{"id":3564,"date":"2026-10-24T09:00:00","date_gmt":"2026-10-24T01:00:00","guid":{"rendered":"https:\/\/cnbygele.com\/?p=3564"},"modified":"2026-10-23T19:04:15","modified_gmt":"2026-10-23T11:04:15","slug":"current-transformer-burden-calculation","status":"publish","type":"post","link":"https:\/\/cnbygele.com\/ar\/blog\/current-transformer-burden-calculation\/","title":{"rendered":"\u062d\u0633\u0627\u0628 \u0639\u0628\u0621 \u0645\u062d\u0648\u0644 \u0627\u0644\u062a\u064a\u0627\u0631: \u0645\u0642\u0627\u0648\u0645\u0629 \u0627\u0644\u0633\u0644\u0643\u060c \u0627\u0644\u0641\u0648\u0644\u062a-\u0623\u0645\u0628\u064a\u0631 \u0648\u0627\u0644\u0623\u0645\u062b\u0644\u0629 \u0627\u0644\u0639\u0645\u0644\u064a\u0629"},"content":{"rendered":"

Current transformer burden calculation adds the impedance of everything connected to the secondary circuit and converts it to apparent power at the stated secondary current. For a predominantly resistive circuit, burden in volt-amperes is S = I\u00b2R<\/strong>. Include the complete outgoing and return cable path, meter inputs, test blocks and connections. Then compare the result with the CT manufacturer’s declared accuracy conditions, not simply the largest VA number on a catalogue page. This calculation supports conventional current-output metering CTs; voltage-output sensors and Rogowski coils require different input arrangements.<\/p>\n

Establish the secondary circuit boundary<\/h2>\n

Begin with the approved drawing and the exact transformer and meter model codes. Identify which core supplies which instruments and whether the secondary is rated for one ampere, five amperes, or another value. These are design inputs, not settings to guess from a photograph. A primary ratio such as 400\/5 does not, by itself, tell you the allowed secondary impedance, accuracy class, frequency range or performance at light load.<\/p>\n

Follow the secondary conductors from the CT terminals through every series device and back to the transformer. Separate this external circuit from the CT’s own winding impedance. External burden is the connected load; winding impedance becomes relevant to a more complete excitation or protection analysis. Do not silently add the winding resistance to an external burden figure and then compare the total against a specification defined differently.<\/p>\n

The OpenEnergyMonitor introduction to CT sensors<\/a> distinguishes current-output and voltage-output arrangements and explains why the secondary circuit matters. Confirm the output architecture before applying a conventional current-output burden calculation; an internal burden resistor in a voltage-output sensor changes the interface being assessed.<\/p>\n

Gather inputs before opening the calculator<\/h2>\n

Record the secondary current, cable conductor material, conductor resistance per unit length at its stated reference temperature, installed one-way route length, expected operating temperature, and every connected input’s burden. Obtain meter input data from its current technical documentation. A communications connector, panel size or display style provides no evidence of an electrical input’s impedance.<\/p>\n

Cable route length should include vertical rises, terminal loops, routing around compartments and spare length that will actually be installed. A straight-line cabinet-to-CT distance can underestimate the real circuit. Use the resistance data for the selected cable construction rather than borrowing a value from an unrelated conductor size. If only a nominal conductor resistance is available, state that limitation and request the final cable schedule before design acceptance.<\/p>\n

Document whether the cable return is dedicated or shared in a particular secondary arrangement. The simple two-wire example below assumes a dedicated outward and return conductor. Three-phase star-connected arrangements need the actual phase and neutral paths analysed; do not apply an arbitrary factor of two to every configuration without checking the drawing.<\/p>\n

\"Reference
Product-reference illustration of the disconnected BY194 exterior; the photograph does not establish a current-input rating or CT compatibility.<\/figcaption><\/figure>\n<\/p>\n

Convert each item to a consistent basis<\/h2>\n

For a purely resistive lead circuit, calculate R_leads = 2 \u00d7 L \u00d7 r<\/strong>, where L is one-way route length in metres and r is resistance in ohms per metre. The factor of two accounts for the return conductor in this stated arrangement. Lead burden is then S_leads = I_secondary\u00b2 \u00d7 R_leads<\/strong>, in VA.<\/p>\n

When an instrument’s burden is specified directly in VA at the same rated secondary current, include it on that basis. When impedance and power factor are supplied, work with resistance and reactance: Z_total = \u03a3R + j\u03a3X<\/strong>, followed by S_total = I\u00b2 \u00d7 |Z_total|<\/strong>. Adding VA magnitudes alone can obscure the phase relationship. A resistive approximation should be identified as such and used only where the component information supports it.<\/p>\n

The NIST technical note on current-transformer calibration<\/a> provides specialist measurement context for current-transformer behaviour and secondary loading. A field burden worksheet is a design record, not a substitute for calibration or an accuracy certificate covering the actual operating conditions.<\/p>\n

\n\n\n\n\n\n\n\n\n\n
Circuit item<\/th>\nRequired evidence<\/th>\nTreatment in the worksheet<\/th>\n<\/tr>\n<\/thead>\n
Secondary leads<\/td>\nActual path, cable resistance and temperature basis<\/td>\nCalculate resistance for the complete circuit<\/td>\n<\/tr>\n
Meter current input<\/td>\nVA or impedance at the declared input rating<\/td>\nConvert on the same current basis<\/td>\n<\/tr>\n
Test block and terminals<\/td>\nDocumented resistance or justified allowance<\/td>\nInclude every series path actually used<\/td>\n<\/tr>\n
Additional instrument<\/td>\nInput specification and approved connection<\/td>\nRecalculate the entire series circuit<\/td>\n<\/tr>\n
CT itself<\/td>\nAccuracy class and permitted burden conditions<\/td>\nCompare external burden with the applicable declaration<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

