A CT ampere rating defines the maximum primary alternating current a current transformer can safely measure and the proportional secondary current it outputs for monitoring or metering. When you select a current transformer (CT) for an energy monitoring project, this rating changes three critical things in your installation: the physical window size required for your conductor, the scaling factor your microcontroller's ADC must apply, and the exact wattage of the burden resistor needed to prevent voltage spikes. Beginners commonly confuse the primary ampere limit (e.g., 100A) with the secondary output, or mistakenly wire a raw current-output CT directly into an ESP32 ADC pin without a burden resistor, instantly frying the microcontroller's input stage.
How CT Ampere Ratings Alter Your Circuit Design
To understand how the rating translates to bench-level component selection, let's look at the most common DIY sensor: the YHDC SCT-013-000. This is a 100A:50mA CT. The primary ampere rating is 100A RMS, and the secondary output is 50mA RMS when the primary is at full scale. The turns ratio is 2000:1.
Think of the CT ampere ratio like a water flow meter with a bypass loop: the main pipe carries the heavy, dangerous flow (primary), while a tiny, precisely scaled capillary tube diverts a measurable, safe trickle (secondary) to your sensor.
If you want to read this 50mA secondary current with an ESP32 (which has a 0-3.3V ADC and requires a biased AC signal centered at 1.65V), you must convert the current to voltage using a burden resistor. Here is the exact math for a 120V/240V split-phase residential panel:
- Target Secondary Voltage: We want 1.65V peak (which is ~1.16V RMS) to maximize the ESP32's 12-bit ADC resolution without clipping the sine wave.
- Ohm's Law: R = V / I
- Calculation: R = 1.16V RMS / 0.050A RMS = 23.2 Ω
- Standard Resistor Pick: 22 Ω or 24 Ω. Let's use 22 Ω.
- Power Dissipation: P = I² × R = (0.050)² × 22 = 0.055W. A standard 1/4W (0.25W) metal film resistor is more than adequate.
If you had instead chosen a 200A CT with a 100mA secondary output for this same circuit, your burden resistor would need to drop to 11 Ω to maintain the same 1.16V RMS ADC input, and the power dissipation would quadruple, requiring a 1/2W resistor to avoid thermal drift.
Where You Meet CT Ampere Ratings in Practice
You will encounter specific CT ampere specifications across several real-world electrical and embedded scenarios:
- Commercial Energy Monitors (IoTaWatt, Emporia Vue): These systems use proprietary or standardized 200A:100mA or 200A:50mA CTs. The firmware assumes a specific turns ratio; if you clamp a 100A CT onto a 200A-rated input channel in the software, your logged kWh will be exactly half of reality.
- Solar Inverter Grid-Tie Metering: Fronius and SolarEdge inverters use specific 100A or 200A CTs to monitor grid export/import. The inverter's internal ADC is hardcoded for the exact secondary milliampere output of the OEM CT. Swapping a 100A CT for a 200A CT without changing the inverter's software configuration will cause the inverter to curtail solar production prematurely.
- Motor Overload Protection: In industrial VFDs (Variable Frequency Drives), CTs monitor phase current to detect motor stalling. Here, the CT ampere rating must be sized to 125% of the motor's Full Load Amps (FLA) to allow for inrush current without saturating the magnetic core.
Current Output vs. Voltage Output: The Burden Resistor Trap
The most frequent mistake makers make when ordering CTs is ignoring the suffix on the part number, which dictates whether the CT outputs raw current or a pre-conditioned voltage. This is entirely determined by the presence of an internal burden resistor.
| Feature | YHDC SCT-013-000 (Current Output) | YHDC SCT-013-030 (Voltage Output) |
|---|---|---|
| Primary Rating | 100A | 30A |
| Secondary Output | 50mA (Raw Current) | 0-1V RMS (Voltage) |
| Internal Burden | None (Requires external resistor) | 62 Ω built-in |
| Microcontroller Wiring | Requires burden resistor + DC bias voltage divider | Plug-and-play into ADC (with DC bias for 3.3V/5V systems) |
| Best Use Case | Whole-home 100A-200A subpanel monitoring | Individual 15A/20A branch circuit appliance tracking |
Decision Tree: Picking the Exact CT Ampere Rating for Your Build
Use this decision path to select the exact part number for your next energy monitoring or IoT build. Do not guess; match the primary ampere rating to your breaker size and the secondary output to your ADC architecture.
| If your target circuit is... | And your microcontroller is... | Then select this exact CT part number | Required External Components |
|---|---|---|---|
| 15A or 20A branch circuit (e.g., fridge, window AC) | Arduino Uno (5V ADC) | YHDC SCT-013-030 (30A:1V) | None (Direct plug-in) |
| 15A or 20A branch circuit | ESP32 / Raspberry Pi Pico (3.3V ADC) | YHDC SCT-013-030 (30A:1V) | DC bias network (2x 10kΩ, 1x 10µF cap) |
| 50A to 100A subpanel feeder or EV charger | ESP32 / Arduino (Any ADC) | YHDC SCT-013-000 (100A:50mA) | 22Ω 1/4W burden resistor + DC bias network |
| 200A main residential service entrance | IoTaWatt or custom high-res ADC | Magnelab SCT-0750-200 (200A:100mA) | 10Ω 1W burden resistor (if not using IoTaWatt internal burden) |
Default Recommendation: If you are building a general-purpose IoT energy monitor on a breadboard and want to avoid soldering surface-mount burden resistors or calculating complex bias networks, default to the YHDC SCT-013-030. Its internal 62-ohm burden resistor and 30A primary rating make it the most forgiving, plug-and-play sensor for measuring standard household appliances without risking ADC saturation.
Frequently Asked Questions
What happens if I pass 150A through a 100A rated CT?
The CT's ferromagnetic core will enter magnetic saturation. Once saturated, the secondary current no longer scales linearly with the primary current. Your microcontroller will read a flat-topped, distorted sine wave, resulting in massively under-reported power calculations. Furthermore, the excessive magnetic flux can cause the core to overheat, degrading the insulation over time.
Does the physical wire gauge passing through the CT window matter?
Electrically, no. The CT measures the magnetic field generated by the current, regardless of whether it's a 14 AWG wire carrying 15A or a 1/0 AWG wire carrying 15A. Physically, however, you must ensure the CT's inner window diameter (typically 13mm for the SCT-013 series) is large enough to snap over the insulation of your specific wire gauge. A 2/0 AWG THHN wire will not fit through a standard 13mm split-core CT.
Can I use an AC current transformer to measure DC solar panel output?
No. Current transformers operate strictly on Faraday's Law of Induction, which requires a changing magnetic field. DC current produces a static magnetic field, which will not induce any secondary current. To measure DC ampere flows from solar panels or battery banks, you must use a Hall-effect sensor (like the ACS712 or INA219) or a flux-gate sensor.
For deeper calibration theory and open-source libraries to process these AC waveforms, refer to the OpenEnergyMonitor CT sensor guide. If you are sourcing high-accuracy, revenue-grade split-core sensors for 200A+ service panels, review the Magnelab split-core current sensor lineup for exact phase-shift and linearity datasheets.
