How to Choose the Right CT Clamp Size for a Smart Energy Meter
What is a CT clamp and why does the rating matter?
A current transformer (CT) clamp measures the current in a conductor without cutting it. The CT rating is the largest current it is designed to measure. A CT that is too small can saturate or be overloaded, while a CT that is much larger than the load gives less accurate readings at low current.
How do I choose a CT rating?
- Find the maximum expected current of the circuit, for example from the breaker rating or load calculations.
- Choose a CT rating that covers that current with headroom.
- Check the accuracy bands in the datasheet. For example, the OWON 80 A CT is rated ±1% from 1 A to 80 A, while the 300 A, 500 A and 750 A CTs are rated ±1% from 6 A.
- Check that the cable or busbar fits through the clamp's inner diameter.
- Confirm which CT sizes are available for your meter model, because defaults and options differ by model.
Which CT options does OWON list?
| CT rating | Clamp inner diameter | Current accuracy (OWON datasheets) |
|---|---|---|
| 20 A | Ø5 mm | ±1% (1–20 A), ±2% (0–1 A) |
| 50 A | Ø10 mm | ±1% (1–50 A), ±2% (0–1 A) |
| 80 A | Ø10 mm | ±1% (1–80 A), ±2% (0–1 A) |
| 120 A | Ø16 mm | ±1% (2–120 A), ±2% (0–2 A) |
| 200 A | Ø20 mm | ±1% (2–200 A), ±2% (0–2 A) |
| 300 A | Ø24 mm | ±1% (6–300 A), ±2% (2–6 A), ±5% (0–2 A) |
| 500 A | Ø36 mm | ±1% (6–500 A), ±2% (2–6 A), ±5% (0–2 A) |
| 750 A | Ø51 mm | ±1% (6–750 A), ±2% (2–6 A), ±5% (0–2 A) |
Availability by model: PC4713 offers 80 to 750 A (120 A default); PC321 and PC473 offer 80 to 750 A (80 A default); PC311 and PC472 offer 20 to 300 A (80 A default); PC341 uses 200 A main CTs (300 A / 500 A optional) and 50 A sub CTs (80 A / 120 A optional).
How should CTs be installed?
- Turn off the circuit breakers and confirm there is no voltage before working in the distribution box.
- Place each CT around the cable of its phase with the arrow pointing towards the loads, as described in the OWON quick start guides.
- Make sure the meter's power supply connection and the CT that measures the same circuit are on the same phase.
- Have a qualified electrician install the meter and CTs.
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Frequently asked questions
What happens if the CT rating is much larger than the load?
The accuracy bands in the OWON datasheets widen at low current: for example the 750 A CT is rated ±1% from 6 A, ±2% from 2 to 6 A and ±5% below 2 A, while the 80 A CT is rated ±1% from 1 A.
Does the direction of the CT matter?
Yes. The OWON quick start guides tell you to place the CT with the arrow pointing towards the loads.
Can I change the CT rating after installation?
The PC4713 datasheet lists a CT setting function, which lets the meter be configured for the CT that is fitted. Check the model's documentation for the exact procedure.
Related guides
- How to Choose a Smart Energy Meter: A Buyer's ChecklistA practical checklist for choosing a smart energy meter: supply type, CT rating, accuracy, communication, integration and certifications...
- Single-Phase vs Three-Phase Smart Energy MetersLearn the difference between single-phase, split-phase and three-phase energy monitoring, and which smart meter type fits a home, building...
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