To size a current transformer (CT) for standard AC power monitoring or protection, select a primary current rating at 125% of your maximum continuous load and a standard 5A secondary. For example, on a 400A continuous feeder, specify a 500:5A CT ratio. For revenue metering, require a 0.3 accuracy class; for protective relaying, specify a C200 or C400 class per IEEE C57.13. Crucially, your total connected wire and meter burden must remain below the CT’s rated Volt-Ampere (VA) limit to prevent core saturation and measurement failure.
How to Read the IEEE C57.13 Current Transformer Sizing Chart
The table below provides baseline sizing parameters derived from IEEE C57.13 standard requirements for instrument transformers. Before selecting a row, understand how the columns interact:
- Primary Current (A): The maximum continuous current the CT is designed to measure on the line side. Always round up to the next standard size above 125% of your actual load.
- Secondary Current (A): The scaled-down output current. 5A is the North American standard for short wire runs; 1A is used for long runs to reduce wire burden.
- Standard Metering Burden (VA): The maximum volt-ampere load the CT can drive while maintaining its stated accuracy. This includes the meter's internal resistance plus the resistance of the connecting wires.
- Accuracy Class (Metering): Defines the percentage error at 100% rated current. A 0.3 class means the reading will be within ±0.3% of true current, which is required for utility revenue metering.
- Relaying Class: The 'C' stands for Calculated (low leakage flux). The number (e.g., C200) is the secondary terminal voltage the CT can deliver at 20 times rated current without exceeding 10% ratio error.
| Primary Current (A) | Secondary Current (A) | CT Ratio | Standard Metering Burden (VA) | Accuracy Class (Metering) | Relaying Class (IEEE C57.13) |
|---|---|---|---|---|---|
| 100 | 5 | 20:1 | 15 VA | 0.3 | C100 |
| 200 | 5 | 40:1 | 15 VA | 0.3 | C100 |
| 400 | 5 | 80:1 | 30 VA | 0.3 | C200 |
| 600 | 5 | 120:1 | 30 VA | 0.3 | C200 |
| 800 | 5 | 160:1 | 45 VA | 0.3 | C400 |
| 1200 | 5 | 240:1 | 45 VA | 0.3 | C400 |
| 2000 | 5 | 400:1 | 45 VA | 0.3 | C800 |
| 3000 | 5 | 600:1 | 45 VA | 0.3 | C800 |
Note: Values represent typical baseline specifications for wound or toroidal CTs. Exact VA burdens and physical window dimensions vary by manufacturer (e.g., Accuenergy, CR Magnetics, Schneider Electric).
Selecting Accuracy Classes: Which Column Applies to Your Installation?
The sizing chart splits into two distinct operational philosophies: metering and protection. Choosing the wrong column results in either destroyed equipment or nuisance tripping.
Metering Columns (Accuracy 0.3 or 0.6): These apply to power quality analyzers, revenue meters, and building management system (BMS) sensors. Metering CTs are intentionally designed to saturate at low overcurrents (typically 120% to 150% of primary rating). This saturation protects the delicate internal shunts of your meter from melting during a short circuit. If you are wiring an IoT energy monitor or a standard digital panel meter, you must use the metering accuracy column.
Protection Columns (Relaying Class C100 - C800): These apply to overcurrent relays and breaker trip units. Protection CTs must do the exact opposite of metering CTs: they must not saturate during massive fault currents. A C400 class CT can push 400 volts across its secondary terminals at 20 times the rated secondary current (100A on a 5A secondary) while keeping the ratio error below 10%. If you use a metering CT for a protective relay, a 10,000A fault will instantly saturate the core, the secondary current will collapse to near zero, and your breaker will fail to trip.
Calculating Real-World Burden and Derating
The 'Standard Metering Burden' column in the chart assumes an ideal installation. In reality, the copper wire connecting the CT to your meter adds resistance, which consumes VA capacity. If your total burden exceeds the chart's base value, the CT derates, accuracy plummets, and the core begins to saturate prematurely.
Burden is calculated using the formula: VA = I² × R, where I is the secondary current (5A) and R is the total resistance of the wire loop plus the meter.
Worked Numeric Example:
You are installing a 400:5A CT (rated for 30 VA) to monitor a 300A chiller. The CT is located in a rooftop disconnect, and the meter is in a basement switchgear room. The one-way wire run is 150 feet.
- Wire Resistance: You pull 14 AWG copper wire. 14 AWG has a resistance of 2.525 ohms per 1,000 feet. The total loop length (out and back) is 300 feet.
Total Wire R = (300 / 1000) × 2.525 = 0.757 ohms. - Wire Burden: VA = 5² × 0.757 = 25 × 0.757 = 18.9 VA.
- Meter Burden: Your digital power meter spec sheet lists an internal burden of 1.5 VA.
- Total Burden: 18.9 VA + 1.5 VA = 20.4 VA.
Because 20.4 VA is well below the 30 VA limit on the chart for a 400A primary, this installation will maintain its 0.3 accuracy class. However, if you had used 16 AWG wire (4.016 ohms/kft), the wire burden alone would jump to 30.1 VA, exceeding the CT's capacity and causing the meter to under-read the chiller's actual power draw.
Derating Fix: If your calculated burden exceeds the chart limit, do not buy a larger primary CT. Instead, switch to a 1A secondary CT (which reduces the I² multiplier from 25 to 1) or increase the wire gauge to 12 AWG or 10 AWG to drop the loop resistance.
What the Standard Sizing Chart Cannot Tell You
While the IEEE C57.13 chart gives you the electrical ratios, it omits three critical physical and thermal parameters you must verify on the manufacturer's specific datasheet before purchasing:
- Physical Window Size: A 2000:5A CT might have a 3-inch window or a 6-inch window. If you are retrofitting a split-core CT over an existing 500 kcmil feeder with thick THHN insulation and a grounding conductor in the same conduit, a standard 3-inch window will not physically close. Always measure the outer diameter (O.D.) of your cable bundle and compare it to the CT's internal window dimensions.
- Thermal Rating Factor (TRF): The chart assumes a 1.0 continuous rating at a standard 30°C ambient temperature. If the CT is installed inside a hot switchgear enclosure where ambient temperatures reach 50°C, you must check the TRF curve. A CT with a 1.33 TRF can safely handle 33% more continuous primary current than its nameplate rating without the insulation degrading, but this is strictly manufacturer-dependent.
- Knee-Point Voltage (IEC vs. IEEE): If you are working with IEC 61869-2 standard equipment (common in European or international solar farms), the chart's 'C-class' relaying column does not directly translate. IEC specifies a 'knee-point voltage' (Vk) where a 10% increase in voltage causes a 50% increase in magnetizing current. You must use the manufacturer's excitation curve to map IEEE C-classes to IEC Vk requirements for differential protection schemes.
