A window current transformer (CT) is a non-invasive, toroidal sensor that clamps around an existing AC conductor to step down high primary current into a safely measurable secondary current or voltage via magnetic induction. In a real circuit or installation, a window CT changes a dangerous, high-amperage mains line into a galvanically isolated, low-voltage signal that you can safely feed into a microcontroller, PLC, or digital meter without ever breaking or splicing the primary wire. Think of a window CT like a clamp-on ultrasonic water flow meter: it reads the flow rate of the main pipe purely by sensing the outside, without requiring you to cut the pipe and install an inline turbine.

⚠️ Mains Voltage Hazard: Installing CTs inside a breaker panel exposes you to lethal 120V/240V AC. Always de-energize the panel, verify dead with a CAT III/IV multimeter, and wear appropriate PPE. If you are not comfortable working around exposed bus bars, hire a licensed electrician to install the CTs and route the low-voltage secondary leads to your enclosure.

Sizing and Specifying Window Current Transformers

Selecting the right window CT requires matching the primary current rating, the secondary output type (current vs. voltage), and the physical window aperture to your specific conductors. Below is a specification matrix of the most common models used in DIY energy monitoring and commercial sub-metering in 2026.

Table 1: Common Window Current Transformer Specifications
Model Primary Current Secondary Output Turns Ratio Burden Aperture Typical Price
YHDC SCT-013-000 0 - 100A 50mA (Current) 1800:1 External Req. 13mm $8 - $12
YHDC SCT-013-030 0 - 30A 1V AC (Voltage) 1800:1 62Ω Internal 13mm $10 - $14
Magnelab CT-0750-100 0 - 100A 0.333V AC 3000:1 Internal 19mm $25 - $35
CR Magnetics CR4110 0 - 100A 4-20mA / 0-5V Custom Internal 21mm $45 - $65

According to CR Magnetics design guidelines, the critical distinction is between current-output and voltage-output CTs. A current-output CT (like the SCT-013-000) acts as a current source and must have a burden resistor connected across its secondary terminals to convert the current into a readable voltage. Operating a current-output CT without a burden resistor (open-circuit) will cause the secondary voltage to spike to thousands of volts, instantly destroying the insulation and posing a severe shock hazard. Voltage-output CTs have this burden resistor built into the epoxy housing, making them safer for beginners but slightly less flexible for long wire runs.

Worked Numeric Example: Interfacing an SCT-013 with an ESP32

Let’s calculate the exact hardware values needed to read a YHDC SCT-013-000 (100A primary, 50mA secondary) using the 12-bit ADC on an ESP32 DevKit v1.

Target Parameters:
Primary Max ($I_p$): 100A RMS
Secondary Max ($I_s$): 50mA RMS
ESP32 ADC Max Safe Voltage: 2.5V Peak-to-Peak (biased at 1.25V)

Step 1: Calculate the Burden Resistor

We need to convert the 50mA secondary current into a voltage that fits within our ESP32 ADC range. To maximize resolution while avoiding clipping, we will target a peak voltage ($V_{peak}$) of 1.25V.

  • $V_{peak} = 1.25V$
  • $I_{s(peak)} = I_{s(rms)} \times \sqrt{2} = 0.050A \times 1.414 = 0.0707A$
  • $R_{burden} = V_{peak} / I_{s(peak)} = 1.25 / 0.0707 = 17.68\Omega$

The closest standard 1% resistor value is 18Ω. Using an 18Ω resistor, our maximum peak voltage will be $0.0707A \times 18\Omega = 1.27V$, which safely fits inside the ESP32's 3.3V absolute maximum when properly biased.

Step 2: Create the DC Bias Circuit

The ESP32 ADC cannot read negative voltages, but AC current alternates positive and negative. We must shift the signal up by half the reference voltage. Using the OpenEnergyMonitor CT interfacing standard, we build a voltage divider:

  1. Connect two 10kΩ resistors in series between the ESP32’s 3.3V pin and GND.
  2. The midpoint provides a stable 1.65V DC bias.
  3. Place a 10µF electrolytic capacitor across the 10kΩ resistors (positive to midpoint, negative to GND) to absorb noise and stabilize the bias voltage during ADC sampling.
  4. Connect one leg of the CT to the bias midpoint, and the other leg to the ESP32 ADC pin (e.g., GPIO 34) through a 1kΩ protection resistor.

Where You Meet Window CTs in Practice

Window current transformers are the backbone of modern non-invasive power monitoring. You will encounter them in three primary real-world scenarios:

  • Home Energy Monitors: Commercial systems like the Emporia Vue or Sense use arrays of 13mm to 19mm split-core window CTs clamped directly onto individual branch circuit THHN wires inside the main service panel to provide appliance-level disaggregation.
  • Solar Net Metering: Grid-tied inverters use revenue-grade window CTs (often solid-core for permanent installations) clamped to the main service feeder to monitor export/import power, ensuring the inverter curtails output if local grid limits are exceeded.
  • Tenant Sub-metering: In commercial multi-tenant buildings, facility managers clamp large-aperture (50mm+) window CTs around multi-conductor feeders to bill tenants for exact HVAC and lighting usage without installing expensive inline shunts.

Common Confusions: Window CT vs. Hall Effect vs. Rogowski

People commonly confuse window current transformers with Hall effect sensors (like the ubiquitous ACS712 module) and Rogowski coils. While all three measure current, their underlying physics, installation requirements, and accuracy profiles are vastly different.

Table 2: AC Current Sensor Technology Comparison
Feature Window CT (Ferromagnetic) Hall Effect (e.g., ACS712) Rogowski Coil (Air Core)
Installation Non-invasive (Clamp-on) Invasive (Must break circuit) Non-invasive (Flexible wrap)
DC Measurement No (AC only) Yes (AC and DC) No (AC / transient only)
Galvanic Isolation Excellent (Magnetic) Poor (Silicon die coupling) Excellent (Air gap)
Low Current Accuracy High (Down to ~100mA) Moderate (Noisy at low mA) Very Poor (Requires integrator)
Saturation Risk High (Core saturates at fault) Moderate (Depends on range) None (Linear to 10,000A+)

When to Choose Which

Choose a Window CT when: You are measuring AC mains (120V/240V/480V), require strict galvanic isolation for safety, and want to monitor existing wiring without cutting conductors. This is the default choice for 95% of home energy and solar monitoring projects.

Choose a Hall Effect sensor when: You are building a low-voltage DC system (like a 12V LiFePO4 battery bank or solar charge controller) where you need to measure both DC charge/discharge current and AC ripple, and you are designing a custom PCB rather than retrofitting an existing panel.

Choose a Rogowski coil when: You are measuring massive, unpredictable AC fault currents (1000A to 5000A+) in industrial busbars where a ferromagnetic CT core would instantly saturate and clip the waveform. Note that Rogowski coils output a voltage proportional to the derivative of the current ($di/dt$), requiring an active analog integrator circuit to reconstruct the actual current waveform.

Bench Tip: If your ESP32 or Arduino readings from a window CT are fluctuating wildly at low currents, check your burden resistor connections. A loose breadboard contact on the burden resistor of a current-output CT will briefly open-circuit the secondary, causing voltage spikes that can permanently damage your microcontroller's ADC pin. Always solder the burden resistor directly to the CT leads or use a screw terminal block.