Quick technical guide: This article explains current transformer selection: why the ct ratio is only the starting point for panel builders, contractors, maintenance teams and industrial electrical buyers.
A panel builder receives a simple requirement:
"Need three CTs, 400/5."
That sounds specific. It is not.
The 400/5 ratio tells you that a rated primary current of 400 A corresponds to a rated secondary current of 5 A. It does not tell you whether the CT physically fits the busbar, whether its accuracy is suitable for the meter, whether its burden is adequate, or whether the application is measurement or protection.
Current transformers are straightforward once the application is defined. Problems start when the ratio is treated as the complete specification.
What does a current transformer actually do?
A current transformer, usually shortened to CT, reproduces a scaled representation of primary current on its secondary circuit.
This allows meters, power monitors and protection equipment to work with a manageable current rather than carrying the full primary conductor current through the instrument.
ABB describes its CT1M range as transforming primary current into a secondary current for indirect measurement of electrical network parameters.
ABB Catalogo Generale
In a window-type CT, the main conductor or busbar passes through the CT opening. The primary circuit itself carries the load current while the secondary terminals connect to the measuring or protection circuit.
What does 400/5 mean on a CT?
Consider a CT marked:
400/5 A
Its transformation ratio is:
400 ÷ 5 = 80:1
At the rated relationship, 400 A in the primary corresponds to 5 A in the secondary.
If the primary current were 200 A under ideal proportional conditions:
200 ÷ 80 = 2.5 A secondary
The meter connected to the CT is configured around this ratio so it can display the corresponding primary current.
Manufacturers offer many ratios because industrial feeders have very different operating currents. ABB's current transformer catalogue, for example, includes products with defined primary and secondary ratings rather than one universal CT.
ABB Catalogo Generale
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Do not automatically oversize the CT ratio
Suppose a feeder normally carries around 180 A.
Choosing a 1000/5 CT simply because "higher is safer" is poor measurement practice.
The connected instrument would use only a small portion of the CT's rated range during normal operation, which can affect the usefulness and accuracy of the measurement depending on the CT class and application.
Schneider Electric's current CT selection guidance recommends choosing a ratio appropriate to the maximum measured current rather than selecting an unnecessarily large primary rating. Its guidance, updated April 15, 2026, recommends the next suitable ratio above the maximum measured current for the applications covered by that product guidance.
Schneider Electric
The exact choice still needs to account for expected load growth, overload conditions and the purpose of the CT.
1A or 5A secondary?
Industrial CTs commonly use standardized secondary currents such as 1 A or 5 A.
Both exist in current manufacturer ranges. Siemens, for example, currently publishes low-voltage measurement CTs with 1 A secondaries, while other models use 5 A.
Siemens Ausschreibungstexte
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The receiving meter or protection relay has to match the chosen secondary current.
You cannot assume a meter configured for a 5 A CT input will accept a 1 A CT without checking its specifications and configuration.
Cable length also matters.
Current flowing through secondary wiring creates a burden on the CT. For longer secondary circuits, a 1 A system can reduce wiring burden because the cable loss for the same resistance is much lower at the lower current.
Siemens gives an example in its protection documentation where a 1 A secondary is selected specifically to reduce CT burden.
Siemens Industry
That does not make 1 A universally preferable. The CT, meter or relay and the complete secondary circuit need to be designed together.
What is CT burden?
The secondary of a CT does not operate without a load.
The meter, relay, connecting cable and terminals together create a burden.
CT burden is commonly expressed in volt-amperes (VA) at the rated secondary current.
Consider why cable length matters.
A longer cable has more resistance. At a given secondary current, that resistance contributes additional burden. Connections and the input impedance of the connected device contribute as well.
If the actual secondary burden falls outside the conditions for which the CT's accuracy is specified, measurement or protection performance can be affected.
Schneider's CT selection guidance therefore specifically tells designers to check both accuracy and burden capabilities after choosing the basic transformer configuration.
Schneider Electric
Siemens product data likewise publishes rated power alongside CT ratio and accuracy class. A current Siemens 200/5 measurement CT, for example, lists its rated secondary current, accuracy class and rated VA as separate specifications.
Siemens Ausschreibungstexte
That is a useful reminder: ratio and burden are different parameters.
Why accuracy class matters
A CT introduces some measurement error.
Accuracy class defines its performance within specified conditions.
For energy measurement or monitoring, the required accuracy may be different from that needed for a protection relay.
Current manufacturer catalogues reflect this distinction. ABB publishes instrument transformers intended for metering as well as devices and secondary windings intended for protection applications. Its IEC indoor CT range lists both metering accuracy classes and protection classes.
ABB Group
Do not select the smallest accuracy-class number simply because it appears "better."
First determine what the connected device requires and under what operating range the specified accuracy must be maintained.
Metering CT and protection CT are not the same job
A metering system normally needs useful accuracy around the currents being measured during operation.
Protection equipment faces another problem: it may need a reliable current representation during fault conditions where primary current is many times normal load current.
Those requirements can lead to different CT characteristics.
Schneider's documentation, for example, separately defines relay accuracy classes for CTs used with protective relays.
Schneider Electric
Siemens protection documentation likewise specifies CT requirements based on the protection function, including ratio, class and burden.
Siemens Industry
A CT selected for an ammeter should therefore not automatically be copied into a protection circuit.
The CT opening has to fit the conductor
This sounds obvious until the panel reaches assembly.
A CT can have the correct ratio and electrical specification but still be useless because the busbar will not pass through its window.
Schneider lists window size as one of the primary CT selection checks.
Schneider Electric
For a cable installation, check the outside dimensions of the conductor arrangement.
For a busbar installation, check both bar width and thickness, including the actual way the bar passes through the CT.
