When sizing a metering setup, the current transformer VA rating is the critical limit that prevents your readings from drifting into inaccuracy. A current transformer's VA rating specifies the maximum apparent power its secondary winding can deliver to a connected metering or protection circuit without exceeding its defined accuracy limits. If you exceed this VA limit (known as the 'burden'), the CT core saturates, and a 200A primary current might falsely read as 180A on your display. This rating directly dictates the maximum wire length, wire gauge, and number of daisy-chained meters you can install on the secondary side before your data becomes unreliable.
Standard Current Transformer VA Burden Limits
To understand how much load your CT can handle, we look to the IEEE C57.13 standard, which defines standard burden ratings for instrument transformers. The VA rating is intrinsically linked to the secondary current (typically 5A in North America, or 1A in IEC regions) and the total impedance (Z) of the connected circuit.
The table below maps the standard IEEE burden designations to their actual VA limits and maximum allowable impedance for a standard 5A secondary CT. This is the data you need when cross-referencing a manufacturer's spec sheet against your field wiring.
| IEEE Burden Designation | Max Impedance (Ohms) | Max VA Rating (at 5A) | Typical Application | Max 14 AWG Wire Loop (Approx) |
|---|---|---|---|---|
| B-0.1 | 0.1 Ω | 2.5 VA | Low-burden digital panel meters | ~12 feet |
| B-0.2 | 0.2 Ω | 5.0 VA | Standard residential energy monitors | ~30 feet |
| B-0.5 | 0.5 Ω | 12.5 VA | Commercial power analyzers | ~90 feet |
| B-1.0 | 1.0 Ω | 25.0 VA | Industrial metering, multiple daisy-chained meters | ~200 feet |
| B-2.0 | 2.0 Ω | 50.0 VA | Heavy-duty protection relays | ~420 feet |
Note: Wire loop lengths assume 14 AWG copper at 75°C (approx. 3.14 Ω/1000 ft). Always calculate using the temperature column that matches your panel's ambient operating environment, as copper resistance increases with heat.
Calculating Real-World Burden: A Worked Example
Let's walk through a real-world installation to see how the current transformer VA rating governs your wiring choices. Suppose you are installing a commercial IoT energy monitor (like a Schneider PowerLogic PM8000 or an IoTaWatt) on a 400A feeder using a 400:5A split-core CT.
The CT spec sheet lists an accuracy class of 0.3 (meaning it is accurate to within 0.3% error) at a 15 VA burden. Your goal is to determine if your planned wiring will keep you under this 15 VA limit.
Step 1: Identify the Meter Burden
Modern digital meters have very low internal burdens. Let's assume the IoT energy meter's internal current input draws 0.25 VA at 5A. (Older analog electromechanical meters could draw 2.0 VA or more, which is why legacy panels often required larger CTs).
Step 2: Calculate the Wire Burden
The CT is located in a main distribution panel, and the meter is mounted in a control enclosure 40 feet away. Because the current must travel to the meter and return, the total wire loop length is 80 feet. You plan to use 14 AWG THHN copper wire.
- Resistance of 14 AWG copper at 75°C = 3.14 Ω per 1,000 feet.
- Resistance for 80 feet = (80 / 1000) × 3.14 = 0.2512 Ω.
Now, we calculate the VA consumed by the wire using the formula VA = I² × R:
- Secondary current (I) = 5A
- I² = 25
- Wire VA = 25 × 0.2512 Ω = 6.28 VA.
Step 3: Sum and Verify
Total Circuit Burden = Meter Burden + Wire Burden
Total Circuit Burden = 0.25 VA + 6.28 VA = 6.53 VA.
Where You Meet CT VA in Practice (and Common Confusions)
You will encounter current transformer VA limits most frequently when retrofitting energy monitoring systems, sizing protective relays for motor starters, or designing subpanels for solar inverters. In solar applications, production meters must be highly accurate to comply with utility interconnection agreements, making proper CT burden sizing a legal and financial necessity.
What People Commonly Confuse with VA
- Confusing VA with the Primary Ratio: A '200:5A' rating describes the current transformation ratio, not the burden capacity. You can buy a 200:5A CT with a 2.5 VA rating or a 50 VA rating. The ratio tells you what the CT scales; the VA tells you how far and to what you can connect it.
- Confusing VA with Watts: VA (Volt-Amperes) measures apparent power. The burden of a circuit includes both resistance (wires) and reactance (the inductive coils inside analog meters or relay inputs). While modern digital meters are almost purely resistive (Power Factor ≈ 1.0, making VA ≈ Watts), older electromechanical relays have high inductance. Always size for VA, not Watts.
- Metering vs. Protection CTs: A metering CT (e.g., Class 0.3) is intentionally designed to saturate at high fault currents to protect delicate meters from burning out. A protection CT (e.g., Class 5P20) is designed to resist saturation at 20 times the nominal current so a protective relay can accurately 'see' a short circuit and trip a breaker. Never use a metering CT for a protection relay circuit.
For deeper technical specifications on instrument transformer accuracy classes and burden limits, refer to the Omega Engineering Current Transformer Technical Reference or the comprehensive guides on Electrical Engineering Portal's CT accuracy breakdowns.
FAQ: Sizing and Selecting Current Transformers
Why do some CTs have a 1A secondary instead of 5A?
A 1A secondary is standard in IEC regions and long-distance industrial runs. Because the VA formula squares the current (I²R), dropping the secondary current from 5A to 1A reduces the wire burden by a factor of 25. This allows you to run significantly smaller wire over much longer distances without exceeding the CT's VA rating, though it requires meters specifically configured for 1A inputs.
What happens if I connect a 10 VA burden to a 5 VA rated CT?
The CT will not immediately fail or catch fire, but it will operate outside its guaranteed accuracy class. The core will experience higher magnetic flux density, leading to saturation. In practical terms, your meter will read low, and the error will be non-linear—it might read 95% accurate at 20A, but only 70% accurate at 100A. For utility billing or solar export monitoring, this is unacceptable.
Can I connect two meters to the same CT secondary?
Yes, you can wire multiple meters in series on the same CT secondary, provided the sum of all meter burdens plus the wire burden remains below the CT's VA rating. Remember that wiring in series adds the resistance of every device in the loop. Never wire CT secondaries in parallel, as this will alter the current division and ruin the accuracy of both meters.