The table is a documentation checklist rather than a list of assumed component ratings. Missing data should remain visible instead of being replaced with convenient zeroes.<\/p>\n

Worked example: five-ampere secondary<\/h2>\n

Assume an illustrative dedicated two-wire circuit with a 20 m one-way route, a cable resistance of 0.008 ohm\/m at the selected calculation temperature, a meter burden of 0.5 VA at 5 A, and a documented additional series resistance of 0.02 ohm for the test path. These inputs are hypothetical; they are not CNBYG product specifications or universal cable values.<\/p>\n

The lead resistance is 2 \u00d7 20 \u00d7 0.008 = 0.32 ohm. At 5 A, the leads contribute 25 \u00d7 0.32 = 8 VA. The additional test-path resistance contributes 25 \u00d7 0.02 = 0.5 VA. With the assumed resistive meter burden, the total is 8 + 0.5 + 0.5 = 9 VA<\/strong>. The equivalent resistive external impedance is 9\/25 = 0.36 ohm<\/strong>.<\/p>\n

That answer does not establish a pass. Compare it with the exact CT accuracy declaration, including burden range, burden power factor, current range and frequency. If the permitted conditions do not cover the calculated circuit, shorten the route, reconsider the conductor, relocate the meter or select a compatible measurement arrangement. Changing the CT ratio simply to obtain a larger catalogue burden can reduce useful performance at the plant’s normal current.<\/p>\n

Why one ampere changes the cable contribution<\/h2>\n

For the same 0.32 ohm lead circuit, a 1 A secondary produces 1\u00b2 \u00d7 0.32 = 0.32 VA<\/strong> of lead burden. The corresponding 5 A calculation produced 8 VA. The ratio is twenty-five because the current is squared. This comparison applies to the same lead resistance; it does not mean that every one-ampere installation has twenty-five times better overall accuracy.<\/p>\n

A one-ampere CT must feed an input designed for that signal, and its own accuracy conditions must be checked. Meter burden may also differ between variants. Recalculate with the specifications of the proposed pair. Do not connect a one-ampere transformer to a five-ampere input and assume that entering a new primary ratio fixes the electrical mismatch.<\/p>\n

Cable heating changes resistance, and resistance changes the external burden. If the calculation is already close to an allowed boundary, a room-temperature cable assumption may hide a practical problem. Use the cable manufacturer’s temperature data or an approved engineering correction, and preserve both the reference and operating-temperature calculations for review.<\/p>\n

Accuracy burden is not a protection design<\/h2>\n

Metering and protection transformers serve different duties. A burden check for an industrial panel meter does not establish protection performance during fault current, transient response, knee-point voltage or resistance to saturation. Those questions require the relevant CT class, excitation data, fault conditions and protection engineering method.<\/p>\n

Likewise, reducing external burden is not a universal remedy. Some accuracy declarations apply over a specified burden range and power factor. Confirm those conditions rather than interpreting the rated burden as a target that every circuit must approach or as an unlimited permission to add devices. A design change that removes an instrument should be evaluated with the same care as a change that adds one.<\/p>\n

For the broader equipment decision, use the CT selection guide for multifunction meters<\/a>. This page provides the detailed burden worksheet; the selection guide also addresses primary-current range, mechanical fit and input compatibility.<\/p>\n

Verify safely and retain the evidence<\/h2>\n

Secondary-circuit inspection and measurement belong in an approved procedure performed by qualified personnel. The HSE electrical safety guidance<\/a> provides general context on competence and safe electrical work. Never open the secondary of a conventional CT while its primary circuit is energised. A resistance measurement requires an appropriately isolated circuit and the correct handling of shorting and test facilities. This article does not provide a live disconnection sequence.<\/p>\n

Before handover, reconcile the calculation with the installed cable length and connected devices. Record the final wiring drawing, cable type, measured or documented resistance, input specifications, calculation temperature, CT nameplate and reviewer. If a comparison test is required, define the reference equipment and acceptance basis before measuring. A plausible meter reading alone does not prove accuracy.<\/p>\n

\"Reference
Product-reference illustration of the disconnected BY194 exterior; the photograph does not establish a current-input rating or CT compatibility.<\/figcaption><\/figure>\n<\/p>\n

The open-secondary safety guide<\/a> explains why the CT circuit must remain controlled. For an installation review, the multifunction meter wiring guide<\/a> covers phase association and scaling. These checks complement the burden calculation without replacing it.<\/p>\n

Specify the measurement chain in the RFQ<\/h2>\n

When enquiring about a CNBYG BY194 instrument<\/a>, provide the exact function and input variant required, proposed CT ratio and class, expected normal and maximum current, route length and calculated external burden. Ask for written confirmation of compatibility and the applicable accuracy conditions. The reference photograph illustrates the product exterior; it does not establish a particular CT input rating.<\/p>\n

Include a change-control note: adding a second indicator, extending the leads or moving the meter requires recalculation. This makes the worksheet useful after commissioning rather than leaving it as an isolated purchasing attachment. A future maintainer should be able to identify which assumptions changed and what evidence is needed to accept the revised circuit.<\/p>\n

Further learning: instrument transformer principles<\/h2>\n

The NPTEL IIT Kharagpur lesson below explains current and potential transformer principles. It provides learning context for the secondary-circuit discussion; the approved equipment documents govern the installation.<\/p>\n