CT Ratio Calculator

CT ratio example: a 400 A maximum load current with a standard 5 A secondary gives a CT ratio of 400:5, or 80:1. A current transformer (CT) scales down a large primary current to a small, standardized secondary current (almost always 5A or 1A) that meters and protection relays are built to accept. Choosing the right CT ratio means picking a standard primary rating that gives good measurement accuracy at your actual operating current, not simply "big enough to be safe" — oversizing a CT relative to actual load is one of the most common and easily avoided sources of poor metering accuracy in the field.

Formula used: CT Ratio = Selected Standard Primary Rating ÷ Secondary Current (5A or 1A). For a basic metering-oriented preliminary selection, this calculator targets approximately 60-100% utilization as a practical sizing heuristic — not an IEC accuracy requirement. Final CT selection should verify the CT's accuracy class, specified accuracy range, burden, application requirements, and manufacturer data.

Worked example: a feeder has a maximum load current of 320 A. Looking at standard CT primary ratings (250, 300, 400, 500...), a 400A primary rating puts 320A at 80% utilization (320÷400 × 100 = 80%) — within the calculator's 60-100% preliminary utilization target. With a 5A secondary standard, CT Ratio = 400:5.

Why utilization percentage matters as much as the ratio number itself: a CT's accuracy performance is specified over a defined operating range relative to its rated primary current, with the exact range depending on the CT's accuracy class and applicable standard — conventional metering classes commonly specify accuracy points beginning around 5% or 20% of rated current and extending through 100% (and often somewhat beyond, for a defined accuracy limit factor), while extended-range classes specify performance at lower currents. Operating well below the CT's specified accuracy range, common when a CT is selected with excessive margin above actual load "to be safe," tends to produce larger measurement error — a CT rated for 1000A serving an actual 200A load (20% utilization) will generally measure less accurately than a 250A-rated CT serving that same 200A load (80% utilization), though the precise accuracy at each utilization level depends on the specific CT's accuracy class.

The trade-off against future load growth: selecting a CT ratio tightly matched to today's exact load current creates a real tension with anticipated future load growth — a CT sized precisely for current load may need replacement if load grows meaningfully beyond its rated primary current, and replacement can require significant electrical isolation, access, and rewiring work depending on the CT's construction (window/bar-type, wound-type, etc.) and installation arrangement. Many designers deliberately select a CT primary rating that comfortably covers a reasonable projected future load, accepting a somewhat lower utilization percentage today as a worthwhile trade-off against avoiding a CT replacement in a few years — this is a legitimate design judgment call, not a mistake, as long as the resulting utilization percentage doesn't drop so low that current accuracy becomes genuinely inadequate for the application's actual metering or protection needs.

Understanding CT accuracy classes: a metering CT's accuracy class (commonly 0.2, 0.5, 1.0, or similar) states the maximum percentage error permitted within its rated operating range at rated burden — a Class 0.5 CT guarantees no more than 0.5% ratio error under specified conditions, tighter (better) than a Class 1.0 CT's 1% guarantee. Higher-accuracy classes (like 0.2, sometimes 0.2S for extended accurate range down to lower currents) are typically specified for revenue/billing metering where financial accuracy matters most, while looser classes suffice for general indication or less critical monitoring purposes. The "S" designation on some accuracy classes specifically indicates extended accuracy performance down to lower current levels (often 1% of rated current), useful for applications needing accurate measurement across a wide load range rather than just near full load.

Protection CT accuracy limit factor (ALF): protection-class CTs (5P10, 5P20, 10P10, and similar designations) specify both an accuracy class (5P or 10P, meaning maximum 5% or 10% composite error) and an accuracy limit factor (the multiple of rated current, like 10 or 20, up to which that accuracy is maintained). A 5P10 CT maintains 5% accuracy up to 10 times its rated current — important because protection relays need to see a reasonably accurate representation of fault current, which is often many times normal load current, not just accuracy near rated load the way metering CTs prioritize.

