Nanocrystalline CT Core Selection Guide: Design Checklist for Engineers

Who this guide is for:

• Electrical engineers designing current transformers for metering and protection
• CT manufacturers selecting cores for 0.1, 0.2S, and 5P accuracy classes
• Power system designers optimizing CT performance in switchgear and substations

What you'll learn:

• Step-by-step nanocrystalline CT core selection process
• Accuracy class requirements and burden calculation methods
• Design examples with real-world specifications

The 3 Biggest CT Core Selection Mistakes I See Every Week

1. Undersized for fault current: Last month, a 630A protection CT failed during commissioning because the engineer used 20kA fault current instead of the actual 31kA available fault current.

2. Ignoring cable burden: A Class 0.2S revenue meter failed accuracy tests because 150m of control cable added 2.3VA to the "1VA" digital meter burden.

3. Wrong material choice: CRGO cores in a compact 11kV switchgear couldn't fit the required turns for 0.2S accuracy – switching to nanocrystalline reduced core size by 40%.

After 15 years designing CTs for utilities and OEMs, I've learned that nanocrystalline CT cores solve most of these problems – but only if you select them correctly. Here's the step-by-step process we use at CenturaCores.

Rajesh Kumar• Magnetics Design Engineer, CenturaCores

15+ years designing current transformers for utilities and OEMs. M.S. Electrical Engineering, IIT Delhi. Holds 8 patents in magnetic core design. Previously Senior Engineer at Schneider Electric and ABB.

How to Calculate Minimum Core Cross-Section (Ae) for Your CT

The core cross-sectional area determines whether your CT will saturate under fault conditions. Here's the formula I use for initial sizing:

Core Area Calculation

Ae (mm²) = (Vs × 10⁴) / (4.44 × f × Bmax × N)

Vs = Secondary voltage at maximum burden (V)

f = Frequency (Hz)

Bmax = Maximum flux density (T) - use 1.0T for nanocrystalline

N = Secondary turns

Real Example: 400/1A Protection CT

For a 5P10 CT with 15VA burden:

• Vs = √(15 × 1²) = 3.87V at rated current
• At 10× overcurrent: Vs = 38.7V
• With 400 secondary turns: Ae = (38.7 × 10⁴)/(4.44 × 50 × 1.0 × 400) = 43.6 mm²
• Result: Minimum 44mm² core cross-section needed

This calculation assumes sinusoidal flux, but fault currents contain DC offset. I add 20% safety margin for protection CTs, 10% for metering CTs.

Nanocrystalline vs CRGO vs Ferrite: Head-to-Head Performance Data

I tested identical 200/1A CTs with different core materials in our lab. Here's what the Omicron CPC100 showed:

Lab Test Results: 200/1A, Class 0.2S, 5VA Burden

Nanocrystalline (Winner)

• Ratio error: 0.05% at 5% In
• Phase error: 1.2 minutes
• Core size: 32mm OD
• Knee point: 180V

CRGO Silicon Steel

• Ratio error: 0.15% at 5% In
• Phase error: 4.8 minutes
• Core size: 45mm OD
• Knee point: 120V

Ferrite (Failed)

• Ratio error: 0.25% at 5% In
• Phase error: 8.1 minutes
• Core size: 38mm OD
• Failed Class 0.2S limits

Material comparison for CT cores

Property	Nanocrystalline	CRGO Silicon Steel	Ferrite
Initial permeability (μi)	80,000 (measured)	8,000 (typical M6X)	15,000 (3C90)
Saturation flux density (Bs)	1.25T @ 25°C	2.0T @ 25°C	0.39T @ 25°C
Core losses @ 1T, 50Hz	12 W/kg	1.3 W/kg	22 W/kg
Temp coefficient (20-80°C)	-0.8%/°C	-0.2%/°C	-2.5%/°C
Cost per CT (relative)	1.4× (but smaller size)	1.0× (baseline)	0.8× (limited accuracy)
Best CT applications	Class 0.1, 0.2S revenue metering	Class 0.5, 5P protection	Class 1.0, low-cost applications

Table: CT core material comparison showing nanocrystalline advantages in permeability and size

When NOT to Use Nanocrystalline (The Trade-offs)

Budget-constrained Class 1.0 applications: If you're building thousands of basic panel meters where 1% accuracy is acceptable, CRGO costs 30% less and the larger size doesn't matter.

Extreme high-frequency noise environments: Above 10kHz, ferrite cores can outperform nanocrystalline in some EMI-heavy industrial settings.

