Step-by-step guide to calculating and selecting the right current transformer ratio for metering and protection
applications
CT ratio calculation & selection is a critical step in designing electrical measurement and
protection systems. The CT ratio determines how primary current is scaled down to a safe, measurable secondary
current. Choosing the correct ratio ensures accurately measuring load currents while maintaining
proper accuracy during normal operation and short circuit conditions.This is part of our Complete CT Guide series.
This guide covers everything you need to know to select CT ratios correctly, including calculation
formulas, standard ratios, and practical selection guidelines based on accuracy class requirements.
50-80% of the CT’s rated primary current. This ensures optimal accuracy while allowing headroom for temporary
overloads.
What is CT Ratio?
CT Ratio is the relationship between the primary current and the secondary current of a current
transformer. It’s expressed as:
CT Ratio = Primary Current / Secondary Current
For example, a 200:5 CT means:
- Primary: 200A (line current being measured)
- Secondary: 5A (output to meters/relays)
- Ratio: 200 ÷ 5 = 40:1
Standard Secondary Currents
| Secondary Rating | Common Applications |
|---|---|
| 5 A | Most common, short cable runs (<50m)< /td> |
| 1 A | Long cable runs, reduced burden |
CT Ratio Calculation Formula
Basic Formula
CT Ratio = Maximum Expected Load Current × 1.25 (safety factor)
Then select the next standard CT ratio above this calculated value.
Step-by-Step Calculation
- Step 1: Determine maximum load current (I_load)
- Step 2: Apply safety factor (typically 1.2-1.25)
Required Primary Rating = I_load × 1.25 - Step 3: Select next higher standard ratio
Calculation Example
📊 Example Calculation:Given:
- Motor full-load current: 180A
- Application: Metering with 0.5 accuracy class
Calculation:
Required Primary = 180A × 1.25 = 225A
Selection: Choose 250:5 (next standard ratio above 225A)
This gives optimal accuracy at normal operating current (180A ÷ 250A = 72% of rated).
Standard CT Ratios
Common Metering CT Ratios
| Primary Current | Secondary | Ratio | Typical Application |
|---|---|---|---|
| 50 A | 5 A | 10:1 | Small panels, lighting |
| 75 A | 5 A | 15:1 | Residential service |
| 100 A | 5 A | 20:1 | Small commercial |
| 150 A | 5 A | 30:1 | Medium loads |
| 200 A | 5 A | 40:1 | Commercial panels |
| 300 A | 5 A | 60:1 | Large commercial |
| 400 A | 5 A | 80:1 | Industrial feeders |
| 600 A | 5 A | 120:1 | Main switchboards |
| 800 A | 5 A | 160:1 | Industrial mains |
| 1000 A | 5 A | 200:1 | Substation feeders |
| 1200 A | 5 A | 240:1 | Large industrial |
| 1500 A | 5 A | 300:1 | Utility applications |
| 2000 A | 5 A | 400:1 | High-capacity systems |
Multi-Ratio CTs
Many CTs offer multiple tap ratios for flexibility:
| CT Designation | Available Ratios |
|---|---|
| 600:5 Multi-Ratio | 100:5, 200:5, 300:5, 400:5, 500:5, 600:5 |
| 1200:5 Multi-Ratio | 200:5, 400:5, 600:5, 800:5, 1000:5, 1200:5 |
Selection Guidelines
For Metering Applications
To select CT ratios for accurately measuring load current:
- Operating Range: Select ratio so normal current is 50-80% of rated primary
- Accuracy Class: Use 0.2, 0.5, or 1.0 class CTs
- Burden: Ensure CT burden matches meter requirements
| Application | Recommended Accuracy Class |
|---|---|
| Revenue metering | 0.2 or 0.2S |
| Industrial metering | 0.5 or 0.5S |
| General indication | 1.0 or 3.0 |
For Protection Applications
To select CT ratios for protection relays considering short circuit requirements:
- Short Circuit Rating: CT must handle fault current without excessive saturation
- ALF (Accuracy Limit Factor): Select 5P10 or 10P20 for protection
- Thermal Rating: Must withstand short circuit thermal effects
| Protection Type | Recommended Class |
|---|---|
| Overcurrent | 5P10 or 5P20 |
| Differential | 5P20 or PX |
| High-impedance | PX or TPY |
Avoiding Common Mistakes
| Mistake | Problem | Solution |
|---|---|---|
| Oversized CT | Poor accuracy at low currents | Select ratio closer to actual load |
| Undersized CT | Saturation during overloads | Allow 25% margin above maximum load |
| Ignoring short circuit | CT damage during faults | Verify short-term thermal rating |
🏭 Factory Experience:Based on our manufacturing and application experience:
- The 70% Rule: For optimal accuracy, size CT so normal operating current is approximately
70% of CT primary rating. This balances accuracy and overload capability. - Future Growth: When sizing for new installations, add 20-30% for anticipated load growth
rather than selecting CT exactly at current load. - Multi-Ratio Preference: When in doubt, specify multi-ratio CTs. They allow field adjustment
at minimal additional cost. - Burden Matching: Many metering inaccuracies result from cable resistance exceeding CT
burden rating. Always calculate total burden including cables.
Frequently Asked Questions
How do I calculate CT ratio?
Calculate CT ratio by taking your maximum expected load current, multiplying by 1.25 safety factor, then selecting
the next higher standard ratio. For example: 180A load × 1.25 = 225A, select 250:5 CT.
What CT ratio should I use for a 100A load?
For a 100A load, 100A × 1.25 = 125A. Select a 150:5 or 200:5 CT ratio. The 150:5 gives better accuracy (100A = 67% of
rated), while 200:5 allows more growth headroom.
What happens if CT ratio is too high?
If CT ratio is too high (oversized), accuracy decreases at low currents. The meter may show significant errors when
measuring currents below 10-20% of the CT’s rated primary current.
Can I use a protection CT for metering?
Protection CTs (5P, 10P) have lower accuracy than metering CTs (0.2, 0.5). While they can work for general
indication, they should not be used for revenue or precision metering applications.
What is the difference between 5P10 and 5P20?
The number after “P” indicates the Accuracy Limit Factor (ALF). 5P10 maintains ±5% accuracy up to 10× rated current.
5P20 maintains ±5% accuracy up to 20× rated current, suitable for higher short circuit
applications.
Related Articles
- Current Transformer (CT):
Complete Guide - Distribution
Transformer Sizing Calculator - Single Phase Distribution
Transformer Guide - Distribution
Transformer Losses Calculation
References
- IEC 61869-2 – Instrument Transformers: Current
Transformers - IEEE C57.13 – Standard Requirements for
Instrument Transformers - Mike Holt Forum – CT Ratio Selection
Discussions
Disclaimer: This information is provided for educational purposes. Always consult with a qualified electrical
engineer for specific CT sizing and selection applications.