What Current Transformer to Choose: Ratios, Burden, and Sizing Explained

A current transformer (CT) is a passive electromagnetic device that steps down high alternating primary currents into a safely measurable, proportional secondary current (typically 5A or 1A) while electrically isolating the measuring instruments from the high-voltage line.

When you ask what current transformer technology actually changes in a real installation, the answer is twofold: it provides galvanic isolation (keeping 480V mains away from your 3.3V microcontroller) and it scales down unmeasurable currents into standardized signals. Think of a CT like a mechanical gear reduction system; the primary wire acts as a single-turn gear driving a secondary coil with hundreds of turns, trading high current for low, manageable current.

Core Specifications: Ratios, Accuracy Classes, and Burden

Selecting the right CT requires matching three primary specifications to your installation: the current ratio, the accuracy class, and the burden rating. If you undersize the burden or pick the wrong accuracy class, your energy monitoring data will drift, or worse, your protection relays will fail to trip during a fault.

Understanding Burden (VA): Burden is the total load (impedance) connected to the CT's secondary terminals, expressed in Volt-Amperes (VA). It includes the internal resistance of your meter's shunt plus the resistance of the secondary wire run. According to All About Circuits, if your connected burden exceeds the CT's rated burden, the core will saturate early, introducing massive ratio errors and phase shifts. Always calculate your wire resistance and add it to the meter's internal burden before buying.

Worked Example: Sizing a CT for a 200A Solar Inverter Feeder

Let's walk through a real-world sizing scenario. You are installing a bidirectional meter to monitor a 200A continuous solar inverter output. The meter is located 15 feet away from the CT in your combiner box.

1. Select the Ratio: The maximum continuous current is 200A. We select a 200:5A CT (like the Accuenergy AcuCT-200A). If the inverter is pushing 150A, the secondary current will be exactly 150 × (5 / 200) = 3.75A.
2. Calculate the Wire Burden: You run 15 feet of 14 AWG copper wire to the meter (30 feet total round-trip). 14 AWG copper has a resistance of roughly 2.52 ohms per 1,000 feet.
   Wire Resistance = (30 / 1000) × 2.52 = 0.075 ohms.
3. Calculate the Meter Burden: The meter's internal current shunt is rated at 0.1 ohms.
4. Total Secondary Impedance (Z): 0.075 + 0.1 = 0.175 ohms.
5. Calculate Total VA Burden: Using the formula VA = I² × Z at maximum secondary current (5A):
   VA = 5² × 0.175 = 25 × 0.175 = 4.375 VA.

The Decision: You must purchase a 200:5A CT with a burden rating of at least 5 VA (standard sizes are 2.5, 5, 10, 15 VA) to maintain the stated 0.5 accuracy class. If you accidentally bought a 2.5 VA CT for this run, the core would saturate near peak solar production, and your app would under-report your generation by 10% or more.

Where You Meet Current Transformers in Practice

You will encounter CTs in almost every modern energy management system. In residential smart panels like Span or Emporia, dozens of miniature split-core CTs clip onto individual branch circuits to provide circuit-level energy disaggregation. In commercial solar and battery storage (like Victron or Schneider Electric systems), large solid-core CTs are bolted onto busbars to feed data to the inverter's grid-following algorithms. In industrial settings, protection-class CTs feed overcurrent relays that trip 480V contactors in milliseconds during a short circuit.

What People Commonly Confuse CTs With

Makers and junior technicians frequently mix up CTs with other current-sensing technologies. Here is how to tell them apart:

• Potential Transformers (PTs): PTs step down voltage (e.g., 4160V down to 120V) for metering. They are wired in parallel with the line, whereas CTs are wired in series (the line passes through the window).
• Shunt Resistors: Shunts measure current by reading the voltage drop across a low-value resistor. They work for both AC and DC but lack galvanic isolation. If you use a shunt on a 240V line, your entire microcontroller ground is elevated to 240V—a lethal hazard.
• Hall-Effect Sensors (e.g., ACS712): Hobbyists love the $2 ACS712 module for Arduino projects because it outputs a simple analog voltage. However, Hall sensors drift with temperature, have poor resolution at high currents, and physically connect the high-voltage mains to the low-voltage IC pins. For reliable, safe mains revenue metering, always use an inductive CT.

Critical Safety Rules and Common Wiring Mistakes

Beyond the open-circuit hazard, polarity is the most common installation mistake. CTs are directional. They feature P1/P2 markings on the primary window and S1/S2 on the secondary terminals. P1 must face the source (the utility or the solar inverter), and P2 must face the load. If you install the CT backward, the secondary current will be 180 degrees out of phase. On a simple ammeter, you won't notice. But on a wattmeter or a solar inverter calculating power factor, a reversed CT will make the system think you are exporting power when you are actually consuming it, causing the inverter's anti-islanding protection to trip offline.

Finally, respect the window size. Forcing a 500 MCM cable through a CT window meant for 2 AWG wire will physically damage the tape wrapping and compromise the dielectric insulation. If your conductor is too large for standard solid-core CTs, specify a split-core CT (like the Magnelab RCK-1000 series), which clamps around the wire without requiring you to disconnect the lugs—saving you from having to de-energize the entire building just to install a sensor.