Understanding Transformer CT Ratios, Burden, and AC Measurement

A transformer CT (Current Transformer) is a specialized instrument transformer that steps down high primary AC current into a safely measurable, isolated secondary current proportional to the primary. Instead of changing voltage to power a load like a standard transformer, a CT changes the current scale in a real circuit so that standard 5A or 1A panel meters, protective relays, and energy monitors can safely measure hundreds or thousands of amps without being directly connected to the high-energy busbar.

How a Current Transformer Steps Down AC Current

In a standard power transformer, the primary and secondary windings are both multi-turn coils, and the core is designed to transfer maximum real power. In a transformer CT, the primary is often just a single turn—typically the thick busbar or heavy-gauge THHN cable passing straight through the center of the toroidal core. The secondary consists of hundreds or thousands of turns of fine magnet wire wrapped around a high-permeability silicon steel or nanocrystalline core.

The fundamental operating principle relies on the ampere-turn balance: $I_p \times N_p = I_s \times N_s$. Because the primary is usually just one turn ($N_p = 1$), the turns ratio is simply the number of secondary turns. If you have 800 turns on the secondary, a 400A primary current will induce exactly 5A in the secondary (an 80:1 ratio). The secondary current flows through a low-impedance path called the "burden," which converts the current back into a measurable voltage or drives a low-impedance analog ammeter.

Think of a CT like a traffic counting system on a massive 10-lane highway. Instead of standing in all 10 lanes to count every car (high current), engineers build a dedicated off-ramp that is geometrically forced to carry exactly 1/100th of the main traffic. You count the cars on the off-ramp, multiply by 100, and know the total highway flow without ever stepping into the high-speed lanes.

When selecting a CT for a specific installation, you must match the accuracy class to the application. Metering CTs (e.g., Class 0.5 or 1.0) are designed to be highly accurate at normal load currents but intentionally saturate during a short-circuit fault to protect delicate 5A panel meters from thermal destruction. Protection CTs (e.g., Class 5P or 10P), on the other hand, are engineered to remain linear up to 20 times their rated current so that protective relays can accurately detect and trip during massive fault events. For a deep dive into CT saturation curves and accuracy limits, the All About Circuits textbook chapter on instrument transformers provides excellent baseline theory.

Worked Numeric Example: Sizing a 400A CT and Burden Resistor

Let’s walk through a real-world bench scenario. You are retrofitting an IoT energy monitor (like an ESP32-based custom board or a commercial Emporia Vue) to measure a 400A commercial service feeder. You select a standard window-type CT with a 400A:5A ratio. However, your microcontroller’s ADC can only read 0–3.3V DC, and you need an AC voltage signal peaking around 1.5V to leave headroom for transients.

First, we calculate the secondary current at a typical continuous operating load. Commercial feeders are rarely loaded to 100% continuously; let's assume a steady baseline load of 250A RMS.

• Secondary Current ($I_s$): $250A / 80 = 3.125A$ RMS.
• Required Burden Resistance ($R_b$): To get 1.06V RMS across the burden at 3.125A, we use Ohm's Law: $R = V / I = 1.06V / 3.125A = 0.339\Omega$.
• Standard Resistor Selection: We select the nearest standard 1% value, which is 0.33Ω.

Now we must verify the power dissipation to ensure the resistor doesn't melt inside the enclosure. Power ($P$) is calculated as $I^2 \times R$. At the maximum rated primary current of 400A, the secondary is pushing a full 5A.

• Max Power Dissipation: $5A^2 \times 0.33\Omega = 25 \times 0.33 = 8.25 Watts.

A standard 1/4W or even 1W through-hole resistor will instantly vaporize. You must spec a 15W or 20W chassis-mount wirewound resistor (like an Ohmite 20J series) and bolt it to a heat sink or the metal enclosure wall. If you want to avoid massive power dissipation, the modern 2026 standard practice is to swap the 400A:5A CT for a 400A:333mV voltage-output CT (such as those from Magnelab or Accuenergy). These have an internal, factory-potted burden resistor and op-amp buffer, outputting a safe, low-power millivolt signal directly compatible with microcontroller ADCs without external heating issues.

Where You Meet Transformer CTs in Practice

You will encounter CTs anywhere high AC current needs to be monitored, metered, or protected without running massive busbars into delicate electronics. Common jobsite and bench encounters include:

• Main Service Metering: The utility company uses large, oil-filled or epoxy-cast CTs inside the metering cabinet to step down 800A–3200A service entrance currents down to 5A for the revenue-grade kWh meter.
• Solar Inverter Export Limiting: Grid-tied solar inverters (like SolarEdge or Fronius) require a CT clamped onto the main utility feed. The inverter reads this CT to ensure it never exports more power to the grid than the local utility allows, throttling solar production in real-time.
• Bidirectional EV Chargers: Modern V2G (Vehicle-to-Grid) and dynamic load-balancing EV chargers (like the Wallbox Quasar) use split-core CTs on the home's main feeders. If you turn on the electric oven and the HVAC, the CT detects the current spike and instantly throttles the EV charging rate to prevent tripping the main 200A breaker.
• Motor Starters and VFDs: Inside industrial motor control centers, three CTs (one per phase) feed into a solid-state overload relay. If a mechanical jam causes the motor to pull locked-rotor current, the CTs detect the imbalance or overcurrent and trip the main contactor in milliseconds.
• Home Energy Monitors: Consumer devices like the Sense monitor or Emporia Vue use dozens of tiny, 50A split-core CTs clipped onto individual branch circuits inside your residential load center to disaggregate appliance usage via machine learning algorithms.

Frequently Asked Questions