CT Ratio, Accuracy Class and VA Burden in an 11kV Panel: Why Oversizing Backfires

A current transformer (CT) in an 11kV panel is specified by three numbers: its ratio, its accuracy class and its rated VA burden. Choose the ratio from the circuit’s full-load current, the class from the job the core does (metering or protection), and the VA from a burden calculation of the actual relay, meter and wiring. A rated burden far above the calculated burden is not a safety margin. It takes the metering core outside its guaranteed accuracy band and lets more fault current reach the meters.

TL;DR

• CT ratio is primary current over secondary current. A 1200/5 CT delivers 5 A to the relay when 1,200 A flows in the busbar.

• Metering cores carry classes such as 0.5 or 0.5S plus an instrument security factor (FS). Protection cores carry classes such as 5P10 or 5P20.

• Under IS/IEC 60044-1, metering accuracy for classes 0.1 to 1 is guaranteed only when the connected burden sits between 25% and 100% of the rated burden.

• ABB’s REF615 application manual states the actual accuracy limit factor is proportional to the ratio of rated to actual CT burden, and notes that a 10 VA rating could have been selected much lower.

• IEC 61869-2:2012 cancels and replaces IEC 60044-1. BIS adopted it as IS 16227 (Part 2):2016.

How do you calculate CT ratio for an 11kV incomer or feeder?

Calculate the circuit’s full-load current, then choose the standard primary rating that carries it within the CT’s accurate range. For a transformer feeder, full-load current is kVA divided by (1.732 x kV). A 2,000 kVA transformer at 11 kV draws about 105 A, so a 150/5 or 200/5 CT suits it, not a 600/5 CT chosen for fault level.

The CT ratio formula is rated primary current divided by rated secondary current: a 200/5 CT has a ratio of 40, so 105 A gives 2.6 A in the secondary.

The harder decision is where normal load sits on the CT’s range. IS/IEC 60044-1 sets class 0.5 limits of 1.5% current error at 5% of rated current, 0.75% at 20%, and 0.5% at 100% and 120%. A feeder that normally runs at 10% of its CT rating is metered at the poor end of that table. Size the primary so normal load sits between roughly 20% and 120% of rated current.

Dual-ratio CTs solve the growth problem. New India Electricals Ltd (NIEL) designs and builds 11 kV VD4 switchgear in stock with this arrangement:

The 600-1200 tap lets the incomer meter accurately on day one and still carry the plant at full load. Every panel CT must also withstand the switchboard’s short-time current: 18.4, 25, 26.3, 31.5 or 40 kA on NIEL’s Power+ V 11 kV panels. At 33 kV, the same rule applies to the Power+ V3 range.

What does a CT ratio of 1200/5 or 100/5 actually mean?

A CT ratio of 1200/5 means 1,200 A flowing in the primary conductor produces 5 A in the secondary winding. A 100/5 CT produces 5 A from 100 A. The secondary current always scales with the primary, so 600 A through a 1200/5 CT gives 2.5 A to the relay and meter.

On NIEL’s incomer CT, “600-1200/5-5-5 A” means two primary ratings selectable by tap and three 5 A secondary cores, typically one for metering and two for protection. In NIEL’s standard 11 kV lineup, the incomer carries a three-core CT and each outgoing feeder a two-core CT, as set out in our guide to specifying an 11kV, 22kV or 33kV MV panel.

Metering CT vs protection CT: what do class 0.5, 0.5S, 5P10 and 5P20 mean?

A metering CT is accurate at normal load and saturates early to protect meters. A protection CT stays accurate up to many times rated current so relays see the fault. The accuracy class tells you which job a core does, and one CT often carries both.

Metering classes. Under IS/IEC 60044-1, classes 0.1, 0.2, 0.5 and 1 set current error and phase displacement limits at 5%, 20%, 100% and 120% of rated current. Classes 0.2S and 0.5S extend accuracy down to 1% of rated current, which suits tariff metering at light load. The rating plate reads, for example, “15 VA class 0.5 FS 10”.