Very high fault current (>100kA): Silicon steel's higher saturation flux density (2.0T vs 1.25T) may be needed for the largest utility CTs.

Where Nanocrystalline Wins Every Time

Revenue metering (Class 0.1, 0.2S): The phase error improvement alone justifies the cost. We've seen 60% reduction in phase error compared to CRGO.

Compact switchgear: 40% smaller core size means the difference between fitting in a standard cubicle or needing custom metalwork.

Multi-ratio CTs: High permeability maintains accuracy across multiple taps where CRGO struggles.

Accuracy class, burden, and core choice

Accuracy class and burden define how demanding the CT application is, and therefore what core performance is required.

Accuracy classes in practice

For a feeder meter at a utility incomer, 0.2S with 3–5 VA burden is common; this is where nanocrystalline really shines compared to CRGO.

For metering CTs (IEC style):

Class 0.1 and 0.2S: Precision and revenue metering; very low ratio and phase angle errors over a wide current range.

Class 0.5: General purpose metering; still accurate but less stringent than 0.2S.

For protection CTs:

5P, 10P: Protection classes where the CT must remain predictable up to a specified multiple of rated current.

PX and similar: Special protection CTs requiring defined knee‑point voltage and excitation characteristics for schemes like differential protection.

Higher‑precision and protection‑grade CTs benefit most from nanocrystalline cores because they need:

• High permeability for low magnetizing current
• Low losses and hysteresis for minimal phase shift
• Sufficient flux capability to avoid saturation during faults

Understanding burden

The burden is the total VA load seen by the CT secondary: instrument input plus wiring and any auxiliary devices. A 1 VA meter plus 2 VA of wiring is not unusual in older switchgear, even though drawings still say '5 VA'.

Core implications:

• Higher burden means higher secondary voltage at a given current, increasing flux in the core and pushing it closer to saturation.
• For a given accuracy class and ratio, higher burden often requires a larger or higher‑grade core.

When selecting a nanocrystalline core, always clarify both the accuracy class and the maximum expected burden in VA at the rated secondary current (e.g., 5 A or 1 A).

Step‑by‑step CT core selection

This section outlines a practical process engineers can follow when selecting nanocrystalline current transformer cores.

Step 1: Gather application data

Before choosing a core, define these parameters:

• Primary current rating (e.g., 200 A, 1000 A) and maximum fault current
• Secondary current rating (e.g., 1 A or 5 A)
• Accuracy class and CT type (metering vs protection)
• Total secondary burden in VA (meter + relays + wiring)
• System frequency (50 Hz, 60 Hz) and expected harmonics
• Installation constraints: available space, busbar size, mounting style, ambient temperature range

Engineers usually underestimate wiring burden and fault level; writing those numbers down now avoids an unpleasant surprise in the lab.

Step 2: Choose material and core family

For most modern low‑voltage and medium‑voltage CTs, nanocrystalline is appropriate when:

• Accuracy requirements are 0.5 or better, especially 0.2 or 0.2S
• CTs must handle high fault currents or require defined knee‑point voltages
• Space is limited and compact cores are needed

At this step, select the nanocrystalline material grade suitable for 50/60 Hz operation and a core family or shape (e.g., toroidal ring core, split core, rectangular core) that matches your mechanical constraints and primary conductor layout.

At CenturaCores, nanocrystalline is the default for any CT targeting 0.5 or better, unless the customer has a legacy design locked to CRGO.

Step 3: Rough sizing of the core

Core sizing balances cross‑section (to carry flux without saturating) and window area (to accommodate primary conductors and secondary turns).

Key considerations:

• Cross‑sectional area must be large enough so that the peak flux density remains within the recommended limits of the nanocrystalline material for both normal and fault conditions.
• Window area must allow the required number of secondary turns and mechanical clearance around the primary conductor or busbar.

A practical approach is to:

• Use empirical rules or manufacturer recommendations for minimum core cross‑section based on ratio, burden, and accuracy class.
• Start with a standard size from a CT core catalogue, then refine based on detailed calculations or simulation.

If you are between two sizes, CT experience says the slightly larger core is almost always cheaper than a redesign after failed tests.

Step 4: Check accuracy and excitation behavior

Once a tentative core size is chosen, evaluate whether it can meet:

• Ratio and phase angle error limits for the specified accuracy class over the intended current range
• Knee‑point voltage or excitation curve requirements (especially for protection CTs)

For precision metering CTs, ensure magnetizing current is low at rated current so the secondary current accurately reflects primary current. For protection CTs, confirm that the core does not saturate prematurely at multiples of rated current relevant to the protection scheme.

