A current transformer (CT) is a passive electromagnetic device that steps down high alternating current (AC) to a safe, proportional, and measurable low-current signal without interrupting the primary circuit. Instead of forcing you to cut a 2/0 AWG feeder and insert a meter in series, a CT simply clamps around the existing wire insulation. In a real installation, it changes a dangerous, unmeasurable high-current line into a safe, galvanically isolated 1A, 5A, or milliamp signal that standard panel meters, protective relays, and microcontrollers can process without risking lethal shock or equipment destruction.
The Core Function: Stepping Down Current Safely
A CT operates on the same magnetic induction principles as a standard voltage transformer, but it is designed to act as a constant current source rather than a constant voltage source. The primary 'winding' is often just a single pass of the load-carrying conductor through the center of the CT's toroidal core. The secondary winding consists of hundreds or thousands of turns of fine magnet wire wrapped around that core.
The governing equation is the ampere-turn balance:
(Primary Current × Primary Turns = Secondary Current × Secondary Turns)
Because the primary is usually 1 turn (Np = 1), a CT with 2,000 secondary turns will step down 100A of primary current to exactly 50mA of secondary current. This provides two massive benefits: it scales the current down to a range that cheap, low-power electronics can read, and it provides galvanic isolation, keeping 120V/240V/480V mains voltage completely separated from your measurement circuit.
Worked Numeric Example: Sizing an SCT-013-000 for an ESP32
Let's bridge power theory and embedded systems. You want to monitor a 100A HVAC compressor circuit using an ESP32 DevKit v1 and a popular YHDC SCT-013-000 split-core CT (typically $12–$15). The SCT-013-000 outputs a current, not a voltage, so we must add a 'burden resistor' to convert the secondary current into a voltage the ESP32's 12-bit ADC (0–3.3V) can read.
1. Identify the Specs:
- Primary Current (Ip): 100A RMS
- Secondary Current (Is): 50mA RMS
- Turns Ratio: 100 / 0.05 = 2,000 turns
2. Calculate Peak Current:
The ESP32 ADC reads instantaneous voltage, so we must calculate the peak AC values, not RMS.
Peak Primary = 100A × √2 = 141.4A
Peak Secondary = 141.4A / 2,000 = 70.7mA (0.0707A)
3. Size the Burden Resistor:
The ESP32 ADC range is 0 to 3.3V. To measure AC, we bias the signal to the midpoint (1.65V) using a voltage divider, meaning our maximum allowable peak voltage swing is 1.65V.
Using Ohm's Law (R = V / I):
R = 1.65V / 0.0707A = 23.33Ω
We select the nearest standard 1% resistor value: 22Ω. This yields a peak voltage of 1.55V, safely keeping the signal within the 0.1V to 3.2V linear range of the ESP32's ADC.
4. Verify Resistor Power Rating:
P = I² × R = (0.05A RMS)² × 22Ω = 0.055W. A standard 1/4W (0.25W) through-hole resistor is more than adequate. For a complete guide on wiring the DC bias network and calculating phase calibration, refer to the OpenEnergyMonitor CT sensor documentation.
Where You Meet Current Transformers in Practice
You will encounter CTs across almost every tier of electrical infrastructure:
- Residential Energy Monitors: Devices like the Emporia Vue or Sense use arrays of small split-core CTs clamped to individual branch circuits to provide real-time appliance-level power tracking.
- Solar Export Limiting: Grid-tied inverters (like Fronius or SolarEdge) require a CT clamped to the main service lateral. If the CT detects power flowing backward to the grid, the inverter throttles its output to comply with utility zero-export rules.
- Utility Revenue Metering: The utility meter on your house uses precision, solid-core CTs with an accuracy class of 0.2s, meaning they are guaranteed to be within 0.2% of the true current value for billing purposes.
- Motor Protection Relays: Industrial 3-phase motors use CTs wired to overload relays. If a mechanical jam causes current to spike, the relay detects the secondary current increase and trips the main contactor before the motor windings melt.