Instrument security factor (FS). FS is the multiple of rated current above which the metering core is designed to saturate, so that a fault does not drive damaging current through the meters. IS/IEC 60044-1 notes that this saturation is desirable for the safety of the apparatus supplied by the measuring transformer.

Protection classes. The standard classes are 5P and 10P, and the standard accuracy limit factors are 5, 10, 15, 20 and 30. A 5P20 core keeps composite error within 5% up to 20 times rated current. The 10P class allows 10%. ABB’s REF615 application manual states both are suitable for non-directional overcurrent protection, with 5P the more accurate.

How to do a CT burden calculation for relays, meters and cable runs

CT burden is the total load on the CT secondary, expressed in VA at rated secondary current: the secondary current squared times the loop resistance of the wiring and connected devices. Calculate it from the relay and meter datasheets and the cable route, not from a habit of writing 15 VA.

ABB’s application note on accuracy limit factors gives the method:

1. Wiring resistance. Resistance equals resistivity times length divided by cross-section. Copper at 75 degrees C is 0.00865 ohm per metre for 2.5 sq mm and 0.00541 ohm per metre for 4 sq mm. For a three-phase connection with a common return, ABB multiplies the one-way distance by 1.2 rather than 2.

2. Device burden. Take the relay’s input impedance from its manual. ABB’s REF615 product guide lists current input impedance under 20 milliohm on the 1/5 A input, which is under 0.5 VA at 5 A.

3. Total burden. Add the wiring and device resistances and multiply by the secondary current squared.

Here is the same calculation for a relay in the panel’s own LV compartment and for a relay in a remote control room, using 2.5 sq mm copper:

A numerical relay in the panel loads a 5 A core with under 2 VA, so specifying 15 VA or 30 VA buys nothing. The remote relay on a 50 m run genuinely needs VA, and a 1 A secondary removes the problem.

Why does an oversized VA burden hurt accuracy and expose your meters?

An oversized rated burden hurts in two ways. The metering core runs below the burden band in which its accuracy class is guaranteed. And because a lightly loaded core saturates later, the meters see more fault current than the FS on the nameplate implies. Oversizing also makes the CT larger and more expensive.

The accuracy band. IS/IEC 60044-1 states that for classes 0.1 to 1 the error limits apply when the secondary burden is any value from 25% to 100% of rated burden, with a 1 VA minimum test burden. A 15 VA class 0.5 core whose meters and wiring total 1.8 VA runs at 12% of its rated burden. Its class 0.5 promise no longer covers that operating point.

The saturation point moves. ABB’s application note shows that the actual accuracy limit factor follows the ratio of rated to actual burden: Fa is approximately Fn x (Sin + Sn) / (Sin + Sa). Sin is the CT’s internal burden, Sn its rated burden and Sa the actual burden. The secondary limiting e.m.f. is fixed by the core, so a lighter burden pushes saturation to a higher current. The same physics applies to a metering core’s FS.

Taking the 0.07 ohm secondary resistance from ABB’s worked example (an internal burden of 1.75 VA at 5 A) and an actual burden of 1.8 VA:

For a protection core, a higher effective factor is extra headroom and does no harm, only cost. ABB’s REF615 manual makes that point in its own worked example: the actual accuracy limit factor came to 59, and the 10 VA rating could have been selected much lower for economical reasons.

For a metering core the higher factor is the problem. An FS 10 core specified at 30 VA but loaded at 1.8 VA behaves closer to FS 90, so the meter sees up to 90 times rated current before the core saturates. The FS on the nameplate stops describing the panel you built.

Burden sits on the short checklist I give buyers comparing quotations: busbar cross-section, the burden on the CTs and PTs, and the class of relay. CTs and PTs are the messengers of the panel; if their numbers are wrong, the relay acts on wrong information.

When does a protection CT need knee point voltage, not a VA rating?

High-impedance differential and restricted earth fault protection need a class PX CT, specified by knee point voltage, secondary winding resistance and magnetising current rather than VA and accuracy limit factor. These schemes must stay stable for through faults, and that stability is calculated from the CT’s magnetising characteristic, not from a VA figure.

