Current Transformer Ratio Selection Guide
Engineering Intent
CT ratio should match feeder current, meter or relay input, accuracy requirement and future load margin.
This article is written for engineers, EPC buyers and procurement teams who need to turn CT ratio selection into a clear technical request instead of a vague product inquiry.
Key Selection Data
| Item | Data to Confirm |
|---|---|
| Check 1 | Primary feeder current |
| Check 2 | Secondary 1A or 5A |
| Check 3 | Metering or protection purpose |
| Check 4 | Expected maximum load |
Standards and Evidence to Request
- IEC 61869 current transformer references
- Relay or meter datasheet
- Panel wiring diagram
For project approval, request a technical datasheet, outline drawing, wiring or terminal diagram where applicable, routine test report and nameplate information before shipment.
Common Specification Mistakes
- Oversizing CT so normal current is too low
- No secondary current stated
- No burden calculation
RFQ Checklist
- Provide feeder current
- State 1A or 5A secondary
- Confirm accuracy class
- Share panel type
Related Product Paths
Ziyao Power can review the single-line diagram, load schedule, panel drawing or tender sheet and convert the requirement into a practical quotation package.
Download This Guide as PDF
Save this technical guide for offline reference. Includes all tables, specifications, and contact information.
Related Articles
CT Accuracy Class 0.5 vs 5P Explained
Metering CTs and protection CTs use different accuracy classes, so 0.5 and 5P should not be treated as interchangeable.
Current Transformer Burden Calculation
CT burden must include meter or relay load plus secondary cable resistance to maintain accuracy.
Protection CT vs Metering CT Selection
Protection CTs must reproduce fault current reliably, while metering CTs focus on measurement accuracy near normal load.