A current transformer (CT) is an instrument transformer that steps down high primary AC current to a safe, standardized secondary current (typically 5A or 1A) for metering and protection relays. In a real installation, a properly sized CT changes the game by isolating your sensitive measurement gear from lethal mains voltage and scaling massive currents down so a standard 5A ammeter or a 3.3V microcontroller ADC can read a 1000A service entrance without melting. If you undersize it, your meter reads garbage; if you pick the wrong accuracy class, your protection relay might fail to trip during a dead short.

The Core Specs: Reading a CT Nameplate

Before calculating wire runs or selecting a relay, you have to decode the nameplate. The most common mistake hobbyists and junior engineers make is treating the CT ratio as the only important spec. In reality, the burden and accuracy class dictate whether the CT will actually perform under load. Here is the data-dense breakdown of what those nameplate stamps actually mean.

Parameter Symbol / Format Typical Values What It Dictates in Practice
Ratio Primary:Secondary (e.g., 400:5) 100:5, 400:5, 1000:1 The scaling factor. A 400:5 CT outputs 5A when exactly 400A flows through the primary window.
Burden Rating VA (Volt-Amps) 2.5 VA, 5 VA, 15 VA The maximum load (meter + wire resistance) the CT can drive while maintaining its stated accuracy.
Accuracy Class Metering (0.3, 0.6) or Protection (C100, 5P10) 0.5, 1.0, C200, 10P20 The maximum percentage error at rated current, and whether the core intentionally saturates during faults.
Thermal Rating Factor (RF) Multiplier (e.g., 1.33, 1.5, 2.0) 1.0 to 4.0 How much continuous overcurrent the CT can handle without overheating. A 400A CT with a 1.25 RF handles 500A continuously.
Knee Point Voltage (Vk) Voltage (e.g., 150V, 300V) 50V to 1000V+ Used mostly in IEC protection CTs; the voltage where a 10% increase in voltage causes a 50% increase in exciting current (saturation onset).

According to the foundational principles outlined by All About Circuits, the secondary winding of a CT acts as a constant current source. If the connected burden (the physical resistance of your wires and meter) exceeds the CT's VA rating, the core saturates prematurely, the secondary current drops, and your meter reads artificially low.

Worked Example: Sizing a CT for a 400A Solar Feeder

Let us move from theory to the workbench. You are installing a solar export-limiting meter on a 400A main feeder supplying a workshop subpanel. The inverter manufacturer requires a 5A secondary CT for its grid-tie monitoring. Here is how you size it correctly.

Step 1: Select the Ratio and Rating Factor

The continuous load is 400A. Following NEC-style guidance for continuous loads (125% multiplier), your design current is 500A. You select a 500:5 CT. Because 500A is the exact primary rating, you want a Thermal Rating Factor (RF) of at least 1.0, though 1.25 is preferred if the panel runs hot.

Step 2: Calculate the Total Burden

Burden is measured in Volt-Amps (VA) and is calculated using the formula: VA = I² × R. Your secondary current (I) is 5A.

  • Meter Burden: The inverter's internal CT input specifies a burden of 0.5 VA.
  • Wire Burden: You are running 12 AWG THHN copper wire from the CT to the inverter. The one-way distance is 30 feet, making the round-trip loop 60 feet. The resistance of 12 AWG copper is roughly 1.588 ohms per 1,000 feet.
    R_wire = (60 / 1000) × 1.588 = 0.095 ohms.
    Wire VA = 5² × 0.095 = 25 × 0.095 = 2.375 VA.

Total Circuit Burden: 0.5 VA (meter) + 2.375 VA (wire) = 2.875 VA.

Step 3: Select the Accuracy Class and Final Spec

Since this is for energy monitoring and export limiting (not fault protection), you need a Metering Class CT. A Class 0.5 (meaning it is accurate to within 0.5% at rated current) is standard for revenue-grade solar metering.
Your final spec to order: 500:5 Ratio, Class 0.5, 5 VA Burden (minimum). Choosing a 5 VA or 10 VA rated CT ensures your 2.875 VA actual load keeps the transformer operating well within its linear region.

Pro-Tip on Wire Sizing: Never use 18 AWG or 16 AWG wire for 5A CT secondaries over long runs. The resistance spikes, eating up your VA budget and causing saturation. Stick to 12 AWG or 10 AWG for runs over 20 feet.

Where You Meet CT Sizing in Practice (and Common Confusions)

You will encounter current transformer sizing in three main areas: solar inverter installations (like Fronius or SolarEdge export limits), smart home energy monitors (Emporia Vue, Sense), and industrial motor control centers. In all these scenarios, the most dangerous confusion is mixing up Metering Class and Protection Class CTs.

Metering CTs (e.g., Class 0.3, 0.6, 1.2) are designed to be highly accurate at normal operating currents (0A to 120% of rating). However, they are intentionally designed to saturate during a massive short-circuit fault. If a 10,000A fault hits a 500:5 metering CT, the core saturates, limiting the secondary current and protecting your delicate $50 digital panel meter from exploding.

Protection CTs (e.g., Class C100, C200, 5P10) are the exact opposite. They are built with larger iron cores and are designed not to saturate during faults. If a 10,000A fault occurs, a C200 protection CT will faithfully reproduce that massive current spike on the secondary side so the protection relay sees the exact fault magnitude and trips the main breaker in milliseconds.

Critical Safety Warning: Never use a Metering CT for a protection relay circuit. During a fault, the metering CT will saturate, the relay will see a artificially low current, and it will fail to trip the breaker, potentially causing a fire or catastrophic equipment destruction. Always consult Fluke's instrument transformer guidelines or your local AHJ when mixing metering and protection circuits.

FAQ: Current Transformer Sizing Pitfalls

Can I leave a CT secondary open-circuited while the primary is energized?

Absolutely not. A CT is a step-up transformer regarding voltage. If the secondary is open-circuited, there is no counter-magnetomotive force to oppose the primary flux. The core saturates instantly, and the secondary terminals will induce lethal voltages (often thousands of volts), which will arc, destroy the insulation, and pose a fatal shock hazard. Always short the secondary terminals (using a shorting block or jumper wire) before disconnecting a meter.

Does the physical direction of the CT matter?

Yes. CTs have polarity markings, usually labeled P1/P2 (primary) and S1/S2 (secondary), or H1/H2 and X1/X2. The P1/H1 side must face the source of the power (the utility or generator), and P2/H2 must face the load. If you install it backward, your power factor readings will invert, and directional protection relays will think power is flowing backward, causing nuisance trips.

What if my primary conductor doesn't fill the CT window?

The physical size of the primary wire does not change the turns ratio, but placement matters. If you have a massive 4-inch window CT and a single 8 AWG wire passing through it off-center, the magnetic flux distribution becomes uneven. This causes ratio errors, especially at low loads. Always center the primary conductor in the CT window. If you are measuring a very small current with a large CT, you can loop the primary wire through the window multiple times; just divide the CT ratio by the number of loops (e.g., looping a 100:5 CT twice makes it a 50:5 CT).

How do I size a CT for a 1A secondary system instead of 5A?

1A secondary CTs are used in large substations or solar farms where the CT is located hundreds of feet from the relay. Because VA = I² × R, dropping the secondary current from 5A to 1A reduces the wire burden by a factor of 25. This allows you to use much smaller, cheaper wire over long distances without exceeding the CT's VA burden rating. For DIY or residential panels, stick to 5A secondaries.