A multi ratio current transformer is an instrument transformer with multiple taps on its secondary winding, allowing you to select different current transformation ratios from a single physical device. In a real installation, this changes your metering and protection flexibility, letting you adapt a single CT to different primary load currents or future panel upgrades without swapping hardware. People commonly confuse it with a multi-core CT (which has physically separate secondary windings for metering versus protection) or a dual-ratio CT with independent windings; a true multi ratio current transformer uses a single continuous winding with intermediate tap points brought out to a terminal block.

How Secondary Taps Alter the Turns Ratio

Inside the casing, the secondary coil is wound with a specific number of turns to achieve the maximum nameplate ratio. For a standard bar-type primary (where the primary conductor passes through the center once, making $N_p = 1$), the secondary turns dictate the ratio. If the full winding has 120 turns, the full ratio is 120:1, which translates to 600:5A.

Manufacturers bring out physical wire connections at specific turn intervals along this single coil. By selecting different terminal pairs, you change the active number of secondary turns ($N_s$), which directly alters the transformation ratio according to the fundamental CT equation:

$I_p \times N_p = I_s \times N_s$

Industry Standard: Secondary current ratings are almost universally standardized at 5A or 1A at full rated primary current. 5A is dominant in North American switchgear, while 1A is common in European IEC installations to reduce $I^2R$ losses over long cable runs.

Terminal naming conventions depend on the standard governing the nameplate. Under IEEE C57.13, taps are typically labeled X1, X2, X3, X4, and X5. Under IEC 61869-2, you will see 1S1, 1S2, 1S3, 1S4, and 1S5. The lowest and highest numbers (e.g., X1 to X5) always represent the full winding and the maximum ratio.

Worked Numeric Example: Sizing and Wiring a 5-Tap CT

Let us look at a real-world switchgear scenario to understand tap selection, secondary current output, and the critical accuracy class derating that catches many technicians off guard.

The Scenario: You are metering a 480V main breaker with a continuous primary load of 380A. You have a multi ratio current transformer installed with the following nameplate data:

  • Full Ratio: 600:5
  • Accuracy Class: C200
  • Available Taps: 100:5, 200:5, 300:5, 400:5, 600:5
  • Total Burden (Meter + Wiring): 2.5 VA

Step 1: Select the Optimal Tap
We want the secondary current to be as close to 5A as possible without exceeding it at full load, maximizing the resolution of our 5A metering scale.

  • If we use the 600:5 tap: $380A \times (5 / 600) = \mathbf{3.16A}$ (Only 63% meter utilization).
  • If we use the 400:5 tap: $380A \times (5 / 400) = \mathbf{4.75A}$ (95% meter utilization — Optimal Choice).

Step 2: Calculate the Burden Impedance
With 4.75A flowing on the secondary, the actual burden impedance ($Z$) presented to the CT is:

$Z = \frac{VA}{I_s^2} = \frac{2.5}{4.75^2} = \mathbf{0.11 \Omega}$

Step 3: The Accuracy Class Gotcha (Derating)
The nameplate states C200. This means the CT can deliver 20 times rated secondary current (100A) at 200 volts without exceeding 10% ratio error. However, this rating only applies to the full 600:5 winding. When you use a lower tap, the accuracy voltage drops proportionally to the turns ratio.

For the 400:5 tap, the new accuracy class is:

$C_{new} = 200 \times \left(\frac{400}{600}\right) = \mathbf{C133}$

CRITICAL SAFETY & WIRING RULE: Never open-circuit the active secondary taps. On a multi ratio current transformer, the unused taps (e.g., X3, X4 if you are using X1 and X5) are simply left floating and isolated. However, the two taps you do select must always be connected to a burden (meter/relay) or shorted together with a shorting block. An open-circuited active CT secondary will induce lethal voltages and destroy the insulation.

Where You Meet This in Practice

You will rarely find multi ratio current transformers in residential or light commercial panels. They are the domain of heavy industrial and utility infrastructure:

  1. Medium Voltage Switchgear: In 5kV to 35kV metal-clad switchgear (like Eaton MAG or ABB UniGear), protection relays require precise CT ratios. Multi-tap CTs allow the factory to build standardized breaker cells that the site engineer can configure in the field based on the final load study.
  2. Utility Metering Cabinets: For large commercial services (e.g., 1200A to 4000A), utility metering cabinets use multi-ratio CTs to accommodate future load growth. If a facility expands its transformer capacity, the utility technician can simply move a wire on the terminal block rather than scheduling a crane to lift a new 300-pound CT into the vault.
  3. Solar Inverter Combiner Boxes: In large-scale photovoltaic arrays, the DC/AC output can vary based on panel degradation or future array expansions. Multi-tap CTs on the AC output feeders allow the metering to remain highly accurate across different phases of the solar farm's lifecycle.

Frequently Asked Questions

How do I wire a multi ratio current transformer to a 5A meter?

Identify the terminal pair that gives you the ratio closest to your maximum expected primary load without exceeding the 5A secondary limit. Connect your meter's current coil directly across those two specific terminals (e.g., 1S1 and 1S4 for a 400:5 ratio). Ensure all other unused terminals on that specific winding are left completely unconnected and insulated. Route the secondary wiring through a test switch or shorting block so you can safely isolate the meter for calibration without open-circuiting the CT.

What happens to the accuracy class when I use a lower tap?

The accuracy voltage rating (e.g., C200, C400) drops linearly with the tap ratio. If a CT is rated C400 on its full 1200:5 winding, and you wire it to the 600:5 tap (half the turns), the accuracy class becomes C200. If your protection relay requires a high knee-point voltage to avoid saturation during fault currents, using a lower tap might cause the CT to saturate prematurely, blinding the relay to downstream faults.

Can I parallel two taps on a multi ratio CT to increase capacity?

No. You must never parallel different taps on the same winding. Because the taps represent different physical points along a single continuous coil, connecting them in parallel creates a shorted turn across the section of the winding between those taps. This will cause massive circulating currents, rapid overheating, and catastrophic failure of the transformer. Paralleling is only permissible if you have two entirely separate, identical secondary cores (e.g., Core 1 and Core 2) with the exact same ratio.

Do I need to short the unused taps on a multi ratio current transformer?

No. This is a very common misconception. You only need to short or burden the active tap pair that you are using for your meter or relay. The intermediate, unused taps on a multi ratio current transformer are electrically isolated from your active circuit and should be left unconnected (floating). Taping them off or wrapping them in heat shrink to prevent accidental contact with grounded metal is good practice, but they do not require shorting blocks.