CT Burden Planning: Count the Return Cable Before Choosing 1 A or 5 A
A current transformer can have the right ratio and still be poorly matched to the circuit between the CT and the meter. Before specifying a remote display or extending a secondary cable, prepare a burden worksheet that includes the complete cable loop and every connected input. The useful question is how much load the selected secondary must drive under its stated accuracy conditions.
This is a desk-based planning guide for conventional AC metering CTs with 1 A or 5 A secondary outputs. It is not a wiring procedure, a protection-system design or approval to work inside a switchboard. Electrical assessment, isolation, installation and validation belong with competent electrical personnel using the exact equipment documentation and current requirements applicable to the site.
Identify the output before doing the calculation
Start with the full CT part number and rating plate, then the exact meter input. Acrel’s split-core range includes ampere, milliampere and millivolt output variants. A similar enclosure does not establish compatibility. Keep 1 A and 5 A CT calculations separate from low-power sensors, built-in-burden voltage-output devices, Rogowski coils and electronic transducers.
The pictured ACREL AKH-0.66 K-S-50x30 is a dual-winding catalogue product. Treat each output as a separately specified circuit: identify its assigned meter, rating and permitted load from the supplied documentation. This photograph does not establish the rating of a particular ordered variant, and the hypothetical example below is not a specification for this model.
Count the whole secondary circuit
Janitza’s CT installation guidance includes both outgoing and return cable length when assessing line losses. Its selection guidance also considers the measurement-device load and cable cross-section. For a resistive cable loop, resistance follows R = ρL/A and its burden contribution at secondary current I is I²R. Use the cable data and operating conditions for the actual design.
Build the worksheet from drawings and verified component data. Record the one-way route, resulting loop length, conductor material, cross-section, resistance basis and expected temperature. Put meter input burden in its own row, with the document and input rating that support it. Identify terminals and other devices in the circuit rather than assuming that the meter is the only load.
A hypothetical 5 A versus 1 A comparison
Assume a 10 m one-way route, giving a 20 m outgoing-and-return loop whose resistance is hypothetically 0.20 Ω. At 5 A, the cable contribution is 5² × 0.20 = 5 W; for this purely resistive contribution, that is 5 VA. At 1 A through the same resistance, it is 1² × 0.20 = 0.20 VA. These are illustrative inputs, not a cable-sizing table.
Now suppose the 5 A design has another 0.5 VA of meter input burden and 0.1 VA of other verified circuit burden. The total is 5.6 VA. A hypothetical CT specified for a maximum 5 VA load would not satisfy that assumed burden limit. Do not describe the mismatch as a 12% measurement error: the calculation identifies an exceeded load allowance, not an accuracy prediction.
The smaller cable loss at 1 A explains why secondary-current choice deserves review over longer routes. It does not authorise changing a meter setting to convert a 5 A installation into a 1 A installation. A different pairing needs its own documented CT, compatible input and burden calculation. The meter’s input burden may differ between its 1 A and 5 A ratings.
Review the result within the accuracy conditions
Keep the manufacturer’s load limits beside the calculated total. Janitza warns that excessive loading affects accuracy and discusses consequences of underloading as well. “Choose the biggest VA rating” is therefore an incomplete selection rule. Ask the designer or supplier to confirm the appropriate operating burden range, accuracy duty and complete CT–meter combination.
Record assumptions explicitly. A resistance value without its temperature basis is an unresolved input. A proposed route that later doubles in length requires review. An additional meter is a design change, not an administrative addition. If the verified load is outside the permitted conditions, refer the route, conductor choice or measurement arrangement for redesign; do not conceal the issue by adjusting the displayed scale.
Keep verification separate from hazardous access
Do not disconnect an operating conventional CT secondary to investigate its burden. Manufacturer guidance warns that an open secondary can develop dangerous voltages. Reading a worksheet is not authority to remove a terminal cover, insert a test instrument or change connections. Have the responsible electrical person define the safe work method and any required outage.
Purchasing can still make useful progress without opening equipment. Request approved drawings, the exact CT output and accuracy data, meter input documentation, the proposed cable schedule and any existing calculation. Flag missing information as missing. Do not infer a burden rating from the product photograph, aperture size, primary current ratio or a different family member’s datasheet.
Leave a calculation someone can review
A good handover identifies the metering point, CT and meter variants, calculation current, loop length, cable assumptions, itemised burdens and document revisions. State who must approve the result and which inputs remain unverified. After authorised installation, retain the accepted design and commissioning evidence together. A burden check supports selection; it does not replace polarity, ratio, phase-allocation or end-to-end measurement validation.
References and pictured equipment
Need help with your application?
For instrument enquiries, visit ProSense Instruments, email sales@prosenseinstruments.com.au or call 1800 560 854. Include the CT and meter models and proposed cable route.
For monitoring and automation projects, visit ProSense Solutions or email sales@prosense.com.au.
For software enquiries, visit ProSight Software or email info@prosightsoftware.com. Device and protocol integration must be confirmed for your application.
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