IS/IEC 60044-1 defines class PX alongside classes 5P and 10P. Class PX suits transformer differential, busbar protection and restricted earth fault on transformer neutrals. The relay manufacturer’s stability calculation sets the minimum knee point voltage, and the panel builder orders the CT to that figure.

Overcurrent and earth fault protection on an 11 kV feeder does not need class PX; a 5P10 or 5P20 core, checked with the actual accuracy limit factor calculation above, is correct. The same numerical relays often take arc flash sensor inputs, covered in our guide to arc flash in MV switchgear. NIEL’s Power+ R control and relay panels up to 230 kV carry the numerical, electromechanical and high-speed relays where PX cores are typically specified.

1A or 5A secondary: which suits your switchroom?

Choose 1 A when relays or meters sit far from the CT, and 5 A when everything sits in the panel and existing meters are 5 A. Wiring burden is current squared times resistance, so a 1 A secondary has 1/25 of the wiring burden of a 5 A secondary on the same cable. ABB’s application note states this ratio directly.

On the 50 m run in the burden table, a 5 A core carries 13.5 VA and a 1 A core 0.54 VA. The ABB REF615 accepts either, as its phase-current inputs are rated 1/5 A. Three practical points decide the rest:

1. Existing meters. Panel meters and energy meters wired for 5 A need a 5 A core unless they are replaced.

2. Remote control rooms. Substations with relays in a separate control room gain the most from 1 A.

3. Open-circuit voltage. A 1 A CT has more secondary turns, so never open-circuit a live CT secondary. Short it at the test terminal block first.

CT and PT in the same lineup: where the voltage transformer sits

In NIEL’s standard 11 kV lineup, only the incomer carries a potential transformer (PT, also called a voltage transformer). Outgoing feeders take their voltage reference from the incomer PT, so they carry CTs but no PT. Removing a PT you do not need is a legitimate saving. Removing a CT core that protection needs is not.

NIEL’s incomer PT is 11kV/sqrt3 : 110V/sqrt3 rated at 50 VA, connected in parallel with the system and protected by HT fuses. Size its rated output from the same kind of burden calculation.

The CT and PT ratios combine in every energy meter. With a 1200/5 CT (ratio 240) and an 11000/110 PT (ratio 100), the meter multiplying factor is 240 x 100, which is 24,000. A wrong multiplying factor goes straight into energy reconciliation, so check it at the factory acceptance test.

Looking ahead, I expect low-power CTs and PTs on Rogowski coils to replace conventional instrument transformers in MV panels. IEC 61869-6:2016 already covers low-power instrument transformers, and the change will shrink panels again.

IS 16227 and IEC 61869-2: why IEC 60044-1 on a specification is out of date

The current international standard for inductive current transformers is IEC 61869-2:2012. It cancels and replaces IEC 60044-1, first published in 1996 with amendments in 2000 and 2002, and IEC 60044-6. BIS adopted it as IS 16227 (Part 2):2016. IEC 61869-2 is read with IEC 61869-1, the general requirements part.

Many tender specifications still call up IS 2705 and IEC 60044-1. Specify IS 16227 (Part 2) / IEC 61869-2 on new orders so that type tests and routine tests reference the standard in force. The same logic applies to switching technology, covered in vacuum or SF6 at 11 kV, 22 kV and 33 kV.

IEC 61869-2:2012 includes the contents of an interpretation sheet issued in January 2022, and a second edition is under development with a forecast publication date of January 2027. The BIS product manual for IS 16227 (Part 2) lists the parameters a CT is certified against, including ratio, rated burden, accuracy class, instrument security factor and short-time thermal current.

CT specification checklist for an 11kV panel order

1. Ratio. Normal load between about 20% and 120% of rated primary current; dual ratio where load will grow.

2. Secondary current. 1 A for remote relays and long runs, 5 A where everything is in the panel and meters are 5 A.

3. Cores. One core per function: metering, overcurrent and earth fault, and a separate core for differential or restricted earth fault.

