A current transformer (CT) is a type of instrument transformer that steps down high primary AC current to a safe, measurable secondary current—typically 5A or 1A—while maintaining precise proportional phase and magnitude for metering and protection. In a real installation, what a CT changes is the physical boundary of the measurement circuit: it allows a standard 5A panel ammeter, a utility revenue meter, or a low-voltage microcontroller ADC to safely measure a 400A mains feeder without breaking the primary conductor or exposing the sensitive electronics to lethal fault currents and line voltage.
Standard Current Transformer Ratings and Specifications
Unlike standard power transformers that step down voltage for distribution, CTs are designed to operate into a near short-circuit condition (a very low impedance burden). Selecting the right CT requires matching the primary current rating, the secondary output, the accuracy class, and the maximum burden capacity. Below is a reference table of common CT configurations you will encounter in both commercial panels and DIY workbenches.
| Primary Rating | Secondary Rating | Ratio | Accuracy Class | Typical Burden Limit | Common Application |
|---|---|---|---|---|---|
| 100A | 5A | 20:1 | 0.5 (Metering) | 15 VA | Residential subpanel monitoring |
| 400A | 5A | 80:1 | 5P20 (Protection) | 30 VA | Main breaker trip relay input |
| 100A | 50mA | 2000:1 | N/A (Uncalibrated) | 18 Ω max | Split-core DIY (e.g., YHDC SCT-013-000) |
| 2000A | 1A | 2000:1 | 0.2S (Revenue) | 10 VA | Utility revenue metering at service entrance |
Accuracy classes dictate the use case. A 0.5 class CT guarantees the secondary current is within 0.5% of the true ratio at rated current, making it ideal for billing and energy monitoring. A 5P20 class is a protection CT; the '5' means 5% composite error, and the '20' means it maintains that accuracy up to 20 times the rated primary current during a fault, ensuring your protective relays trip correctly without the core saturating prematurely. For a deep dive into instrument transformer standards, the Electronics Tutorials guide on CTs provides an excellent breakdown of the equivalent circuits and phasor diagrams.
The Math: Working Through a 400A CT Ratio and Burden Calculation
To understand how a CT behaves in a circuit, let us work through a numeric example using a standard 400A:5A panel-mount CT with an 80:1 ratio.
Imagine the primary conductor (the thick cable passing through the CT window) is carrying exactly 320A of AC current. Because the ratio is 80:1, the secondary terminals will push exactly 4A (320 / 80 = 4). We connect this secondary output to a digital panel ammeter that has an internal shunt (burden) resistance of 0.2 ohms.
- Secondary Voltage Drop: Using Ohm's Law (V = I × R), the voltage developed across the meter is 4A × 0.2Ω = 0.8V.
- Power Dissipated (Burden): Using P = I²R, the power dissipated as heat in the meter shunt is (4²) × 0.2 = 16 × 0.2 = 3.2W.
This 3.2W dissipation is well within a standard 15 VA (Volt-Ampere) burden rating for this CT, meaning the core will not saturate, and the measurement will remain highly accurate.
If you disconnect the ammeter (leaving the secondary terminals open) while the primary conductor is still carrying 320A, the secondary current has nowhere to go. The CT will attempt to drive 4A across an infinite resistance. The magnetic core will instantly saturate, and the secondary voltage will spike to several kilovolts. This will arc across the terminal block, destroy the CT insulation, and poses a lethal electrocution hazard. Always short-circuit the secondary terminals (using a dedicated shorting switch or terminal block) before removing a connected meter.
Where You Meet CTs in Practice (and What People Confuse Them With)
You will find current transformers in almost every modern power monitoring scenario. In residential solar installations, inverters like those from Fronius or SolarEdge require a CT clamped around the main utility service conductor. This CT tells the inverter exactly how much power the house is consuming versus exporting to the grid, enabling 'zero-export' limiting. In the smart home space, whole-home energy monitors like the Sense or Emporia Vue rely on a pair of split-core CTs installed in your main panel to calculate real-time wattage.
For makers and hobbyists building DIY power monitors with an ESP32 or Arduino, the YHDC SCT-013-000 is the standard choice. This is a 100A:50mA split-core CT. Because it outputs a current, not a voltage, you must wire a burden resistor (typically 22Ω to 33Ω for a 3.3V microcontroller ADC) across the output to generate a measurable analog voltage. The Adafruit CT Sensor guide details the exact biasing network required to center this AC waveform on a DC offset for microcontroller ADCs.
Common Confusions: CTs vs. Hall Effect Sensors vs. PTs
A frequent mistake among DIYers is confusing a Current Transformer with a Hall Effect sensor (like the popular ACS712 breakout board). While both measure current, their operating physics and limitations are vastly different:
- Current Transformers (CTs): Operate strictly on AC. They rely on Faraday's law of induction, requiring a continuously changing magnetic field. They provide excellent galvanic isolation (often rated for 3kV+), meaning the high-voltage mains is physically separated from your low-voltage logic. They require no external power supply to operate.
- Hall Effect Sensors: Can measure both AC and DC current by sensing the static magnetic field generated by charge carriers. However, they require a VCC power supply, introduce insertion loss (the load current passes directly through the silicon chip), and offer minimal galvanic isolation compared to a wound transformer.
- Potential Transformers (PTs/VTs): People sometimes confuse CTs with PTs. A PT steps down voltage (e.g., 480V to 120V) for metering, while a CT steps down current. A PT secondary must never be short-circuited, whereas a CT secondary must never be open-circuited.
Frequently Asked Questions
Can a current transformer measure DC current?
No. Standard CTs rely on alternating magnetic flux to induce a secondary current. A steady DC current creates a static magnetic field, which will not induce any secondary current and will simply magnetize (and potentially saturate) the CT core. To measure DC, you must use a Hall Effect sensor, a fluxgate sensor, or a shunt resistor.
What happens if I wire a CT backwards?
CTs have strict polarity markings, usually denoted as P1/P2 on the primary and S1/S2 on the secondary, or a physical dot on the schematic. If you reverse the secondary leads, the output waveform will be shifted by exactly 180 degrees. While an ammeter will still read the correct RMS current magnitude, a wattmeter or energy monitor calculating Real Power (P = V × I × cos θ) will see the phase shift and calculate negative watts, assuming power is flowing in the opposite direction.
Why do some DIY CTs output 1V instead of 5A?
Manufacturers like YHDC produce 'voltage-output' split-core CTs (e.g., the SCT-013-030). Internally, these are still current transformers, but the manufacturer has permanently installed a high-precision burden resistor inside the plastic casing. This converts the secondary current to a safe 0-1V AC output, making them plug-and-play for microcontrollers without requiring the user to solder surface-mount burden resistors or risk open-circuiting the raw coil.
