Current Transformer Connection Basics: Wiring, Ratios, and Burden Calculations

A current transformer (CT) connection is a wiring configuration that steps down high primary AC current to a safe, measurable secondary current (typically 5A or 1A) for metering and protective relaying, while electrically isolating the measurement circuit from the high-voltage primary line. In a real installation, it changes a dangerous, high-magnitude primary current into a low, proportional secondary signal that standard panel meters, PLCs, and microcontrollers can safely read without being destroyed by fault currents or exposed to lethal mains voltages.

The Core Mechanics of a Current Transformer Connection

Unlike a standard voltage transformer where both primary and secondary windings have many turns, a CT's primary winding is often just a single conductor passing through the magnetic core. The secondary winding consists of hundreds or thousands of turns of fine wire wrapped around that same core. The relationship is governed by the inverse turns ratio: I_primary × N_primary = I_secondary × N_secondary.

Standard secondary ratings are almost universally 5A or 1A in industrial power systems (per IEEE C57.13), though hobbyist split-core CTs often output 50mA or 100mA for direct PCB integration.

CTs are classified by accuracy standards based on their application. A metering CT might be rated Class 0.3, meaning it guarantees less than 0.3% ratio error at rated current, but it is deliberately designed to saturate early during a fault to protect the delicate metering equipment. Conversely, a protection CT (e.g., Class 5P20) is built to remain linear up to 20 times the rated current so the protective relay sees the exact fault magnitude and trips the breaker.

Worked Example: Sizing and Burden Calculation

A CT does not output a voltage; it acts as a current source. To get a measurable voltage for an analog-to-digital converter (ADC) or a panel meter, you must connect a "burden resistor" across the secondary terminals (S1 and S2). Sizing this resistor correctly is where most DIY builds fail.

Scenario: You are monitoring a 400A industrial feeder using a standard 400:5A CT. The actual primary load is 320A. You need to calculate the secondary current, the required burden resistor for a 1V full-scale meter, and the resistor's power dissipation.

1. Calculate Secondary Current: The turns ratio is 400:5 (or 80:1). I_secondary = 320A / 80 = 4A.
2. Calculate Burden Resistance: To get a 1V drop at a maximum 5A secondary current (which corresponds to 400A primary), use Ohm's Law: R = V / I = 1V / 5A = 0.2 Ω.
3. Calculate Voltage at Actual Load: At our 4A secondary current, the voltage drop is V = 4A × 0.2 Ω = 0.8V.
4. Calculate Power Dissipation: P = I² × R = 5² × 0.2 = 5W (using the maximum 5A rating for safety margin).

Where You Meet This in Practice

You will encounter CT connections in several distinct environments, each with different hardware expectations:

• Solar Inverters (Zero-Export Logic): Inverters like the Fronius Symo or SolarEdge require a CT on the main service entrance. If the CT detects current flowing backward to the grid, the inverter throttles its DC-AC conversion to match the home's exact load, avoiding utility export penalties.
• Home Energy Monitors: Systems like Emporia Vue or Sense use split-core CTs with 3.5mm TRRS jacks. These contain built-in burden resistors and output a low-voltage AC signal directly to the monitor's internal ADCs.
• Secondary Wiring Runs: In industrial panels, the physical distance between the CT and the relay adds wire resistance to the burden. If you run 50 feet of 14 AWG wire, you add roughly 0.12 Ω of lead burden. Jobsite practice dictates using 10 AWG or 12 AWG stranded wire for long CT secondary runs to keep lead resistance under 0.1 Ω, preventing the CT from saturating prematurely under fault conditions.

Common Confusions: CTs vs. PTs and Shunts

People frequently confuse the safety rules of Current Transformers (CTs) with Potential Transformers (PTs/VTs). This is a lethal mistake. A CT must never be open-circuited while primary current is flowing. Conversely, a PT must never be short-circuited. As explained in the All About Circuits AC theory guide, an open CT secondary forces the core into deep saturation, inducing kilovolts across the open terminals.

Additionally, hobbyists often confuse CTs with shunt resistors. While a 50mV/100A shunt is cheap and provides a direct DC/AC voltage drop, it offers zero galvanic isolation. If your microcontroller's ground reference shifts or a fault occurs, the mains voltage will travel directly through the shunt into your low-voltage logic, destroying your board and posing a severe shock hazard.

Current Transformer Connection FAQ