Power Systems

How to Size and Select Appropriate Current Transformers

Current transformer sizing and selection are among the most critical and demanding tasks in designing a power system. A current transformer (CT) is more than just a coil of wire around a conductor or busbar. Get the sizing wrong, and you’re looking at billing issues, nuisance trips, or a protection relay that can’t see a fault. In this post, we will walk through key topics, including ratio, class, burden, and saturation. When you see a designation C400 or 0.15B1.8, what comes to mind?

Why CT Sizing and Selection Matter in Power System Design

Current transformers do two things: they step down dangerously high currents to a standard secondary level (usually 5 A or 1 A) so that instruments and relays can safely operate on the signal.

They provide galvanic isolation between the high-voltage primary circuit and the low-voltage secondary equipment. It sounds simple in concept; however, pick the wrong CT and the consequences range from inaccurate energy billing to a relay that fails to trip during a real fault.

The two most misunderstood decisions in CT selection are the ratio and the accuracy class. The accuracy class is where most engineers either over-specify (wasting money) or under-specify (risking system integrity). So let’s break it all down.

Current Transformer Ratio

The CT ratio defines how much the primary current gets stepped down. A 400:5 CT, for example, steps 400 A down to 5 A — a turns ratio of 80:1. Your goal is to select a primary rating that keeps the normal operating current within a predictable, accurate band on the secondary.

Size the CT so your normal operating current falls between 60–80% of the CT’s rated primary current.

For example, if your feeder or circuit carries a maximum continuous current of 320 A, you need a primary rating of at least 400 A. The next standard size, a 400:5 CT, is your pick. Why not a 350:5A? That will be over 91% of the maximum continuous load. As such, a 350:5A CT is not preferred.

Similarly, if your full load current is 80A, you can pick a CT rated 100A primary and a typical 5A secondary (100:5). This leaves room for load growth (25%) while keeping you comfortably within the CT’s accurate operating range.

Quick Tip:

Undersizing the ratio causes the secondary to be overdriven and saturate. However, oversizing also creates the opposite problem. That is, your normal current is such a small fraction of the rated that you’re measuring in the low-accuracy range of the CT’s performance curve.

Metering Class vs. Protection Class: Know the Difference

This is where the biggest misunderstandings live. Metering-class and protection-class CTs are optimized for completely different operating conditions and are not interchangeable. Verify the specifications of the CT you select, bearing in mind the need or application. Below are some key differences between metering and protection-class CTs for your quick reference.

Feature	Metering Class CT	Protection Class CT
Primary purpose	Accurate measurement at normal load	Reliable output during fault conditions
IEC accuracy classes	0.1, 0.2, 0.2S, 0.5, 0.5S, 1.0	5P, 10P, PX (IEC); C, K, T (IEEE/ANSI)
IEEE/ANSI classes	0.15, 0.15S, 0.3, 0.6, 1.2 (per C57.13-2016)	C100, C200, C400, C800
Accurate range	5% to RF × rated current (RF typically 1.33–4.0)	Up to 20× rated (Accuracy Limit Factor)
Behavior at fault current	Saturates early — intentional by design	Remains linear up to the ALF limit
Core design	Small, high-grade grain-oriented silicon steel	Larger core to avoid saturation at high multiples
Typical use	Revenue meters, energy analyzers, PQ monitors	Overcurrent, differential, distance relays

Metering Class - Current Transformer

A metering-class CT is designed to be precise under normal operating conditions — specifically between 5% and the rating factor of its rated primary current. The class numbers (0.2, 0.5, 1.0) indicate the maximum allowable ratio error at rated current. A Class 0.5 CT means the error in reproducing the primary current cannot exceed ±0.5%.

The “S” designation (0.2S, 0.5S) extends that accuracy down to 1% of rated current, useful in applications with highly variable loads where you need accurate measurement even at low current levels.

The saturation of a metering CT at fault current is actually a feature, not a flaw. It protects the connected instruments — meters and analyzers, from being driven far beyond their rated input.

Protection Class — Performance When It Matters Most

A protection-class CT must faithfully reproduce the primary current even during fault conditions, when it can reach 10, 20, or even 40 times the rated value. The key parameter here is the Accuracy Limit Factor (ALF) under IEC, or the C-class voltage rating under ANSI/IEEE.

IEC 5P and 10P classes — The designation “5P20” means the CT will maintain a composite error of no more than ±5% at currents up to 20 times rated. So, on a 100:5 CT rated 5P20, the CT stays accurate up to 2,000 A primary (that is 100 A primary x 20). “10P20” allows ±10% composite error at 20× rated.

ANSI/IEEE C-class ratings — The C100, C200, C400, and C800 designations represent the maximum secondary terminal voltage the CT can produce at 20 times rated secondary current without exceeding 10% ratio error. C200 means 200 V at 100 A secondary — implying a design burden of 2 ohms (that 200V/100A). Select the C-class rating that ensures the relay’s required voltage is met at maximum fault current.

Insight

The ALF (or C-class) must be selected so the relay trip point falls on the linear portion of the CT’s secondary current curve — between 50% and 100% of the ALF limit. If a fault current will drive the CT into saturation before the relay can operate, the relay will see a distorted secondary current and may fail to trip, delay its trip, or produce erroneous measurements.

Calculate the CT Burden

Burden is the total impedance (and thus VA demand) that the secondary circuit places on the CT. It includes the resistance of the relay or meter coils, the connecting cable, and the CT secondary winding.

If the total burden exceeds the CT’s rated burden, the CT will see a higher excitation voltage than it was designed for — driving up magnetizing current, increasing ratio error, and potentially causing premature saturation.

How do you calculate the burden of a CT?

