1. How to Size a Current Transformer
CT sizing is the most critical step in selecting a current transformer. The primary current rating must be equal to or slightly higher than the maximum load current of your circuit. A common rule of thumb:
CT Primary Rating ≥ 1.25 × Maximum Load Current
(Select the next standard CT ratio above your calculated value)For a load drawing 75A maximum, you would select a 100/5A CT (not 75/5A, which is non-standard). Standard CT primary ratings: 50A, 75A, 100A, 150A, 200A, 250A, 300A, 400A, 500A, 600A, 800A, 1000A, 1500A, 2000A, 3000A, 5000A.
Key rule: A CT should operate between 20% and 100% of its rated primary current for best accuracy. Oversizing (e.g., 500/5A for a 50A load) degrades accuracy at low currents.
2. CT Ratio Selection: 5A vs 1A Secondary
The CT ratio defines the relationship between primary and secondary current. For example, a 200/5A CT outputs 5A when 200A flows through the primary. The two standard secondary ratings are:
- 5A secondary — Standard for most metering applications. Shorter cable runs (<10m). Higher burden capability.
- 1A secondary — Preferred for long cable runs (>10m) between CT and meter. Lower line losses. Common in utility substations.
3. Accuracy Class Guide: Which Class Do You Need?
CT accuracy class determines measurement precision. The class number represents the maximum percentage error at rated current:
| Accuracy Class | Max Error at 100% In | Phase Displacement | Typical Application |
|---|---|---|---|
| Class 0.2S | ±0.2% | ±10 minutes | Revenue metering, utility billing, critical energy audit |
| Class 0.5S | ±0.5% | ±30 minutes | High-accuracy industrial metering, sub-billing |
| Class 0.5 | ±0.5% | ±30 minutes | Standard commercial metering |
| Class 1.0 | ±1.0% | ±60 minutes | General monitoring, non-billing applications |
| Class 3.0 | ±3.0% | — | Protection relays (not for metering) |
"S" designation (0.2S, 0.5S): Means the CT maintains stated accuracy down to 20% of rated current. Standard CTs without "S" only guarantee accuracy from 100% to 120% of rated current. Always specify "S" class for metering applications with variable loads.
4. Split Core CT vs Solid Core CT: Which to Choose?
The choice between split core CT and solid core CT depends on your installation scenario:
| Feature | Split Core CT | Solid Core CT |
|---|---|---|
| Installation | Clamp-on — no cable disconnection needed | Requires cable feed-through during wiring |
| Best accuracy | Class 0.5 — Class 1.0 | Class 0.2S — Class 0.5 |
| Cost | $2.50–$6/unit | $1.50–$4/unit |
| Use case | Retrofit, energy audits, existing panels | New installations, panel building |
| Output options | 5A, 1A, 333mV | 5A, 1A |
| Size range | 50A — 5000A | 5A — 3000A |
Need current transformers for your project? 148 models in stock — split core, solid core, busbar type. Class 0.5 accuracy. From $1.50/unit at 100+ quantities. Free samples available.
Browse CT Products →5. CT Burden: Why It Matters
CT burden is the maximum impedance (in VA or ohms) the CT can drive while maintaining stated accuracy. The connected load — meter input impedance + cable resistance — must not exceed the CT's rated burden.
Total Burden = Meter Burden + Cable Burden < CT Rated Burden
Standard CT burdens: 2.5VA, 5VA, 10VA, 15VA, 30VA. For typical energy meter connections (meter burden 0.3-1VA + 5-10m cable), a 5VA CT is sufficient. Increase to 10VA or 15VA for longer cable runs or multiple meters.
6. CT Types by Construction
- Molded Case CT (MCT) — Encapsulated in epoxy or plastic, ring-type window. Most common for LV metering. Easy DIN rail or busbar mounting.
- Rectangular Window CT — Larger window to accommodate busbars or multiple cables. Common in switchgear and industrial panels.
- Busbar CT — Designed to mount directly onto rectangular busbars. Compact, minimal footprint.
- Wound Primary CT — Has an actual primary winding (not just a window). Used for very low primary currents (1A to 50A).
- High Voltage CT (10kV-35kV) — Oil or resin-cast insulated for medium voltage switchgear and outdoor substations.
7. Quick Selection Table
| Application | CT Type | Accuracy | Ratio | Approx. Price |
|---|---|---|---|---|
| Residential sub-metering | Split Core | Class 1.0 | 100/5A | $2.50 |
| Commercial building | Molded Case | Class 0.5 | 200/5A | $3.00 |
| Industrial panel metering | Rectangular Window | Class 0.5S | 400/5A | $4.50 |
| Utility revenue metering | Molded Case | Class 0.2S | 600/5A | $6.00 |
| Retrofit energy audit | Split Core | Class 0.5 | 500/5A | $4.00 |
| Solar PV monitoring | Split Core | Class 0.5 | 250/5A | $3.50 |
Frequently Asked Questions
What happens if I select the wrong CT ratio?
If the CT primary rating is too low, the CT core saturates during high current — causing severe measurement errors and potential overheating. If it is too high, the secondary current at normal loads is very small, leading to poor accuracy. Always aim for the load current to be 50-80% of the CT rated primary.
What is the difference between metering CT and protection CT?
Metering CTs saturate at lower overcurrent to protect connected meters from damage during faults. They are optimized for accuracy (Class 0.2S–1.0). Protection CTs must remain linear at very high fault currents (5P, 10P, 20P class) and are not suitable for accurate energy measurement. Never use a protection CT for metering.
Can I connect multiple meters to one CT?
Yes, but the total burden of all connected meters plus cabling must not exceed the CT's rated burden. For example, a 5VA CT can support two meters each with 0.5VA burden plus cable losses up to 4VA. Never connect meters in parallel with the CT secondary — always in series.
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