A current transformer (CT) is a type of instrument transformer designed to produce an alternating current in its secondary winding that is accurately proportional to the much larger current flowing through its primary winding. In a real electrical installation, a CT changes a potentially lethal, high-amperage primary current (like 400A on a main service feeder) into a safe, low-amperage secondary current (typically 5A, 1A, or milliamp-level) that standard multimeters, microcontrollers, and protection relays can safely measure, all while providing vital galvanic isolation. Beginners frequently confuse CTs with potential transformers (PTs, which step down voltage), shunt resistors (which measure current via voltage drop but lack isolation and dissipate heat), or Hall-effect sensors (which can measure DC, whereas standard magnetic CTs only measure AC).
Standard CT Current Transformer Ratios and Specifications
When selecting a CT for metering or protection, the ratio and burden rating dictate its real-world performance. The ratio defines the step-down factor, while the burden (measured in Volt-Amperes or Ohms) defines the maximum load the secondary can drive without losing accuracy or saturating the core. Below is a reference table of common CT specifications you will encounter in both industrial panels and DIY energy monitoring projects.
| CT Type / Common Model | Primary Rating | Secondary Rating | Turns Ratio | Typical Burden | Primary Application |
|---|---|---|---|---|---|
| Industrial Solid-Core (e.g., ABB 400/5) | 400 A | 5 A | 80:1 | 10 VA (0.4 Ω) | Utility metering, switchgear protection relays |
| Industrial Solid-Core (e.g., Schneider 800/1) | 800 A | 1 A | 800:1 | 5 VA (5.0 Ω) | Long-distance relay wiring (lower I²R loss) |
| Split-Core DIY (YHDC SCT-013-000) | 100 A | 50 mA | 2000:1 | External (e.g., 33 Ω) | ESP32/Arduino home energy monitors, Emporia Vue |
| Voltage-Output DIY (YHDC SCT-013-030) | 30 A | 1 V RMS | N/A (Internal) | Internal 62 Ω burden | Direct plug into ADC without external resistors |
| Rogowski Coil (Flexible) | 2000 A | 100 mV/A | Air-core | Integrator circuit | Retrofit busbars, high-current VFD harmonics |
The Math: Interfacing a CT with an ESP32 ADC
Let’s walk through a worked numeric example using the ubiquitous YHDC SCT-013-000, a 100A:50mA split-core CT heavily used in maker projects to build custom energy dashboards. Microcontrollers like the ESP32 cannot measure alternating current directly; their Analog-to-Digital Converters (ADCs) only read DC voltages between 0V and 3.3V. We must convert the CT's secondary current into a voltage, then bias it to sit in the middle of the ADC's range.
The Scenario: You are monitoring a 240V EV charger circuit drawing 40A RMS on the primary side.
- Calculate Secondary Current: The CT ratio is 100A primary to 50mA (0.05A) secondary.
I_secondary = I_primary × (50mA / 100A) = 40A × 0.0005 = 0.02A (20mA RMS). - Select a Burden Resistor: To convert this 20mA current into a readable voltage, we pass it through a burden resistor. For a 3.3V ESP32 ADC, we want our peak-to-peak voltage to stay under 3.3V. A 33 Ω resistor is standard.
V_rms = I_secondary × R_burden = 0.02A × 33Ω = 0.66V RMS. - Calculate Peak Voltage: The ADC reads instantaneous peaks, not RMS.
V_peak = V_rms × √2 = 0.66V × 1.414 = 0.93V. - Apply DC Bias: Because the AC waveform swings positive and negative, a negative voltage will fry the ESP32's ADC pin or read as zero. We use a voltage divider (two 10kΩ resistors across 3.3V) to create a 1.65V DC bias. The AC signal now oscillates between 0.72V (1.65 - 0.93) and 2.58V (1.65 + 0.93), perfectly centered for the 3.3V ADC.
Where You Meet CTs in Practice (and the Open-Circuit Hazard)
In the field, you will encounter CTs in three primary locations: grid-tied solar inverters (where they monitor export/import to the utility), home energy monitors (like Sense or Emporia Vue, which use arrays of 15+ split-core CTs in your panel), and industrial motor control centers (feeding overload relays and power quality analyzers).
While CTs are incredibly robust, they harbor one catastrophic failure mode that every electrician and engineer must memorize: never open-circuit a CT secondary while the primary is energized.
For a deeper dive into the magnetic saturation curves and instrumentation standards, refer to the ABB Instrument Transformer guides or the foundational texts on practical transformer designs at All About Circuits.
CT vs. Shunt vs. Hall Effect: Choosing the Right Sensor
Not every current measurement requires a magnetic transformer. When designing a custom PCB or specifying a panel meter, you must weigh isolation, power loss, and AC/DC requirements. Here is how the big three current sensing technologies compare.
| Criteria | Current Transformer (CT) | Shunt Resistor | Hall-Effect Sensor (e.g., ACS712) |
|---|---|---|---|
| AC / DC Capability | AC Only (relies on changing flux) | AC and DC | AC and DC |
| Galvanic Isolation | Excellent (magnetic coupling) | None (direct electrical contact) | Good (magnetic field sensing) |
| Power Dissipation (Loss) | Negligible (milliwatts in burden) | High (I²R heating, requires wattage rating) | Very Low (mA draw for IC logic) |
| Cost & Complexity | Moderate (bulky, needs burden/bias) | Low (cheap, but needs isolation amps for high V) | Low (single IC, but susceptible to EMI) |
| Best Use Case | Mains AC metering, high-voltage safety | Low-voltage DC battery BMS, precision lab gear | Motor control, DC solar strings, mixed AC/DC |
Frequently Asked Questions
Does the physical direction of the CT matter?
Yes. CTs follow the "dot convention" for polarity. The primary wire must flow from P1 to P2, and the secondary wires must be wired S1 to S2. If you clamp a split-core CT backward on a circuit, the current waveform will be 180 degrees out of phase with the voltage. While a simple ammeter won't care, a power analyzer calculating Real Power (Watts) and Power Factor will read negative values or fail entirely.
Why do some industrial CTs output 1A instead of 5A?
It comes down to wire distance and I²R losses. In a large substation, the protection relays might be hundreds of feet away from the switchgear CTs. Pushing 5A over long, thin control wires generates significant heat and voltage drop. By using an 800:1 CT (outputting 1A instead of an 800:5 CT), the line losses are reduced by a factor of 25, allowing for smaller gauge control wiring.
Can I use a standard CT to measure the DC output of a solar panel array?
No. Standard CTs rely on a constantly changing magnetic field (AC) to induce a voltage in the secondary winding. A steady DC current creates a static magnetic field, resulting in zero induced secondary current. For DC solar strings, you must use a shunt resistor with an isolated amplifier, or a closed-loop Hall-effect sensor.