Allow for practical installation clearance as well. A theoretically exact fit can make assembly unnecessarily difficult.
Orientation and polarity matter
CT terminals and primary direction are normally identified by polarity markings.
Those markings become especially important in three-phase power measurement.
If one CT is installed in the opposite orientation or its secondary connections are reversed relative to the others, the current magnitude may still look plausible while power, power factor or energy readings become incorrect.
For a simple standalone ammeter, the effect may be less obvious.
For a multifunction power meter, incorrect CT polarity can produce confusing results.
Follow the polarity markings and wiring diagram supplied with the CT and measuring instrument rather than deciding orientation from the physical shape of the device.
One safety rule matters more than the others
Never leave the secondary of an energized current transformer open-circuited while primary current is flowing.
This is not a minor wiring preference.
When primary current flows and the CT secondary circuit is opened, dangerous voltage can develop across the secondary terminals.
Schneider Electric warns that an open-circuited CT secondary can produce very high voltage and presents shock, burn and equipment hazards. Its guidance states that the secondary should remain shorted or connected to an appropriate burden when primary current can flow. The FAQ was last updated October 7, 2024.
Schneider Electric
This is why CT circuits can use shorting terminal blocks.
They allow the secondary to be safely shorted according to the equipment procedure before a meter or relay is disconnected for service.
CT secondary work belongs with qualified electrical personnel following the equipment manufacturer's instructions and site isolation procedures.
Why you should not install a normal fuse casually in a CT secondary
Anything that can unexpectedly open the secondary circuit needs careful consideration.
A conventional fuse that operates while primary current continues flowing could leave the CT secondary open, creating the hazard just described.
Protection and isolation arrangements for CT circuits should follow the meter, relay, CT and applicable electrical design requirements rather than treating the secondary like an ordinary low-current control circuit.
Three-phase metering needs more than three matching ratios
For a three-phase panel, CTs are commonly used together with a multifunction meter.
The CT ratios need to match the meter configuration, but installation also needs correct phase association.
The CT measuring L1 must correspond with the voltage input the meter interprets as L1. The same applies to the remaining phases.
Swapping current channels can produce incorrect power calculations even when individual current readings appear reasonable.
Commissioning should therefore include more than checking whether the display turns on.
Verify phase currents, power factor and power readings against the expected operating condition.
Unexpected negative power or unusual power-factor values can be clues to CT polarity or phase-association errors.
Split-core vs solid-core CT
Not every CT requires the primary conductor to be disconnected during installation.
A solid-core or window CT requires the conductor to pass through its opening.
A split-core CT can open mechanically so it can be fitted around an existing conductor in suitable retrofit applications.
That can make split-core designs useful when adding energy monitoring to an existing installation.
There are trade-offs, however. Accuracy, physical size, cost and available ratios vary by product.
Use the application requirements and manufacturer data rather than assuming one construction is always superior.
A common panel-building mistake
Imagine a new main distribution panel rated for a feeder expected to operate around 500 A.
The drawing states only:
CT: 500/5
Purchasing finds the cheapest CT carrying that ratio.
When the components arrive, the CT opening does not fit the specified busbar.
Even if it did, nobody has checked its VA burden or accuracy class against the multifunction meter.
The ratio was correct.
The specification was incomplete.
A better drawing would define the ratio, secondary current, accuracy requirement, rated burden, physical window, application and any other project-specific CT requirements.
Replacing an existing CT
For maintenance work, do not start by removing the old unit.
Record its complete label first.
Capture:
Primary/secondary ratio
Rated secondary current
Accuracy class
Rated burden
Product reference
Physical window dimensions
Primary polarity markings
Secondary terminal markings
Connected meter or relay
Installation type
Also photograph the existing secondary wiring before any authorized disconnection.
If a meter has been replaced recently and readings became incorrect afterward, check the meter's programmed CT ratio before assuming the CT itself has failed.
A 400/5 CT paired with a meter configured for a different ratio can create consistently incorrect displayed values even when the transformer is working normally.
What to send when requesting a CT from INTEKS
For a new panel, provide the expected primary current, required secondary current, application, accuracy requirement, burden requirement, conductor or busbar dimensions and the meter or protection device being used.
For a replacement, send a clear photograph of the complete CT label and its installation.
INTEKS supplies industrial electrical protection, control and measurement-related components for panel builders, contractors, maintenance teams and industrial users. Providing the complete electrical and mechanical requirement is far more reliable than requesting a CT by ratio alone.
Frequently asked questions
What does 200/5 mean on a current transformer?
It describes a rated transformation relationship where 200 A primary current corresponds to 5 A secondary current. The connected meter must use the appropriate CT ratio to calculate or display the primary value.
Should I choose a 1A or 5A CT?
Use the secondary current required by the connected meter or protection equipment and the overall circuit design. A 1 A secondary can reduce wiring burden in longer secondary circuits, but it should not be substituted for 5 A without verifying compatibility.
Siemens Industry
Can I leave CT secondary wires disconnected?
Not while primary current may flow. An open CT secondary can develop dangerous voltage. Appropriate shorting procedures and terminals should be used during authorized servicing.
Schneider Electric
Does CT accuracy class matter for a normal panel meter?
Yes. The CT contributes to the overall measurement error. Select an accuracy class and burden that suit the connected instrument and required measurement performance rather than choosing by ratio alone.
Schneider Electric
Why is my power meter showing strange power-factor or negative-power readings?
Possible causes include reversed CT polarity, current channels associated with the wrong voltage phases, incorrect meter configuration or actual reverse power flow. Verify the installation and configuration before replacing components.
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For project requirements, component identification or a panel enquiry, contact the INTEKS team with the relevant rating, application and existing product details.
For the applicable standard and current edition, consult the official IEC publication search.
