Understanding Current Transformers: CT Ratios, Burden, and Safety

A current transformer (CT) is an instrument transformer designed to produce a secondary alternating current strictly proportional to the primary current flowing through its window or core. Unlike standard power transformers that transfer wattage to run loads, a CT transfers a measurement signal, stepping down hundreds of amps to a standardized, safe output for metering and protection relays while providing galvanic isolation from lethal mains voltages.

In a real circuit, a CT changes a high-amperage, high-voltage primary environment into a safe, isolated, low-amperage secondary signal (typically 5A or 1A) that standard microcontrollers, digital meters, and solid-state relays can read without being destroyed. Makers and junior technicians commonly confuse CTs with Potential Transformers (PTs/VTs) which step down voltage, or assume they function like standard step-down power transformers. The critical distinction is that a CT is a series device acting as a constant current source; its secondary voltage will spike to lethal, insulation-destroying levels if left open-circuited.

What a Current Transformer (CT) Actually Changes in a Circuit

Think of a CT like a clamp-on flow meter on a massive municipal water main: it does not restrict the water flow, it simply reads the magnetic field generated by the flow to give you a scaled-down measurement. In electrical terms, the primary winding is often just a single pass of a thick busbar or THHN wire through the center of the toroidal core. The alternating current in this primary wire generates a magnetic flux in the core, which induces a proportional current in the multi-turn secondary winding wrapped around that core.

When sourcing instrument transformers, CT specifications must be matched not just to the current, but to the physical window size and the burden of the connected wiring and meters. If you undersize the core or ignore the burden, the core will magnetically saturate during a fault condition, flattening the output waveform and blinding your protection relays exactly when you need them to trip.

Standard CT Ratios, Accuracy Classes, and Burden Ratings

Selecting the right unit requires understanding the relationship between the primary rating, the accuracy class, and the allowable burden. Below is a reference table for common window-type CTs used in commercial subpanels and solar inverter installations.

Reading the Accuracy Class: A class 0.3 means the CT is guaranteed to be within 0.3% of the true value at its rated current, which is mandatory for utility billing. A class C100 (ANSI/IEEE C57.13 standard) means it is a fully distributed winding that will not saturate until the secondary voltage reaches 100V at 20 times the rated current, ensuring protection relays see the true fault current. For a deeper dive into these classifications, refer to the accuracy class definitions on the Electrical Engineering Portal.

Worked Example: Calculating Secondary Current and Burden Limits

Let us look at a real-world scenario. You are installing a commercial solar PV system with a zero-export controller. The main service breaker is 400A, and you install a 400:5A, Class 0.6 CT with a rated burden of 2.5 VA to feed the inverter's grid-tie meter.

Step 1: Calculate the Secondary Current
During peak afternoon production, the building draws 280A from the grid before the solar inverters ramp up. What is the secondary current?
I_secondary = I_primary × (Secondary Rating / Primary Rating)
I_secondary = 280A × (5 / 400) = 3.5A
The meter will read 3.5A and scale it back up internally to display 280A.

Step 2: Check the Burden Limit
Burden is the total impedance (resistance + reactance) of the secondary circuit, expressed in Volt-Amps (VA) at the rated secondary current. If your wire run is too long or too thin, the resistance exceeds the CT's capability, causing saturation.
Max Allowable Impedance (Z) = Rated Burden (VA) / I_secondary_rated²
Z = 2.5 VA / (5A)² = 2.5 / 25 = 0.1 Ω

Your total secondary circuit resistance must be less than 0.1 Ω. If you run 40 feet of 14 AWG copper wire (round-trip length 80 feet), the wire resistance is roughly 0.2 Ω. This exceeds the 0.1 Ω limit. The CT will saturate, the waveform will clip, and the inverter's export limiter will fail to curtail power, potentially causing the utility to fine the facility. The fix is to upgrade to 12 AWG wire or move the meter closer to the CT.

Where You Meet CTs in Practice (and How to Choose)

You will encounter CTs across a wide spectrum of electrical work, from DIY smart home upgrades to industrial switchgear.

• Home Energy Monitors: Systems like the Emporia Vue or Sense use small, split-core CTs (often 50A to 200A) that clamp directly over individual branch circuit wires in your main panel. These are low-accuracy (Class 1.0 or worse) but perfectly adequate for tracking appliance usage trends.
• Solar Inverters & Battery Systems: Grid-tied inverters (like SolarEdge or Fronius) require highly accurate, solid-core CTs installed at the main service point to comply with utility export limits. These must be Class 0.5 or better to prevent billing errors.
• Industrial Motor Starters: Protection relays use C-class CTs to detect short circuits. If a motor draws 10x its rated current during a locked-rotor fault, the CT must output a perfectly scaled 50A secondary spike so the relay trips the contactor in milliseconds.