What People Commonly Confuse With CTs
When designing measurement circuits, engineers and hobbyists frequently confuse CTs with two other sensing methods:
1. Shunt Resistors:
A shunt is a precise, low-value resistor placed in series with the load. You measure the voltage drop across it (e.g., 50mV at 100A). While excellent for DC battery monitoring, shunts are terrible for high-voltage AC mains. They dissipate heat (I²R losses), require breaking the circuit to install, and offer no galvanic isolation, meaning your microcontroller is directly tied to 120V/240V mains potential.
2. Hall Effect Sensors (e.g., ACS712):
Hall sensors measure the magnetic field generated by current. They can measure both AC and DC and provide isolation. However, they suffer from temperature drift, lower resolution at high currents, and susceptibility to external magnetic interference. For pure AC mains metering above 30A, a CT is vastly superior in accuracy and noise immunity.
3. Potential Transformers (PTs / VTs):
PTs step down voltage (e.g., 4160V to 120V) for metering. The safety rules are inverted: PT secondaries must be fused to prevent short-circuit fires, whereas CT secondaries must never be fused, because a blown fuse creates an open circuit, leading to the lethal voltage spikes mentioned earlier. For deeper physics on transformer distinctions, see Electronics Tutorials on Current Transformers.
Decision Tree: Picking the Right CT for Your Project
Use this decision matrix to select the exact CT topology and part number for your application. Do not guess; matching the core type and output to your burden circuit is critical for accuracy.
| Application Scenario | Key Requirement | Concrete Pick (Part Number) | Approx. Cost |
|---|---|---|---|
| DIY IoT Energy Monitor (ESP32/Arduino, <100A) |
Split-core (retrofit), mA output, 3.5mm audio jack termination | YHDC SCT-013-000 (100A:50mA) |
$12 - $15 |
| Whole-Home Subpanel Metering (IoTaWatt / Emporia, 200A) |
Split-core, large aperture for 2/0 AWG, high linearity | AccuEnergy ACCT-013-200 (200A:50mA) |
$25 - $35 |
| Industrial Motor Protection (3-Phase, 480V, Panel Meter) |
Solid-core (new build), 5A standard secondary, 5P accuracy class | CR Magnetics CR4100-50-5 (50A:5A) |
$40 - $60 |
| PCB-Mount AC Sensing (Smart Plugs, Appliances) |
Through-hole PCB mount, compact, 10A max | ZMCT103C (10A:5mA) |
$2 - $4 |
FAQ: Burden Resistors, Saturation, and Accuracy
Q: What happens if I use a CT with an internal burden resistor on a circuit that exceeds its rating?
A: Some CTs (like the SCT-013-030) have an internal burden resistor and output 0-1V directly. If you push 150A through a 100A-rated CT, the core will magnetically saturate. Once saturated, the secondary current stops increasing linearly and flattens out. Your microcontroller will read a falsely low current, potentially failing to trigger an over-current alarm while the physical wire melts. Always size the CT primary rating to at least 125% of the continuous expected load.
Q: What do the 'Accuracy Classes' (0.5, 1.0, 5P) mean on industrial CT datasheets?
A: According to CR Magnetics and IEEE C57.13 standards, measurement classes (0.5, 1.0) define the maximum percentage error at nominal current for revenue metering. Protection classes (like 5P or 10P) define the error at overcurrent multiples. A 5P20 CT guarantees 5% accuracy up to 20 times the rated current, ensuring the protection relay sees the true fault current during a dead short.
Q: Can I use a current transformer to measure DC current?
A: No. Standard CTs rely on a changing magnetic field (dΦ/dt) to induce a secondary current. DC current creates a static magnetic field, which induces zero secondary current. For DC measurement (like a 12V LiFePO4 battery bank), you must use a shunt resistor or a closed-loop Hall effect sensor (like a Victron SmartShunt).
Selecting the right current transformer comes down to matching the physical aperture to your wire gauge, calculating the exact burden resistor for your ADC's voltage reference, and respecting the lethal physics of an open secondary circuit. Pick the split-core SCT-013-000 for your next ESP32 energy monitor, calculate your 22Ω burden, and you will have a safe, highly accurate measurement system running in an afternoon.
