Bar Type Current Transformer Market: Theory and Sizing for Switchgear

A bar-type current transformer is a measurement device where the primary "winding" is simply the solid copper busbar or cable passing directly through the center of a toroidal magnetic core, stepping down high AC currents to a safe, measurable level. Unlike wound-type CTs that have physical primary coil terminals, the bar-type relies on the existing conductor as its primary. This changes a real installation by eliminating series resistance and heat generation at the measurement point, allowing for compact, high-current metering without introducing voltage drop. People commonly confuse bar-type CTs with split-core (clamp-on) CTs; while both slip over a wire, a true bar-type CT usually features a solid, continuous iron core requiring the busbar to be threaded through it during assembly, offering vastly superior accuracy and saturation limits.

Where You Meet This in Practice

You will find bar-type CTs bolted directly onto busbars inside industrial switchgear, motor control centers (MCCs), and main distribution boards. Because the primary conductor is just the busbar itself, these CTs handle massive continuous currents without the thermal bottlenecks of wound primary coils.

When evaluating the bar type current transformer market for modern electrical infrastructure, the dominant drivers are retrofitting aging 480V/3-phase switchgear and building high-density EV fast-charging depots. These applications require precise 800A to 3000A metering in extremely tight physical footprints. Solid-core bar types remain the gold standard here because their continuous magnetic grain orientation minimizes hysteresis losses, keeping the 5A secondary standard perfectly linear even during heavy motor starting inrushes.

Worked Numeric Example: Sizing a 400A Feeder

Let us size a bar-type CT for a new 400A, 480V 3-phase feeder feeding a CNC machine shop. We need to select the ratio, accuracy class, and burden rating.

• Primary Current: 400A maximum continuous load.
• Selected CT Ratio: 400:5. (At 400A primary, the secondary outputs exactly 5A).
• Meter Burden: The digital power meter requires 2.0 VA.
• Wire Run: 25 feet of 12 AWG copper from the CT to the meter (50 feet round-trip).

First, calculate the wire resistance. 12 AWG copper is approximately 1.588 ohms per 1,000 feet. For a 50-foot round trip, the resistance is 0.079 ohms. The burden imposed by the wire is calculated using P = I²R: Wire Burden = (5A)² × 0.079Ω = 1.975 VA.

Total circuit burden = Meter Burden (2.0 VA) + Wire Burden (1.975 VA) = 3.975 VA.

According to NEMA C57.13 standards, standard CT burden ratings (at 5A) are 2.5, 5.0, 10.0, and 20.0 VA. Since our total calculated burden is 3.975 VA, we must select a CT with a minimum rating of 5.0 VA. For revenue-grade metering, specify an accuracy class of 0.5 or 0.2s. If this were purely for overcurrent protection relays, we would specify a 5P or 10P protection class instead.

Real-World Scenario Walkthrough: The Open-Circuit Arc Flash

The most dangerous mistake you can make with any current transformer is leaving the secondary open while primary current is flowing. Here is how a simple meter swap turns into a catastrophic failure.

The Setup: An electrician is upgrading an analog ammeter to a digital power analyzer on a 600A main breaker panel. The existing 600:5 bar-type CT is left in place on the busbar. The panel is energized, and the 600A load is actively running.

The Numbers: Under normal operation, 600A on the primary induces 5A on the secondary. The secondary current creates a magnetic flux that perfectly opposes and cancels the primary flux in the iron core. The net flux in the core is very low, keeping the induced secondary voltage to just a few volts (V = IR, roughly 5A × 1Ω burden = 5V).

What Went Wrong: The electrician disconnected the secondary wires from the old meter before installing the new one, creating an open circuit. With infinite resistance on the secondary, secondary current drops to zero. Without secondary ampere-turns to oppose the primary, the full 600A primary current acts as pure magnetizing current. The core instantly saturates.

The Outcome: The massive, unopposed flux swings induce a voltage spike governed by Faraday's law (V = N × dΦ/dt). The secondary voltage rockets from 5V to over 3,000V. An arc flash erupts across the open terminal block, melting the 12 AWG wire insulation, destroying the terminal block, and permanently magnetizing the CT core (ruining its accuracy). The electrician narrowly avoids severe burns.

1. Locate the CT shorting block mounted between the CT and the meter.
2. Insert the shorting knife or jumper to bridge the secondary terminals.
3. Verify the short is secure, then safely disconnect the meter wires.
4. Wire the new meter, and only then remove the shorting jumper.

Navigating the Bar Type Current Transformer Market in 2026

When sourcing components, understanding current market shifts prevents you from buying the wrong spec. According to application guides from manufacturers like Hammond Power Solutions, the physical window size is the most common point of failure in procurement. A 400:5 CT might have the right electrical specs, but if the busbar is 4 inches wide and the CT window is only 3 inches, the job stops.

Furthermore, the market is shifting toward 1A secondary standards for large campuses. Pushing 5A over 200 feet of wire requires thick, expensive 10 AWG cable to keep the burden under the CT's VA rating. By specifying a 400:1 CT (1A secondary), the wire burden drops by a factor of 25 (since P = I²R), allowing the use of cheap 14 AWG wire over long distances without saturating the core.

Frequently Asked Questions