4. Metering class and FS. Class 0.5 for monitoring or 0.5S for tariff, with FS 5 or 10.

5. Protection class and ALF. 5P10 or 5P20 for overcurrent, checked against maximum fault current and relay setting.

6. PX cores. Knee point voltage, winding resistance and magnetising current from the relay maker’s stability calculation.

7. Rated burden. From a burden calculation, keeping actual burden above 25% of rated burden on metering cores.

8. Short-time rating. Thermal current matched to the panel’s short-time rating, such as 25 kA or 31.5 kA for 3 seconds.

9. Standard. IS 16227 (Part 2) / IEC 61869-2, with type test reports for the exact rating.

10. Construction. Resin cast, low smoke and flame retardant, as on NIEL’s medium voltage panels.

    Frequently asked questions

    How do I calculate the CT ratio for an 11kV feeder?

    Divide the feeder’s maximum continuous load in kVA by 1.732 times 11 to get the full-load current, then pick the standard primary rating that keeps normal load between about 20% and 120% of rated current. A 2,000 kVA transformer feeder at 11 kV draws about 105 A, so a 150/5 or 200/5 CT fits. Check the short-time thermal rating against the switchboard fault level separately. Where the load will grow, a dual-ratio CT such as NIEL’s 600-1200/5 incomer CT covers both stages.

    What is the difference between a metering CT and a protection CT?

    A metering CT is accurate at normal load and saturates early to protect meters. A protection CT stays accurate at many times rated current so relays measure the fault. Metering cores carry classes such as 0.5 or 0.5S with an instrument security factor such as FS 5 or FS 10. Protection cores carry classes such as 5P10 or 5P20, where 5P means 5% composite error and the number is the accuracy limit factor. On an 11 kV panel one CT usually carries both kinds of core, as on NIEL’s three-core incomer CT.

    What is CT burden and how is it calculated?

    CT burden is the total load on the CT secondary, expressed in VA at rated secondary current. It equals the secondary current squared times the loop resistance of the wiring plus the input impedance of every relay and meter on that core. ABB’s method uses copper resistance at 75 degrees C, such as 0.00865 ohm per metre for 2.5 sq mm cable, and a 1.2 multiplier on the one-way length for three-phase wiring. A numerical relay in the panel’s LV compartment with 5 m of 2.5 sq mm cable gives about 1.8 VA at 5 A.

    Is a higher VA burden rating safer for a CT?

    No. For a protection core, extra VA only adds cost, and ABB’s REF615 manual notes a 10 VA rating could have been selected much lower in its own example. For a metering core it causes harm. IS/IEC 60044-1 guarantees accuracy only between 25% and 100% of rated burden. A lightly loaded core also saturates later, so meters are exposed to more fault current than the FS rating implies. Specify rated burden from a burden calculation with a modest margin, not as a round number.

    What does 5P20 mean on a current transformer?

    5P20 is a protection class. The 5P means the composite error stays within 5% at the rated accuracy limit current, and 20 is the accuracy limit factor, so the CT holds that accuracy up to 20 times rated current with rated burden connected. Under IS/IEC 60044-1, a 5P core also keeps current error within 1% at rated current. The actual accuracy limit factor in service depends on the real burden. Using ABB’s formula and the internal burden from ABB’s worked example, a 5P20, 10 VA core loaded at under 2 VA reaches an effective factor above 60.

    Which standard applies to current transformers in India?

    IS 16227 (Part 2):2016, which is identical to IEC 61869-2:2012, is the current standard for inductive current transformers. IEC 61869-2 cancelled and replaced IEC 60044-1, the basis of the older IS/IEC 60044-1 and of specifications that still cite IS 2705. The BIS product manual for IS 16227 (Part 2) lists ratio, rated burden, accuracy class, instrument security factor and short-time thermal current among the parameters a CT is certified against. Specify IS 16227 (Part 2) on new MV panel orders.

Send us your single line diagram, fault level and relay schedule, and New India Electricals Ltd will specify the CT ratio, class and burden for each core of your 11 kV lineup. You can reach the team through our contact page.

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