A current transformer (CT) is a type of instrument transformer that steps down high AC primary current to a safe, measurable secondary current—typically 5A, 1A, or 50mA—while providing galvanic isolation from the mains. In a real circuit, a CT changes a dangerous, unmeasurable 200A bus bar into a safe, low-current signal wire that standard panel meters, protection relays, or microcontrollers can read without direct electrical contact with the high-voltage source.
Builders and students commonly confuse CTs with shunt resistors or Hall-effect sensors. Unlike a shunt resistor, which introduces a voltage drop and lacks galvanic isolation, a CT passes the primary conductor straight through its core with virtually zero insertion loss. Unlike Hall-effect sensors (like the ACS712), which can measure DC but suffer from temperature drift and noise, CTs rely strictly on AC magnetic induction, offering high precision and excellent noise immunity for alternating current.
The Core Function: Scaling and Isolating AC Current
A CT operates on the exact same magnetic principles as a standard voltage transformer, but with a critical structural difference: the primary "winding" is often just a single straight conductor passing through the center of the toroidal core. The secondary winding consists of hundreds or thousands of turns of fine magnet wire wrapped around that same core.
Because the secondary current is strictly dictated by the primary current and the turns ratio, a CT acts as a current source, not a voltage source. This means the voltage across the secondary terminals is entirely determined by the impedance of the load connected to it—known in the industry as the burden.
Standard CT Specifications and Accuracy Classes
When selecting a CT for industrial or commercial panel work, you must match the accuracy class and burden rating to your measuring device. A metering CT is designed to be highly accurate at normal load currents but will intentionally saturate during a short circuit to protect the meter. A protection CT does the opposite: it sacrifices normal-load precision to remain accurate during massive fault currents.
| Accuracy Class | Typical Burden (VA) | Primary Application | Performance Characteristic |
|---|---|---|---|
| 0.2 | 5 VA - 15 VA | Revenue / Utility Metering | ±0.2% error at rated current; saturates early to protect meters. |
| 0.5 | 10 VA - 30 VA | Industrial Panel Meters | ±0.5% error at rated current; standard for facility monitoring. |
| 5P10 | 15 VA - 30 VA | Protection Relays | ±1% error up to 10x overcurrent; designed to measure fault currents accurately. |
| 10P20 | 15 VA - 30 VA | Fault Detection / Tripping | ±3% error up to 20x overcurrent; used for heavy industrial breaker tripping. |
For hobbyist and home energy monitoring, you will rarely see these industrial classes. Instead, you will encounter split-core ferrite CTs like the YHDC SCT-013-000, which output a raw current (e.g., 50mA) or a built-in voltage output (e.g., 1V) via an internal burden resistor.
Worked Example: Sizing a Burden Resistor for an ESP32 ADC
Let’s bridge theory and bench work. You want to monitor a 30A, 240V water heater circuit using an SCT-013-000 (100A primary to 50mA secondary ratio) and an ESP32 DevKit v1. The ESP32’s ADC reads 0V to 3.3V. Because AC current alternates, you must bias the CT signal to the midpoint of the ADC range (1.65V) using a voltage divider, allowing the signal to swing ±1.65V.
Step 1: Determine maximum secondary current.
We want to measure up to 30A RMS. The SCT-013-000 ratio is 100A : 50mA.
I_sec_rms = 30A * (50mA / 100A) = 15mA RMS
Step 2: Convert RMS to Peak current.
I_sec_peak = 15mA * √2 (1.414) = 21.21mA (0.02121A)
Step 3: Calculate the ideal burden resistor.
We want the peak voltage to hit exactly our 1.65V swing limit.
R_burden = V_peak / I_sec_peak = 1.65V / 0.02121A = 77.79 Ω
Step 4: Select a standard E12 resistor and verify.
The nearest standard values are 75Ω and 82Ω. If you choose 82Ω, the peak voltage at 30A will be 0.02121A * 82Ω = 1.739V. Added to the 1.65V bias, that equals 3.389V—which exceeds the ESP32’s 3.3V maximum and will clip your waveform, causing measurement errors. Therefore, you must select the 75Ω resistor. At 75Ω, the peak swing is 1.59V, yielding a maximum ADC reading of 3.24V. This safely accommodates the full 30A load without clipping.
Where You Meet CTs in Practice
You interact with CT instrumentation constantly, even if the devices hide the raw components inside proprietary enclosures:
- Home Energy Monitors: Devices like the Emporia Vue or Sense use arrays of small split-core CTs clamped onto individual branch circuit wires in your main panel to calculate real-time power consumption.
- Solar Inverters: Grid-tied inverters (like SolarEdge or Fronius) use revenue-grade CTs installed on the main utility feed to enforce export limits, ensuring the system doesn't push more power back to the grid than the utility allows.
- Industrial Motor Starters: VFDs and soft starters use protection-class CTs to monitor for phase imbalance or ground faults, tripping the contactor within milliseconds if a motor begins to draw locked-rotor current.
- Digital Multimeters: High-end clamp meters (like the Fluke 376 FC) use a specialized, highly linear CT built into the jaw mechanism to measure AC current without breaking the circuit.
The Lethal Open-Circuit Hazard and Installation Rules
In a standard voltage transformer, removing the secondary load simply stops current flow. In a CT, the primary current is forced by the external mains circuit, not by the transformer itself. If the secondary circuit is opened (e.g., a disconnected wire or a blown fuse on the secondary side), the counter-magnetomotive force collapses. The core instantly saturates, and the rapid, uncontrolled collapse and reversal of the magnetic flux induces massive voltage spikes across the open secondary terminals.
These spikes can easily exceed 2,000V to 5,000V, which will instantly destroy connected microcontrollers, shatter terminal blocks, and deliver a lethal shock to anyone working on the panel. According to instrument transformer safety guidelines, if a CT must be disconnected from its meter while the system is live, the secondary terminals must first be physically shorted together using a dedicated shorting block.
Frequently Asked Questions
Can I use a current transformer to measure DC current?
No. CTs rely on Faraday’s Law of Induction, which requires a changing magnetic field to induce a secondary current. DC current creates a static magnetic field, resulting in zero secondary output. To measure DC, you must use a Hall-effect sensor or a shunt resistor.
Does the direction the CT is clamped matter?
Yes. CTs have polarity markings (usually an arrow or a 'P1/P2' designation). If you clamp the CT backward, the secondary current will be 180 degrees out of phase with the primary. In a simple ammeter, this doesn't matter. But in a wattmeter or energy monitor calculating real power (Watts = Volts × Amps × cosθ), a reversed CT will result in negative power readings, making the system think you are generating power instead of consuming it. For deeper integration theory, the OpenEnergyMonitor project provides excellent documentation on phase alignment and calibration.
What happens if my burden resistor wattage rating is too low?
If you pass 100A through a 100:5A CT, the secondary outputs 5A. If your burden resistor is 1Ω, it will dissipate I²R = 25 Watts. A standard 1/4W through-hole resistor will instantly catch fire. Always calculate the maximum expected fault current, not just the nominal operating current, when sizing burden resistor wattage. For more on sizing and thermal limits, refer to the Electrical Engineering Portal's guide on CT burden and accuracy.
