A multi-ratio (MR) current transformer is an instrument transformer featuring a single secondary winding with multiple intermediate wire taps, allowing you to change the primary-to-secondary current ratio without replacing the physical hardware.
What an MR Current Transformer Actually Changes in Your Panel
In a real installation, an MR CT changes the effective turns ratio and the maximum allowable secondary burden impedance, letting you scale the measurement range to match varying load profiles on the same feeder. Instead of swapping out a solid-core or window CT when a facility's load grows, an electrician simply moves the secondary leads to a different terminal pair (e.g., from X1-X3 to X1-X5) on the terminal block.
Beginners often confuse MR CTs with dual-core CTs. A dual-core CT has two physically separate secondary windings (one optimized for high-accuracy metering, one for high-saturation protection). An MR CT has only one continuous winding with access points spliced out at different turn counts. They also get confused with Rogowski coils, which are air-core, flexible, and inherently linear but require active electronic integrators.
The Math: Tap Ratios and Burden Scaling (Worked Example)
The most dangerous trap with an MR current transformer is assuming the rated burden (in ohms or VA) stays the same when you switch to a lower tap. It does not. According to IEEE C57.13 standards, the maximum allowable secondary burden impedance drops with the square of the turns ratio reduction.
The Worked Example:
You have a 600:5A full-winding MR CT (120:1 turns ratio) with a rated standard burden of 2.0 Ω (B-2.0). You need to measure a smaller load, so you wire your relay to the 300:5A tap (60:1 turns ratio).
- Full Winding Turns ($N_{full}$): 120
- Tap Turns ($N_{tap}$): 60
- Full Winding Burden ($Z_{full}$): 2.0 Ω
Formula: $Z_{tap} = Z_{full} \times (N_{tap} / N_{full})^2$
Calculation: $Z_{300} = 2.0 \times (60 / 120)^2 = 2.0 \times 0.25 = 0.5\ \Omega$
Where You Meet MR CTs in Practice
You will rarely see MR CTs on small branch circuits. They are heavily specified in medium-voltage switchgear, main service entrances, and large motor control centers (MCCs).
- Main Service Feeder Breakers: Spec'd as MR so the utility or facility owner can adjust the main breaker trip settings as the building's tenancy changes without buying new CTs.
- Solar Inverter Combiner Boxes: Used on the grid-tie point of common coupling (PCC) to scale metering if the solar array is expanded in phases.
- Generator Paralleling Switchgear: Used to match the CT ratios of mismatched generator sets so the differential protection relays see balanced secondary currents.
Decision Tree: Fixed-Ratio vs. Multi-Ratio (MR) CT Selection
Use this matrix to terminate your design phase with a concrete part specification. Do not default to MR for everything; the extra copper and terminal complexity add cost and failure points.
| Application Scenario | Load Certainty | Required Action | Concrete Pick / Spec |
|---|---|---|---|
| Main Service Entrance (>800A) | Low (future tenant loads unknown) | Spec MR to allow future ratio scaling without busbar modifications. | Pick: Multi-Ratio C200 class, 1200/5 full, tap initially at 800/5. |
| Fixed Motor Starter (e.g., 50HP) | High (nameplate FLA is 65A) | Spec fixed-ratio. MR adds unnecessary burden limitations and cost. | Pick: Fixed-ratio 100/5 window CT, C50 class. |
| Utility Revenue Metering | High (billing accuracy mandated) | Never use partial taps on MR CTs for billing; utilities reject them due to ratio correction factor (RCF) drift. | Pick: Fixed-ratio Revenue Metering CT, 0.3 accuracy class, B0.1 burden. |
| Differential Protection (87) | Medium (matching two different sources) | Use MR CTs on the higher-current side to perfectly match the secondary amps of the lower-current side. | Pick: MR C400 class, tap selected to match the exact secondary current of the opposing CT. |
Common Mistakes: The Partial-Tap Saturation Trap
The most frequent error NETA-certified technicians find during commissioning is an engineer specifying an MR CT, wiring it to a partial tap, and failing to recalculate the knee-point voltage.
The knee-point voltage (the point where the CT core saturates and stops accurately reproducing the primary current) scales linearly with the number of turns. If your 600:5 CT has a knee-point of 400V on the full winding, the 300:5 tap only has a knee-point of 200V. If your relay requires a 250V drive to push fault current through the wiring burden, the 300:5 tap will saturate, clipping the waveform and potentially blinding an overcurrent relay. Always verify the symmetrical fault current against the reduced knee-point voltage of your chosen tap.
FAQ: MR Current Transformer Basics
Can I use multiple taps on an MR CT simultaneously for different relays?
No. An MR CT has a single continuous winding. If you wire a meter to X1-X3 and a relay to X1-X5, you will create a shorted turn on the winding between X3 and X5. This will cause massive circulating currents, overheating, and catastrophic CT failure. You must use a dual-core CT if you need simultaneous, isolated secondary outputs.
Does the physical window size change when I change taps?
No. The primary conductor still passes through the same physical window. The 'ratio' change happens entirely on the secondary side by utilizing fewer turns of the fine magnet wire wrapped around the core.
What is the default rule of thumb if I am unsure?
For any feeder breaker rated 800A or higher where the final load profile is not 100% locked in, always specify a Multi-Ratio (MR) C-class CT with a full-winding rating 20% above the breaker trip setting, and wire it to the tap that matches the breaker's long-time pickup setting.
