A current transformer (CT) is an instrument transformer that steps down high primary AC current to a safe, measurable secondary current while electrically isolating the measurement circuit from the high-voltage line.
The Core Mechanics: What a CT Actually Changes
In a real circuit or installation, a current transformer changes the magnitude of the current and provides galvanic isolation. It does not step down voltage—that is the job of a potential transformer (PT). By wrapping a primary conductor (or passing it through a toroidal window) and coupling it to a multi-turn secondary winding, the CT forces the secondary current to be a precise fractional ratio of the primary current. This allows you to measure a 400A feeder using a standard 5A analog panel meter or a low-voltage microcontroller ADC without ever making direct electrical contact with the 480V bus.
The relationship is governed by the turns ratio. If a CT has a 1:100 turns ratio (one primary pass, 100 secondary turns), a primary current of 100A will induce exactly 1A in the secondary winding. For a deeper look at the magnetic flux principles driving this, the Electronics Tutorials guide on current transformers provides excellent foundational math.
Worked Numeric Example: Sizing and Burden Calculation
The most critical concept in CT theory is burden—the total impedance of the secondary circuit (wires + meter + internal winding resistance). If the burden exceeds the CT's rating, the core saturates, the ratio becomes non-linear, and your readings drop off a cliff.
Let's calculate the maximum wire run for a standard industrial CT monitoring a motor feeder.
| Parameter | Value | Notes |
|---|---|---|
| Primary Current | 400A | Measured line |
| Secondary Current | 5A | Standard industrial output |
| CT Ratio | 80:1 | 400 / 5 |
| Burden Rating | 2.5 VA | From manufacturer datasheet |
| Wire Gauge | 14 AWG Copper | Standard for 5A CT loops |
Step 1: Calculate Maximum Allowable Impedance (Z)
Using the formula VA = I² × Z, we solve for Z:
Z = 2.5 VA / (5A)² = 2.5 / 25 = 0.1 Ω
Step 2: Account for the Meter
Assume the digital ammeter has an internal burden of 0.02 Ω. This leaves 0.08 Ω for the wiring loop.
Step 3: Calculate Maximum Wire Length
14 AWG copper wire has a resistance of approximately 0.00252 Ω per foot. Because the current must travel to the meter and back, the loop resistance is double the one-way distance.
Max Loop Length = 0.08 Ω / 0.00252 Ω/ft = 31.7 feet.
Divide by 2 for the one-way run: 15.8 feet maximum distance from the CT to the meter.
If your panel is 30 feet away from the meter, 14 AWG will cause core saturation. You must step up to 10 AWG (0.001 Ω/ft) or use a CT with a higher VA burden rating (e.g., 10 VA).
Where You Meet This In Practice
You will encounter current transformers across several distinct domains in modern electrical work and DIY electronics:
- Home Energy Monitors: Systems like the Emporia Vue or Sense use split-core CTs that output 50mA or 333mV at full scale. These clamp directly onto your breaker panel's branch circuits to track real-time wattage via a WiFi-connected hub.
- Grid-Tied Solar Inverters: Revenue-grade CTs are installed on the main service feeder to measure net export/import. The inverter uses this data to throttle its output and ensure it never pushes power backward if the grid drops (anti-islanding).
- Variable Frequency Drives (VFDs): Industrial VFDs use internal or external CTs on the output phases to monitor motor load, detect ground faults, and trigger overcurrent protection before the IGBTs blow.
- Smart Electrical Panels: Next-gen panels (like Span) embed microscopic Rogowski coils or solid-core CTs on every single breaker to provide circuit-level control and monitoring via an app.
Critical Safety: The Open-Circuit Hazard
According to Fluke's safety guidelines on instrument transformers, an open secondary circuit removes the counter-magnetomotive force that normally limits core flux. Without this opposing force, the core saturates instantly, and the transformer attempts to force its rated current across an infinite impedance. This generates lethal voltage spikes (often exceeding 2,000V to 5,000V), which will arc across the open terminals, melt the winding insulation, and potentially cause an explosive fire.
If you need to disconnect a 5A CT ammeter while the circuit is live, you must use a CT shorting block. Shorting blocks are wired in series with the CT and feature a sliding link or screw that safely shorts the secondary terminals (X1 to X2) before you break the connection to the meter. Note: Modern 50mA or 333mV voltage-output CTs have an internal burden resistor, making them inherently safe to open-circuit, but you must verify the datasheet before assuming this.
Current Transformer Wiki FAQ
Can I use a current transformer to measure DC current?
No. Standard current transformers rely on Faraday's law of induction, which requires a changing magnetic field to induce a secondary current. DC current produces a static magnetic field, resulting in zero secondary output once the initial transient settles. To measure DC, you must use a Hall-effect sensor (like the ACS712 or ACS758), a fluxgate sensor, or a shunt resistor with an instrumentation amplifier.
What is the difference between a split-core and solid-core current transformer?
A solid-core (toroidal) CT is a continuous ring of magnetic material. The primary conductor must be disconnected and threaded through the center. They offer higher accuracy (often 0.1% to 0.5% error) because there are no air gaps in the magnetic path. A split-core CT has a hinge and a latch, allowing you to clamp it over an existing, energized wire without disconnecting it. The physical air gap at the mating surfaces slightly reduces accuracy (typically 1% to 3% error) and lowers the saturation threshold, but the installation convenience makes them the standard for retrofit energy monitoring.
How do I wire a current transformer to an Arduino or ESP32?
Microcontrollers like the ESP32 have ADC pins that only read positive DC voltages (0V to 3.3V). A standard AC CT outputs an alternating current that swings positive and negative. To interface them:
- Add an external burden resistor across the CT secondary (e.g., 33Ω for a 50mA CT to get ~1.65V peak).
- Create a DC bias voltage divider using two equal resistors (e.g., two 10kΩ resistors) between 3.3V and GND to create a 1.65V midpoint.
- Connect one side of the burden resistor to the 1.65V bias, and the other side to the ESP32 ADC pin.
- In your code, subtract the 1.65V DC offset from your ADC readings to isolate the AC waveform, then calculate the RMS value.
Why do some current transformers output 5A while others output 50mA?
The 5A secondary standard dates back to the early 20th century when analog moving-iron panel meters required significant current to physically move the needle against the spring tension. Today, 5A CTs are still used in heavy industrial switchgear for legacy compatibility and to drive electromechanical overload relays. Conversely, modern digital meters, IoT energy monitors, and microcontrollers use high-impedance ADCs that require almost zero current. Therefore, modern CTs use a 50mA, 1A, or 333mV output. This drastically reduces the VA burden, allows for thinner wiring (22 AWG instead of 12 AWG), and eliminates the lethal open-circuit explosion hazard inherent to 5A systems.
