A current transformer (CT) is an instrument transformer that steps down high AC line current to a safe, measurable secondary current—typically 5A, 1A, or a low milliamp voltage signal—while maintaining precise phase and proportional accuracy. In a real installation, it changes everything: it allows standard low-voltage meters, microcontrollers, and protection relays to safely monitor hundreds of amps on the primary side without any galvanic connection to lethal mains voltage. Think of it like a step-down gearbox on a winch: the primary wire is the high-torque input, and the secondary winding is the scaled-down output that your delicate instruments can safely handle.
The Core Mechanism and a Worked Burden Resistor Calculation
Unlike a standard voltage transformer where the primary and secondary are both multi-turn windings, a CT's primary is often just the single conductor passing through the core window (a 1-turn primary). The secondary has hundreds or thousands of turns. The fundamental relationship is Ip × Np = Is × Ns. If you have a 200A primary and a 5A secondary, the turns ratio is 40:1.
The most critical concept to grasp is burden. A CT is a constant-current source, not a constant-voltage source. It will push its secondary current through whatever impedance is connected to it. If you connect a 5A secondary directly to a microcontroller's ADC pin, you will destroy the chip. You must convert that current into a voltage using a burden resistor.
Suppose you are installing a 200A:5A split-core CT on a subpanel feeder to feed a commercial IoT energy monitor (like an Emporia or Accuenergy hub) that expects a maximum 333mV RMS input signal.
1. Calculate Resistance: Using Ohm's Law (R = V / I), R = 0.333V / 5A = 0.0666 Ω.
2. Calculate Power Dissipation: Using P = I² × R, P = (5A)² × 0.0666 Ω = 1.665 Watts.
3. The Concrete Pick: A standard 1/4W or 1/2W resistor will overheat and fail. You must source a 3-Watt, 0.068 Ω precision power resistor (like a Vishay Dale RS005 series) to handle the thermal load safely with margin.
If you are interfacing directly with an ESP32 or Arduino ADC (which reads 0-3.3V DC), you cannot use the AC signal directly. You must use a voltage divider to bias the AC signal at 1.65V (half of VCC) so the microcontroller can read both the positive and negative half-cycles of the AC wave. For deep-dive ADC sampling code and biasing schematics, refer to the open-source OpenEnergyMonitor project documentation.
Metering vs. Protection: The Most Expensive Confusion
The most common mistake DIYers and junior technicians make is confusing metering CTs with protection CTs, or confusing passive CTs with active Hall-effect sensors. Getting this wrong either destroys your equipment or leaves your circuit unprotected during a fault.
Metering CTs (e.g., ANSI Class 0.3 or 0.6) are designed for extreme accuracy at normal operating currents (80% to 120% of rated current). However, they are intentionally designed to saturate (core magnetic flux maxes out) at roughly 2 to 3 times their rated current. Why? To protect the delicate 5A metering equipment downstream from the massive thermal and magnetic forces of a 10,000A short-circuit fault.
Protection CTs (e.g., ANSI Class C100 or C200) are the opposite. They sacrifice a bit of low-end accuracy to remain strictly linear up to 20 times their rated current. If a 10,000A fault occurs on a 400A feeder, a protection CT will accurately scale that down to 125A on the secondary so the overcurrent relay sees it and trips the breaker. If you mistakenly used a metering CT here, it would saturate during the fault, the relay would see a clipped, lower current, and the breaker would fail to trip.
CTs vs. Hall-Effect Sensors: People also confuse CTs with Hall-effect sensors (like the Allegro ACS712). Hall-effect sensors can measure both AC and DC current and output a direct voltage, but they require an external power supply, suffer from temperature drift, and lack the robust galvanic isolation of a transformer. CTs are strictly AC, entirely passive, and offer superior isolation for mains voltage.
Where You Meet Current Transformers in Practice
You will encounter CTs in several specific real-world scenarios, each with distinct wiring and sizing requirements:
- Grid-Tied Solar Export Limiting: Inverters like SolarEdge or SMA require a CT clamped onto the main service entrance conductors. The inverter reads this CT to ensure you aren't pushing more power back to the grid than your utility agreement allows. These almost always use 5A secondary split-core CTs.
- Home Energy Monitors: Systems like Sense or Emporia Vue use dozens of tiny, low-current CTs (often 50A:50mA) clamped onto individual branch circuits in your panel to disaggregate appliance loads via machine learning.
- Variable Frequency Drives (VFDs): Industrial VFDs use internal or external CTs to monitor motor phase current for torque control and to trigger overcurrent faults if the motor jams.
- Subpanel Metering: When adding a workshop subpanel, landlords or makers often install a simple 100A:5A CT and an analog ammeter on the panel door to monitor load without opening the enclosure.
Decision Tree: Picking the Right CT for Your Build
Use this decision matrix to terminate your part selection process. Do not guess; match your exact use case to the required secondary output and core type.
| Your Application | Primary Current | Required Secondary / Output | Core Type | Concrete Part Pick |
|---|---|---|---|---|
| ESP32/Arduino DIY Energy Monitor (Custom PCB) | 0 - 100A | 50mA AC (Requires external burden) | Split-Core | YHDC SCT-013-000 (Do NOT buy the -100 version if your PCB expects a current input) |
| Commercial Solar Inverter Export Limiter | 0 - 200A | 5A AC (Standard metering) | Split-Core | Accuenergy ACTL-0200-050 or Magnelab 8122 |
| Industrial Motor Protection Relay | 0 - 400A | 5A AC (Protection class C200) | Solid Ring Core | ABB CTD200 series (Must be threaded onto busbar during assembly) |
| DC Battery Bank / Solar Charge Controller | 0 - 500A DC | Analog Voltage (Hall-Effect) | N/A (Hall IC) | LEM HTFS 200-P or Allegro ACS770 (CTs do not work for DC) |
Installation Rules and the 'Never Open the Secondary' Mandate
When wiring CTs in a panel, follow these field-proven practices to ensure safety and accuracy:
- Use Shorting Blocks: In any professional installation where a meter might be removed for calibration or replacement, wire the CT secondary through a shorting terminal block. This allows you to safely short the CT leads together before unplugging the meter.
- Ground the Secondary: Per NEC Article 250 and standard IEC practices, the secondary circuit of an instrument transformer must be grounded at exactly one point. This prevents the secondary wiring from floating up to primary voltage potentials in the event of an internal insulation failure. Ground it at the shorting block or the meter terminal, never at both ends (which creates a ground loop).
- Observe Polarity (P1/P2 and S1/S2): CTs are directional. The primary side will have an arrow or P1/P2 markings indicating current flow direction. The secondary will have S1/S2. If you install the CT backward, your digital power meter will read negative watts, and a directional overcurrent relay will fail to operate. Always point the P1 arrow toward the load.
- Keep Lead Length Short: The resistance of the wire connecting the CT to the meter adds to the total burden. If you run 50 feet of thin 18 AWG wire, the wire resistance might exceed the CT's rated VA burden, causing the core to saturate early and read low. For long 5A secondary runs, step up to 12 AWG or 10 AWG control wire, or switch to a 1A secondary CT to reduce I²R line losses.
For further reading on the mathematical derivation of CT saturation curves and ANSI class definitions, the Electronics Tutorials guide on Current Transformers provides excellent baseline theory, while manufacturer datasheets from Magnelab or Accuenergy will give you the exact VA burden limits for specific physical models.