Total Burden = Device (meter or relay) + lead wire + winding resistance of CT

Always select a CT whose rated burden is greater than or equal to the calculated total secondary burden. Standard IEC burden ratings are 1, 2.5, 5, 10, and 15 VA. Standard ANSI burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0, B-1.8, and B-4.0 (in ohms).

Practical Tip:

Long cable runs between a CT and the relay panel are the most common cause of excess burden. If the run is long, consider using a 1 A secondary CT instead of 5 A, since burden scales with I², reducing the secondary current by 5× reduces the cable burden by 25×.

Check Saturation and the Knee-Point Voltage

For protection-class CTs, you must verify that the knee-point voltage (Vk) is high enough to prevent saturation before the relay operates. Saturation occurs when the core can no longer support increasing magnetizing flux. At that point, the secondary current no longer tracks the primary, and the relay sees a clipped, distorted waveform.

Required Vk ≥ K × I_fault_secondary × (R_lead + R_relay)

K = system factor (typically 2 for simple overcurrent, higher for differential)

Per IEEE C57.13, the excitation curve must be plotted on log-log graph paper to locate the knee point — defined as the point where a 10% increase in voltage results in a 50% increase in excitation current. Make sure Vk comfortably exceeds the required relay knee-point voltage across the worst-case fault scenario.

Critical Safety Rule:

Critical Safety Rule — Never Open-Circuit an Energized CT Secondary

When a CT secondary is open-circuited while the primary carries current, the full primary MMF drives the core into deep saturation. The resulting flux collapse generates extremely high voltage transients — potentially several kilovolts — at the secondary terminals. This can destroy the CT’s insulation, damage connected equipment, and pose a lethal hazard to personnel. Always short the CT secondary before disconnecting any burden.

The CT Selection Checklist

Determine maximum continuous primary current: Include load growth factor (×1.25 to ×1.5). Match to the next standard CT primary rating.

Identify the application — metering or protection?: Revenue billing → Class 0.2 or 0.3. Industrial sub-metering → Class 0.5. Overcurrent relay → 5P or C-class. Differential or distance relay → higher ALF or PX class.

Calculate total secondary burden: Sum instrument VA, relay VA, and cable I²R losses. Select rated burden ≥ total calculated burden.

Verify protection performance: For protection CTs, confirm ALF or C-class rating ensures linear output up to at least the relay’s trip current. Check Vk against relay requirements.

Confirm thermal withstand (Ith): The CT must survive fault currents without core or winding damage. Ith is rated in kA for 1 second — verify against the available fault level at that point in the system.

Never mix metering and protection on the same secondary winding: High-voltage CTs often have multiple secondary windings or cores. Use a dedicated core for each function — one for the meter, one for the relay. Otherwise, seek approval or confirmation of use from the manufacturer of the connected device (meter or relay). This protects instruments from fault-level currents and prevents the meter’s burden from degrading protection accuracy.

Example: Understanding Metering Class CT

What does a 0.3B0.9 on a current transformer (CT) mean?

If a CT core is marked 0.3B0.9 and another is C400, then we know that the 0.3B0.9 is for metering and the latter is for protection application. What does this mean? It means that, connect the 0.3B0.9 core to meter(s) and use the C400 core for protection relays.

Now, what does 0.3B0.9 actually mean?

The accuracy class is 0.3% with a burden (B) of 0.9 ohms.

However, here is the mistake most people overlook.

For an accuracy class of 0.3%, it doesn’t mean the measurement accuracy is 0.3% for any current flowing through the CT.

Now, let’s break it down further for better understanding. Let us assume the rated current flowing through our circuit is 100A. Remember that in some cases, less than 100A (or 100%) may flow depending on load/demand. Assume the CT rating factor is 2.0.

0.3% accuracy means that the actual accuracy is 99.7% (min) to 100.3% (max) at 100% rated current. Simply, your metering CT will have 0.3% accuracy from 100% of the rated current (100A) to 200% of the rated current at the rating factor.

If the rated current is less than 100% (e.g., 90A), the accuracy of the same CT drops to the 0.6% range. This means the measured current will be (100 – 0.6)%, giving you an actual accuracy of 99.4% (min) and 100.6% (max).

This may seem confusing, but let’s use an example to explain.

If the actual current is 90A < 100A (rated), then CT measures:

0.994*90A = 89.46A (min)

1.006*90A = 90.54A (max)

If the current falls below 10% of the rated current, no accuracy is guaranteed. i.e., 9A, won’t have an accuracy guarantee.

For most applications, such as variable resources (solar or wind), where the likelihood of operating at the rated current is significantly lower, it is critically important to select high-accuracy CTs to ensure accurate revenue measurement and achieve the best return on investment (ROI).

Key Takeaways

Size the primary rating so normal load sits at 60–80% of rated current.
Use metering class (0.2, 0.5 IEC / 0.3, 0.6 ANSI) for revenue and indication — they saturate early to protect instruments.
Use protection class (5P/10P IEC / C-class ANSI) for relays — they must stay linear at multiples of fault current.
Burden must be calculated, not assumed — cable resistance is often the biggest contributor on long runs.
Knee-point voltage must exceed the relay’s requirement under worst-case fault conditions.
Never open-circuit a CT secondary while the primary is energized — it is a life safety issue.

Conclusion

CT selection is one of those things that looks straightforward on paper but hides real consequences inside the details. A metering CT used on a differential relay won’t saturate the way the relay’s algorithm expects. A protection CT with an undersized burden rating will clip the relay’s current during a fault. The standards — IEEE C57.13, IEC 61869-2, and IEEE C37.110 — exist for good reason, and they give you the framework to get this right every time.

If you found this helpful, share it with a colleague who is specifying CTs on a substation project or studying for the PE exam.

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How to Size and Select Appropriate Current Transformers

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Why CT Sizing and Selection Matter in Power System Design