Physical CT types and their sizing implications: window-type (or bar-type) CTs, where the primary conductor simply passes through a central opening, are common for lower to medium current ratings and are often available with multiple ratio taps or even multiple physical window sizes to accommodate different conductor sizes. Wound-type CTs, where the primary winding is a dedicated coil within the CT itself, are more common for lower current ratings where a bar-type design isn't practical. The physical type doesn't change the underlying ratio selection principle, but availability of specific standard ratios can vary somewhat between physical CT construction types.

Summary: select a standard CT primary rating that puts your maximum normal load current at roughly 60-100% utilization for good metering accuracy, use protection-class CTs with an appropriate accuracy limit factor where relay accuracy at fault current matters, verify burden compatibility for all connected devices, and always short a CT's secondary before disconnecting any load while its primary remains energized.

Working with an existing CT installation: when reviewing or troubleshooting an existing metering or protection installation, checking actual measured utilization (comparing typical operating current against the installed CT's rated primary current, readable from its nameplate) is a quick, valuable diagnostic step — unexpectedly poor metering accuracy, inconsistent readings, or relay sensitivity concerns can sometimes trace directly back to a CT that's significantly oversized relative to actual operating current, a straightforward finding once you know to check for it specifically.

Coordinating CT selection with meter and relay input specifications: whatever CT ratio is ultimately selected, the connected meter or relay needs to be configured (or programmed, for a numerical device) with the matching ratio so it correctly scales the CT's secondary current back into an actual primary current reading — a CT ratio change without a corresponding update to every connected device's configured ratio produces systematically incorrect readings or settings, a surprisingly common and easily overlooked step when a CT is replaced or its ratio is changed during a system modification.

Building a simple habit of checking and documenting both the physical CT ratio and every connected device's configured ratio together, any time either one changes, prevents this class of error from persisting undetected — a mismatch here doesn't usually cause an obvious fault or alarm, just silently incorrect readings, which can go unnoticed for a long time until a billing discrepancy, an audit, or an unexpected protection response during an actual fault reveals the underlying configuration error.

1. Oversizing CT primary rating far beyond actual load "to be safe." This is the most common CT selection mistake — a CT operating well below the calculator's 60-100% preliminary utilization target can suffer degraded accuracy at low current, with the specific onset point depending on the CT's accuracy class and specified accuracy range, which defeats the purpose of accurate metering or sensitive protection.

2. Confusing CT ratio selection criteria for metering versus protection applications. Metering-optimized and protection-optimized CTs have different design priorities (accuracy at normal load vs accuracy at fault current) — a ratio and class chosen well for one purpose may not suit the other; confirm which application (or both, via separate cores) your specific installation needs.

3. Ignoring CT burden compatibility when connecting multiple meters or relays. Connecting more devices, or devices with higher input impedance than the CT's rated burden accounts for, can push total burden above the CT's rating and degrade accuracy — verify total connected burden against the CT's rated burden, not just the ratio selection alone.

4. Leaving a CT secondary circuit open while the primary remains energized. This is a genuine safety hazard specific to current transformers, capable of producing dangerously high open-circuit voltage — always short the secondary circuit via a shorting terminal block before disconnecting any load from an energized CT.

5. Not revisiting CT ratio selection after significant load growth. A CT well-matched to load at installation can become poorly utilized (if load dropped) or risk saturation (if load grew significantly) over time — periodically checking CT utilization against actual current operating data catches this drift before it degrades measurement quality.

6. Assuming a CT selected for good metering accuracy is automatically adequate for protection relay accuracy at fault current. A metering-class CT is often specifically designed to saturate (limit output) at high fault current, precisely to protect connected metering equipment — this same saturation behavior can make it unsuitable for accurately relaying true fault current magnitude to a protection relay, which instead needs a protection-class CT designed to remain accurate well beyond normal rated current.

7. Selecting accuracy class without matching the actual application requirement. A tighter (lower-number) accuracy class than genuinely needed adds cost without practical benefit for non-revenue-critical applications, while too loose a class can be genuinely inadequate for revenue metering — match accuracy class to the actual criticality of the specific measurement.

8. Not accounting for multiple loads or devices when calculating total CT burden. Each connected meter, relay, or instrument adds to total burden on the CT secondary circuit — tallying total connected burden across every device sharing that CT, not just the single most obvious load, is necessary for an accurate burden compatibility check.