class Explained
- Distribution
Transformer Sizing Calculator
References
- IEC 61869-2 – Instrument Transformers: Current
Transformers - IEEE C57.13 – Standard Requirements for
Instrument Transformers - CT
Burden Chart – Flex-Core
Disclaimer: This information is provided for educational purposes. Always verify burden calculations with CT
manufacturers and applicable standards for critical applications.