CT Accuracy Class Explained: 0.2, 0.5, 1.0 and What They Mean for Billing

Direct answer: a CT's accuracy class is the maximum permitted ratio error, expressed as a percentage, under defined conditions of burden and current. Class 0.5 means the ratio error stays within ±0.5% at rated current. Metering CTs use Class 0.2, 0.5 or 1.0; protection CTs use a different system entirely (5P, 10P).

This is a small line on a datasheet that decides whether a metering point passes utility acceptance. Class 0.5 and Class 1.0 units look identical on a purchase order and often sit in the same housing. They are not interchangeable when someone is billing against the measurement.

Metering Classes: What Each One Is For

What the "S" Actually Means

This is the detail that catches people out. Class 0.5 guarantees its accuracy from 5% of rated current upward. Class 0.5S extends that guarantee down to 1% of rated current.

Why it matters: a building with a 400 A service that idles overnight at 8 A is running at 2% of rating. A standard Class 0.5 CT is outside its guaranteed accuracy band there. A 0.5S is not. If your load profile has deep troughs — and most commercial buildings do — the S variant is what keeps the overnight hours billed correctly.

Burden: The Specification Everyone Forgets

A CT's accuracy class is only valid within its rated burden range, measured in VA. Burden is the total impedance the secondary has to drive: the meter, plus the secondary wiring, plus every terminal in between.

Two ways this goes wrong in the field:

• Over-burdened: a long secondary cable run adds resistance. A Class 0.5 CT rated 5 VA driving a 7 VA load is no longer a Class 0.5 CT — the error rises outside its stated band even though nothing is faulty.
• Under-burdened: less common but real. Some CTs are calibrated for a minimum burden and drift when run far below it.

Rule of thumb: keep secondary leads short, size the conductor properly, and add up the actual VA before you specify. On long runs, a 1 A secondary CT loses far less to cable resistance than a 5 A unit.

Metering vs Protection Classes

These are different systems solving opposite problems and should never be substituted for each other.

A metering CT deliberately saturates under fault conditions — that is a feature, shielding the meter from destructive current. Put one in a protection circuit and the relay may not see the fault correctly. Put a protection CT on a revenue meter and your billing accuracy is poor at normal load.

How to Specify Correctly

1. Get the requirement from the utility, not from the panel drawing — utilities set the class for revenue points.
2. Check the load profile. Deep overnight troughs mean you want an S-class.
3. Calculate the total burden: meter VA + cable VA + terminations.
4. Confirm the ratio against actual peak demand, not the breaker size.
5. Ask for the test certificate at time of order — utilities usually require it for acceptance.

FAQ

Is Class 0.5 good enough for revenue metering?

In most markets yes, and it is the common standard for commercial and industrial billing. High-value interchange metering between utilities often requires Class 0.2.

Can I improve a CT's accuracy after installation?

You cannot change its class, but you can stop it from performing below class: shorten the secondary run, clean the mating faces on split-core units, verify the burden and centre the conductor.

Does accuracy class change with temperature?

Class is specified over a defined operating range. Outside that range performance is not guaranteed, which matters for outdoor and high-ambient installations.

Do you supply test certificates?

Yes. Every unit is tested before dispatch and we supply documentation for utility submission on request.

Talk to the Factory

Yomin Electric manufactures metering CTs in Class 0.5 and Class 1.0 across the 5–4000 A range at 50/60 Hz, including the NLH split-core, NSQ toroidal and MES series. See the range or send your utility's specification and we will confirm the correct class, ratio and burden.