Step 5: Validate thermal and mechanical constraints

A core that is magnetically adequate must still fit physically and operate safely.

Check that:

• The core and winding fit inside the mechanical envelope of the CT housing or switchgear compartment.
• Temperature rise under continuous and fault conditions remains within acceptable limits.
• Mounting, insulation, and creepage distances meet applicable standards and your company's internal design rules.

If any of these fail, move to the next larger core size or consider a different core shape while keeping the same material.

Why Is My CT Saturating? Troubleshooting Core Selection

When CTs fail in the field, it's usually one of these four issues. Here's how to diagnose and fix them:

Problem 1: "CT works fine until fault current flows"

Symptoms:

• Normal operation: accurate
• During faults: protection doesn't operate
• Secondary current clips at ~20× primary

Solution:

• Recalculate with actual fault current (not nameplate)
• Include DC offset (multiply by 1.6)
• Increase core cross-section by 25%

Problem 2: "Accuracy tests fail at 5% and 10% rated current"

Symptoms:

• Ratio error >0.2% at light load
• Phase error >10 minutes
• Passes at 100% and 120% current

Solution:

• Switch to higher permeability core
• Nanocrystalline instead of CRGO
• Reduce secondary turns if possible

Problem 3: "CT heats up during normal operation"

Symptoms:

• Core temperature >80°C
• Accuracy drifts over time
• Higher losses than calculated

Solution:

• Check actual burden vs design
• Verify flux density <1.0T
• Consider larger core cross-section

Problem 4: "CT won't fit in the allocated space"

Symptoms:

• Core OD exceeds compartment
• Window too small for busbar
• Height doesn't fit with terminals

Solution:

• Switch to nanocrystalline (40% smaller)
• Consider split-core design
• Rectangular core for tight spaces

Real Project: 630A Feeder CT That Almost Failed Commissioning

The Problem

A major utility needed 630/1A Class 0.2S CTs for revenue metering on 11kV feeders. The original CRGO design passed factory tests but failed on-site commissioning with 0.35% ratio error at 10% load current.

Root Cause Analysis:

• Factory test used 1VA electronic burden
• Site had 2.1VA actual burden (1.5VA meter + 0.6VA cable)
• CRGO core saturated at higher secondary voltage
• Magnetizing current increased 4× at light load

The Solution

Switched to our NC-80 nanocrystalline core with same outer dimensions:

Original CRGO Core

• 85mm OD × 45mm ID × 25mm H
• 630 secondary turns
• Ratio error: 0.35% @ 10% In
• Phase error: 12 minutes
• Failed Class 0.2S limits

Nanocrystalline Replacement

• 85mm OD × 45mm ID × 25mm H
• 630 secondary turns (same)
• Ratio error: 0.08% @ 10% In
• Phase error: 3.2 minutes
• Passed Class 0.2S with margin

Result: All 24 CTs passed commissioning tests. Project delivered on time with no redesign costs.

Design checklist before contacting a supplier

If you send a core supplier only the ratio and accuracy class, you are inviting 3–4 rounds of back‑and‑forth. When requesting nanocrystalline current transformer cores, having a clear specification accelerates design and sampling.

Prepare the following:

• CT type and application (metering, protection, combined)
• Primary and secondary current ratings
• Accuracy class and any special requirements (e.g., PX, knee‑point voltage)
• Total secondary burden in VA
• System voltage and frequency
• Expected maximum fault current and duration
• Mechanical constraints: window size, outer diameter/height limits, split or solid core preference
• Operating temperature range and environment

Providing this information allows the core to be correctly sized and optimized for your application from the start.

Work with nanocrystalline CT core specialists

Selecting the right nanocrystalline core is critical for CT performance, safety, and long‑term reliability. An optimized core can reduce overall CT size, improve accuracy, and simplify compliance with metering and protection standards.

CenturaCores specializes in nanocrystalline current transformer cores for metering, protection, and monitoring applications. If you share your application data and drawing constraints, a suitable standard core can often be recommended quickly, or a custom design can be developed where required.

If you are unsure whether your current CT spec is realistic, send over your one‑line diagram, burden, and fault level. It is usually possible to say within a day whether a standard nanocrystalline core will work or a custom design makes more sense.

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