Current transformer taps are accessible connection points on a multi-ratio CT's secondary winding that allow installers to select different turns ratios to match varying primary current ranges and relay requirements. Unlike fixed-ratio CTs, a multi-ratio unit features a single continuous secondary winding with multiple wire leads brought out to a terminal block. Selecting a different tap changes two critical parameters in your protection or metering circuit: the secondary current output for a given primary load, and the effective accuracy class (C-class voltage rating) of the transformer.

Understanding how to read, select, and safely terminate these taps is the difference between a relay that trips accurately during a fault and one that fails due to core saturation. Below is the definitive guide to configuring multi-ratio CTs in the field.

The Core Function of Current Transformer Taps

The most common multi-ratio CTs in North American switchgear and motor control centers are built to the IEEE C57.13 standard. The standard defines a baseline full-winding ratio (usually 600:5) and a C-class accuracy rating (like C800). When you move down the terminal block to a lower tap, you are using fewer secondary turns. This increases the secondary current for a given primary load, but it proportionally reduces the CT's ability to drive current through high-burden relay circuits without saturating.

The Golden Rule of CT Taps: The effective C-class voltage rating scales linearly with the turns ratio. If you tap down to 50% of the full winding, your C-class rating drops to 50%.

Here is the standard tap chart for a typical C800, 600:5 multi-ratio window-type CT. Keep this table handy when setting up protection relays like the SEL-751 or GE Multilin series.

Tap Designation Ratio Secondary Turns Effective C-Class Rating Max Continuous Thermal Current
X1 - X2 100:5 20 C133 500A primary
X1 - X3 200:5 40 C266 500A primary
X1 - X4 300:5 60 C400 600A primary
X1 - X5 400:5 80 C533 800A primary
X1 - X6 500:5 100 C666 1000A primary
X1 - X7 600:5 (Full) 120 C800 1200A primary

Worked Example: Calculating Ratio and Burden Across Taps

Let's run the numbers on a real-world installation. You are commissioning a 400A feeder circuit protected by a microprocessor relay. The relay requires a 5A nominal secondary input for optimal resolution. The switchgear contains a C800, 600:5 multi-ratio CT.

Scenario A: Correct Tap Selection (X1 - X5)

  • Primary Load: 400A
  • Tap Used: X1 - X5 (400:5 ratio)
  • Secondary Current: 5.0A exactly.
  • Effective Accuracy Class: (80 turns / 120 turns) * 800V = C533.
  • Result: The relay sees perfect nominal current. The CT can push 100A of secondary fault current (20x rated) through up to 5.33 ohms of burden (wiring + relay impedance) before exceeding the 10% ratio error threshold.

Scenario B: Incorrect Tap Selection (X1 - X3)

  • Primary Load: 400A
  • Tap Used: X1 - X3 (200:5 ratio)
  • Secondary Current: The CT attempts to push 10.0A into a 5A relay input.
  • Effective Accuracy Class: (40 turns / 120 turns) * 800V = C266.
  • Result: The relay's input transformers may saturate or the CT core will heavily saturate during a fault. Because the C-class rating dropped to C266, the CT can only handle 2.66 ohms of burden at 100A secondary. If your wire run is long (e.g., 12 AWG wire adding 1.5 ohms), the CT will saturate well before the 20x fault current mark, causing the relay to under-read the fault and delay tripping.
Safety & Code Caveat: Always verify the CT nameplate before wiring. Assuming a CT is 600:5 when it is actually a 300:5 fixed-ratio unit will result in secondary currents double what the relay expects, potentially damaging the relay's input shunts. De-energize and lock out the primary circuit, and verify the secondary is properly shorted before altering tap connections on an energized system.

Where You Meet This in Practice (and Common Wiring Mistakes)

You will encounter multi-ratio CT taps primarily in medium-voltage switchgear, utility metering cabinets, and large motor control centers (MCCs). They are heavily used by utility companies to standardize inventory; instead of stocking 100:5, 200:5, and 400:5 CTs, they stock 600:5 multi-ratio units and tap them in the field as needed.

However, this flexibility introduces a specific, dangerous wiring mistake that catches even experienced technicians off guard.

The 'Unused Tap' Shorting Myth

The most common confusion with multi-ratio CTs is mixing them up with dual-secondary CTs. A dual-secondary CT has two completely isolated windings on the same core (e.g., Core 1 for metering, Core 2 for protection). If you are not using Core 2, you must short its terminals to prevent lethal open-circuit voltages.

Multi-ratio taps are not separate windings. They are a single continuous wire. If you wire your relay to X1 and X4, the wire extending from X4 to X5, X6, and X7 is simply the unused tail of the same winding.

Critical Hazard: Never short the unused taps on a multi-ratio CT. If you are using X1-X4 and you place a shorting jumper across X5-X6 'just to be safe', you are creating a shorted turn on the transformer's magnetic core. This will induce massive circulating currents in that specific segment of the wire, leading to localized boiling of the insulation, core overheating, and eventual catastrophic CT failure. Cap and tape the unused taps; leave them open.

Open-Circuit Risks During Tap Changes

When changing taps on an energized CT (which should only be done using specialized shorting blocks or test switches), the circuit must never be left open. An open secondary on a CT with primary current flowing will step up the voltage to thousands of volts, arcing across the terminal block and posing a fatal electrocution hazard. Always use a NETA-certified test switch that automatically shorts the CT secondary before breaking the connection to the relay.

FAQ: Troubleshooting CT Tap Configurations

Why is my relay reading 2.5A when the primary load is 300A?
Check your tap. If the CT is a 600:5 full winding but you are wired to X1-X7 (600:5), a 300A primary will yield exactly 2.5A secondary. You need to move your wiring to the X1-X4 (300:5) tap to get a 5A secondary output, and update the CT ratio setting in your relay's software configuration to match.

Can I use X2 and X5 instead of X1 and X4 to get a 300:5 ratio?
No. Standard IEEE C57.13 multi-ratio taps are almost universally referenced from X1. The turns between X2 and X5 do not equal the turns between X1 and X4. Always use X1 as your common reference point unless the manufacturer's specific nameplate diagram explicitly states otherwise.

Does tapping down affect the CT's continuous thermal rating?
Yes. As shown in the table above, the continuous thermal current rating (the maximum primary current the CT can handle continuously without overheating the secondary wire) drops on the lower taps. The X1-X2 (100:5) tap uses very thin wire to achieve 20 turns in the window. Pushing 600A primary through that tap will overheat the secondary winding, even if the core doesn't saturate.

How do I test the ratio of a specific tap in the field?
Use a CT analyzer (like a Doble or Megger unit) to perform a turns-ratio test. Inject a known voltage into the secondary tap (e.g., X1-X4) and measure the induced voltage on the primary conductor (or vice versa). Verify the measured ratio matches the nameplate within 1-2% before putting the relay into service.