A bar-type current transformer is a solid, single-piece toroidal or rectangular magnetic core with a fixed primary busbar passing directly through its center, used to step down high AC currents into measurable, isolated secondary signals. In a real installation, this component changes a dangerous, unmeasurable 400A+ primary circuit into a safe, standardized 5A or 1A secondary loop that standard digital multimeters, protective relays, and energy loggers can read without breaking galvanic isolation. Beginners frequently confuse bar-type (solid core) CTs with split-core CTs (which hinge open for retrofits) or wound-type CTs (which have physical primary wire windings rather than a single pass-through busbar).
Where You Meet Bar-Type CTs in Practice
You will almost exclusively encounter bar-type current transformers in main distribution switchboards, Motor Control Centers (MCCs), and utility revenue metering cabinets. Because the magnetic core is a continuous, unbroken loop of grain-oriented silicon steel, these CTs offer superior magnetic efficiency. This translates to higher accuracy classes—typically 0.3% or 0.15% for revenue metering—at a lower manufacturing cost compared to split-core equivalents.
You specify a bar-type CT when a panel is being built from scratch, or when a busbar section is completely de-energized and dismantled for an upgrade. The primary conductor (the 'bar') is physically threaded through the CT window before the busbar is bolted to the adjacent sections. If you are doing a retrofit on a live panel or an existing fixed busbar, you cannot use a bar-type CT; you must use a split-core CT that clamps around the existing conductor.
Worked Example: Sizing and Burden Calculation for a 400A Feeder
Let us size and verify the burden for a 400A main feeder to ensure our CT will not saturate and will maintain its stated accuracy. We will assume an operating temperature of 75°C inside the switchgear, which increases wire resistance compared to standard 20°C tables.
- Primary Current: 400A
- CT Ratio: 400:5
- Secondary Current at Full Load: 5A
- Secondary Wire: 12 AWG THHN, 40 feet total loop (20 ft out, 20 ft back to the meter)
- Meter Burden: AccuEnergy AcuRev meter rated at 0.15 VA
Step 1: Calculate Wire Resistance at 75°C
The resistance of 12 AWG copper at 75°C is approximately 1.98 ohms per 1,000 feet. For a 40-foot round-trip loop:
R_wire = (1.98 / 1000) * 40 = 0.0792 ohms
Step 2: Calculate Wire Burden (VA)
Burden is the power consumed by the secondary circuit, calculated as I²R.
VA_wire = 5² * 0.0792 = 1.98 VA
Step 3: Calculate Total Circuit Burden
Total VA = VA_wire + VA_meter = 1.98 + 0.15 = 2.13 VA
Step 4: Verify Against CT Rating
A standard 400:5 bar-type CT is typically rated for 15 VA or 30 VA at 60Hz. Our total burden of 2.13 VA is well within the 15 VA limit. The CT will easily maintain its 0.3% accuracy class without saturating. If the total VA had exceeded the CT rating, we would need to step up to 10 AWG wire or switch to a 1A secondary CT to reduce I²R losses.
Decision Path: Choosing the Right CT Topology
Selecting the wrong physical CT style is the most common cause of delayed panel build-outs. Use this decision tree to lock in your hardware specification.
| Installation Scenario | Primary Current | Panel Status | Required CT Type |
|---|---|---|---|
| New switchgear build or full busbar replacement | > 50A | De-energized / Unassembled | Bar-Type (Solid Core) |
| Retrofit metering on existing infrastructure | > 50A | Live or Fixed Busbar | Split-Core |
| Monitoring small branch circuits or control wiring | < 50A | Any | Wound-Type |
| Long wire runs to remote relay cabinets (>50ft) | Any | Any | 1A Secondary (Bar or Split) |
The Concrete Pick: For new 400A switchgear build-outs, specify a solid bar-type CT like the CR Magnetics CR4500 series (specifically the CR4500-400-5) or an equivalent 400:5, 15VA, 0.3% accuracy solid core. Do not default to split-core CTs for new builds; you will pay a 30-50% premium and sacrifice a fraction of your accuracy for a hinged mechanism you do not need.
Installation Realities: Window Sizing and Lethal Polarity Risks
When threading a busbar through a bar-type CT, physical clearance and electrical polarity are your primary failure points.
Window Sizing and Thermal Clearance: The CT window must accommodate the physical dimensions of the busbar plus a minimum of 0.5 inches of clearance on all sides. This is not just for physical fit; busbars expand under thermal load, and tight clearances can trap heat, degrading the CT's epoxy or tape insulation over time. If you are using multiple copper strips per phase (e.g., two 1/4" x 4" bars to carry 800A), ensure the combined stack fits the window with spacing.
Never open the secondary circuit of a current transformer while primary current is flowing. Without a secondary load to create a counter-magnetomotive force, the entire primary current acts as an excitation current. This drives the core into deep saturation, inducing voltage spikes of 2,000V to 10,000V across the open secondary terminals. This will instantly destroy connected meters, arc across terminal blocks, and can be fatal to anyone touching the wiring. Always short the secondary terminals (X1 to X2) before removing a meter.
Polarity Markings: Bar-type CTs use H1/H2 for primary and X1/X2 for secondary. H1 must face the power source, and H2 must face the load. On the secondary side, X1 connects to the meter's H1 (or I1) terminal, and X2 connects to H2 (or I2). Reversing polarity will not damage the equipment, but it will cause digital power meters to read negative kW and negative power factor, rendering energy logging useless.
Frequently Asked Questions
Can I use a 600:5 bar-type CT on a 400A breaker to leave room for future upgrades?
You can, but it is a poor engineering choice. At a 400A load, a 600:5 CT is only operating at 66% of its capacity. More importantly, at light loads (e.g., 50A), the CT is operating at roughly 8% of its rated primary current, where accuracy drops significantly. Always match the CT primary rating to the breaker trip rating or the maximum expected continuous load. If you upgrade the breaker later, swap the CT at the same time.
Why do some bar-type CTs have a rectangular window instead of a round one?
Default Recommendation: If you are designing a new panel or replacing a bus section, default to a solid bar-type CT sized exactly to your breaker frame rating (e.g., 400:5 for a 400A trip), using 12 AWG secondary wiring for runs under 50 feet, and terminating into a 5A-rated digital meter. Reserve split-core CTs strictly for live retrofits where de-energizing the main bus is impossible.
