CT Ratio Chart: Standard Ratios from 5/5 to 6000/5 | Selection Guide

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Comprehensive reference chart for current transformer (CT) ratios from 50A to 6000A. Use this guide to select the
correct CT ratio for metering, protection, and monitoring applications per IEC 61869-2 standards.

What is CT Ratio?

The CT ratio (Current Transformer ratio) defines how much a current transformer reduces the primary
current. For example, a 400:5 CT steps down 400 Amps to 5 Ampsβ€”an 80:1 reduction. This allows high
currents to be safely measured with standard 5-Ampere meters.

πŸ“ Quick Formula

Primary Current = Secondary Current Γ— Turns Ratio

Example: If a 400:5 CT shows 2.5A on the meter: 2.5 Γ— 80 = 200
Amps (primary)

Low Current CT Ratios (50A – 200A)

These CTs are commonly used for residential, light commercial, and branch circuit monitoring.

CT RatioTurns RatioAccuracy ClassBurden (VA)Window SizeTypical ApplicationSpecs
50:510:10.5S / 5P202.50.75″Small branch circuits, sub-panelsView
β†’
75:515:10.5S / 5P202.50.75″Small three-phase panelsView
β†’
100:520:10.5S / 5P2051.0″Residential 100A main panelsView
β†’
150:530:10.5 / 5P2051.25″Small commercial buildingsView
β†’
200:540:10.5 / 5P2051.5″200A service entranceView
β†’

Medium Current CT Ratios (250A – 800A)

Used for commercial buildings, industrial feeders, and motor control centers.

CT RatioTurns RatioAccuracy ClassBurden (VA)Window SizeTypical ApplicationSpecs
250:550:10.5 / 5P2051.5″Medium commercial, HVAC feedersView
β†’
300:560:10.5 / 5P20102.0″Motor control centers, industrial feedersView
β†’
400:580:10.5 / 5P20102.0″Industrial main switchboardsView
β†’
500:5100:10.5 / 5P20102.5″Large industrial distributionView
β†’
600:5120:10.5 / 5P20152.5″Large commercial, industrial substationsView
β†’
750:5150:10.5 / 5P20153.0″Bus duct monitoring, large feedersView
β†’
800:5160:10.5 / 5P20153.0″Main distribution switchgearView
β†’

High Current CT Ratios (1000A – 6000A)

Used for utility metering, substations, power plants, and high-voltage applications. These are
typically bar-type CTs with larger window sizes.

CT RatioTurns RatioAccuracy ClassBurden (VA)Window SizeTypical ApplicationSpecs
1000:5200:10.5 / 5P20153.5″Substation metering, MV applicationsView
β†’
1200:5240:10.5 / 5P20154.0″Transformer primary side meteringView
β†’
1500:5300:10.5 / 5P20154.0″Large industrial plants, substation feedersView
β†’
2000:5400:10.5 / 5P20304.5″Utility-grade meteringView
β†’
2500:5500:10.5 / 5P20304.5″High capacity substationsView
β†’
3000:5600:10.5 / 5P20305.0″Generator monitoring, substation mainsView
β†’
4000:5800:10.5 / 10P20305.0″Power plant outputsView
β†’
5000:51000:10.5 / 10P20306.0″Main bus monitoring, HV substationsView
β†’
6000:51200:10.5 / 10P20306.0″Utility main bus, major industrialView
β†’

How to Select the Right CT Ratio

Step 1: Determine Maximum Load Current

Calculate or measure the maximum expected current in the circuit. The CT primary rating should be equal to or
slightly higher than this value.

πŸ’‘ Best Practice: Size the CT so the normal operating current is 60-80% of the CT
rating. This ensures optimal accuracy within the CT’s linear range.

Step 2: Check Conductor Size vs. Window Size

Ensure your conductor (cable or bus bar) will physically fit through the CT window:

Window SizeSuitable Conductor (Approx.)
0.75″Up to #4 AWG
1.0″Up to #1 AWG
1.5″Up to 250 MCM
2.0″Up to 500 MCM
2.5″Up to 750 MCM or small bus bar
3.0″ – 4.0″Large cables or bus bars

Step 3: Select Accuracy Class

  • Revenue Metering (Billing): Use Class 0.2S or 0.5S for accurate billing over varying loads.
  • Energy Management: Class 0.5 is sufficient for monitoring and control.
  • Protection: Use 5P20 or 10P20 class for relay applications requiring high fault current
    accuracy.

Understanding CT Accuracy Classes (IEC 61869-2)

ClassRatio Error at Rated CurrentTypical Application
0.2Β±0.2%High-precision laboratory, utility revenue metering
0.2SΒ±0.2% (from 20% to 120% In)Revenue metering with variable loads
0.5Β±0.5%Standard energy management, sub-metering
0.5SΒ±0.5% (from 20% to 120% In)Extended range monitoring
1.0Β±1.0%Indication, general monitoring
5P205% composite error up to 20Γ— InOvercurrent protection relays
10P2010% composite error up to 20Γ— InHigh-fault-current protection

Understanding CT Burden (VA)

The burden is the total load connected to the CT secondaryβ€”including meters, relays, and wiring
resistance. Exceeding the rated burden degrades accuracy.

⚠️ Important: According to IEC 61869-2, the CT maintains its accuracy class when the burden is
between 25% and 100% of its rated VA. Operating below 25% may cause positive errors.

Standard IEC Burden Ratings

2.5 VA | 5 VA | 10 VA | 15 VA | 30 VA

Modern electronic meters typically have very low burden (0.1-0.5 VA), so low-burden CTs (2.5-5 VA) are often
sufficient for new installations.

Frequently Asked Questions

❓ What does CT ratio 400:5 mean?

A 400:5 CT has an 80:1 turns ratio. It reduces 400 Amps of primary current to 5 Amps of secondary
current. This allows safe measurement with a 5A ammeter.

❓ Can I use a CT with a higher ratio than needed?

Yes, but accuracy may suffer at low currents. For best accuracy, size the CT so normal load is 60-80% of
the CT’s rated primary current.

❓ What is the difference between 5P20 and 10P20 protection
classes?

5P20: 5% composite error at up to 20Γ— rated current. Used for standard overcurrent
relays.
10P20: 10% composite error at up to 20Γ— rated current. Used for high-fault-current
applications where wider tolerance is acceptable.

Standards & References

Current transformer specifications are governed by the following international standards. For official documentation,
visit the respective standards organizations:

  • IEC
    61869-2    :     Instrument transformers – Part 2: Additional
    requirements for current transformers
  • IEEE
    C57.13    :     IEEE Standard Requirements for Instrument Transformers
  • ANSI C12.1: American National Standard for Electric Meters – Code
    for Electricity Metering
  • NIST Handbook
    44    :     Specifications for Current Transformers Used in
    Metering

For more background on current transformer principles and operation, see
the Wikipedia article on
Current Transformers.

Related Resources

Disclaimer: Specifications shown are typical reference values per IEC 61869-2. Actual specifications vary by
manufacturer. Always consult manufacturer datasheets for exact values. Last updated: January 2026

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