How to Choose a Current Transformer: Ratio, Class and Burden
Current transformers (CTs) let meters, relays and instruments measure high currents safely by stepping them down to a standard, isolated 1 A or 5 A secondary. Choosing the wrong CT — wrong ratio, wrong class, wrong size — produces readings that are subtly wrong for years. This guide covers the five decisions that matter.
How a Current Transformer Works
A CT is a transformer with a primary winding (or a busbar passing through the window) and a secondary winding. The primary current induces a proportional secondary current; with a 100/5 CT, 100 A in the primary produces exactly 5 A in the secondary. Meters and relays are designed around these standard secondaries, so the CT is what makes "100 A" readable by a panel meter rated for 5 A.
Decision 1: Ratio (Primary Rating)
The primary rating should be close to the full load current, so that normal operation sits between 60–100% of the CT primary. If the primary is oversized, accuracy at normal load suffers; if undersized, the CT saturates and distorts readings.
- 50/5, 100/5, 200/5, 300/5, 400/5, 600/5 and 1000/5 cover most commercial and industrial feeders.
- For motor circuits, choose a primary around 120–150% of motor full-load current to survive starting current.
- For generators and transformers, match the CT primary to the equipment nameplate current.
Decision 2: Accuracy Class
Accuracy class defines the maximum error. The most common classes are:
| Class | Error | Typical use |
|---|---|---|
| 0.5S | ±0.5% | Billing and energy audits |
| 1.0 | ±1.0% | General industrial metering |
| 3.0 | ±3.0% | Panel indication only |
| 5P10 / 10P10 | Protection | Relay and protection circuits |
For revenue or tenant billing, use Class 0.5S (or Class 1.0 minimum). For protection, use the appropriate 5P/10P class so the CT stays accurate well beyond rated current during faults.
Decision 3: Burden (VA)
Burden is the load the CT secondary must drive — the meter, wiring and any relay. The CT's rated burden (in VA) must be higher than the connected burden. A 100/5 CT rated 5 VA driving a meter needing 0.5 VA plus 1 VA of wiring is fine; one driving a long cable run plus several relays may need 10 or 15 VA.
Keep secondary wiring short, use 2.5 mm² conductors, and never leave a CT secondary open-circuit while primary current flows — the induced voltage can be lethal and destroys the CT.
Decision 4: Split-Core vs Solid-Core
- Solid-core CTs — highest accuracy and stability; the primary cable must be disconnected to install. Best for new installations and metering points that rarely change.
- Split-core CTs — clamp around an existing cable without disconnection. Ideal for retrofits, energy audits and temporary monitoring, with slightly lower accuracy.
For permanent billing installations, use solid-core. For adding monitoring to existing switchboards, split-core saves hours of downtime.
Decision 5: Physical Form and Window Size
Measure the primary conductor or busbar first. A CT window must clear the cable or bar with a little room. Common window sizes range from 10–40 mm for cable CTs to large rectangular windows for busbar CTs in switchgear. Also confirm mounting — DIN-rail, busbar direct-mount, or through-panel — and that terminals accept the secondary wire gauge.
Wiring the Secondary Correctly
- Terminals are marked S1 and S2 (or K and L). Connect S1 toward the source; polarity matters for directional meters.
- Use twisted pair or screened cable for secondary runs over a few metres.
- Short and earth the secondary before energizing — never work on a live CT.
FAQ
Can one CT feed a meter and a relay?
Yes, if the combined burden stays within the CT rating. Series-connected devices add their burdens; verify the sum against the CT's VA rating.
Why is my reading 3% high?
Check the CT ratio matches the meter's programmed ratio, secondary wiring is not swapped, and the CT is not saturated by over-current. If the CT primary is oversized, readings can also be inaccurate at light load.
Do I need a CT for a direct-connected meter?
No — meters rated up to 100 A connect directly. CTs are needed above 100 A, for busbar systems, or where metering must be isolated from the live conductor.
Conclusion
Pick the ratio from full-load current, the class from the application (0.5S for billing), the burden from the wiring, the form from the installation, and the window from the conductor. Get those five right and the metering will be accurate for the life of the panel. Yomin Electric supplies Class 0.5S solid-core and split-core CTs from 50 A to 1000 A. Browse the CT range or read our CT basics guide, then ask our engineers to confirm your selection.
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