A current transformer (CT) ratio defines the mathematical relationship between the high primary line current and the standardized, safe secondary output current (typically 5A or 1A) sent to your meter or protective relay. If you are metering a 400A main service panel, the direct answer is to use a 400:5A CT. The secondary current will output exactly 5A when the primary conductor carries 400A, allowing standard 5A-rated metering equipment to read the load safely.
Selecting the correct ratio is only half the battle. The accuracy class, burden rating, and physical window size dictate whether your CT will provide precise revenue-grade metering or reliably trip a protection relay during a fault. Below is the definitive reference for sizing and specifying instrument transformers.
How to Read the Current Transformer Ratio Table
Before selecting a part number from a distributor like CR Magnetics or NK Technologies, you must understand how to read the columns in a standard CT specification sheet. The table below is governed by the IEEE C57.13 Standard Requirements for Instrument Transformers.
- Primary Current (I_p): The maximum continuous line current the CT is designed to measure. This should match or slightly exceed your panel's main breaker rating (e.g., a 400:5 CT on a 400A breaker).
- Secondary Current (I_s): The output current. 5A is standard for short wire runs (under 30 feet). 1A is used for long wire runs to minimize I²R voltage drop and burden on the secondary circuit.
- Accuracy Class: Defines the maximum percentage error at rated current. 0.3 is for revenue metering (high precision). 0.6 is for general sub-metering. C100/C200 are protection classes designed to maintain accuracy during massive fault currents without saturating.
- Standard Burden (VA): The maximum volt-ampere load the secondary circuit (wires + meter) can impose before the CT loses its stated accuracy.
Standard Current Transformer Ratio Table (IEEE C57.13)
The following table lists standard ANSI/IEEE preferred ratios for 60Hz commercial and industrial applications. Source: IEEE C57.13 Standard Requirements for Instrument Transformers.
| Primary Current (A) | Secondary (5A) Ratio | Secondary (1A) Ratio | Typical Accuracy Class (Metering) | Typical Accuracy Class (Protection) | Standard Thermal Rating Factor (TRF) |
|---|---|---|---|---|---|
| 50 | 50:5 | 50:1 | 0.6 / 1.2 | C50 | 2.0 @ 30°C |
| 100 | 100:5 | 100:1 | 0.6 / 1.2 | C50 / C100 | 2.0 @ 30°C |
| 200 | 200:5 | 200:1 | 0.3 / 0.6 | C100 | 1.5 @ 30°C |
| 400 | 400:5 | 400:1 | 0.3 / 0.6 | C100 / C200 | 1.5 @ 30°C |
| 600 | 600:5 | 600:1 | 0.3 / 0.6 | C200 | 1.33 @ 30°C |
| 800 | 800:5 | 800:1 | 0.3 | C200 / C400 | 1.33 @ 30°C |
| 1200 | 1200:5 | 1200:1 | 0.3 | C400 | 1.33 @ 30°C |
| 2000 | 2000:5 | 2000:1 | 0.3 | C400 / C800 | 1.0 @ 55°C |
| 3000 | 3000:5 | 3000:1 | 0.3 | C800 | 1.0 @ 55°C |
| 4000 | 4000:5 | 4000:1 | 0.3 | C800 | 1.0 @ 55°C |
Decision Path: Selecting Your CT Ratio and Class
Use this decision tree to terminate your selection process with a concrete part specification. Do not mix metering and protection classes; they are engineered with different core steel alloys to handle saturation differently.
| Application Goal | Primary Load / Fault | Secondary Wire Run | Concrete Selection (Ratio & Class) | Why This Pick Wins |
|---|---|---|---|---|
| Revenue Metering (Utility billing) | 380A continuous | < 25 feet (12 AWG) | 400:5, Class 0.3, 1.0 VA | Class 0.3 guarantees <0.3% error at 100% load. Core saturates safely during faults to protect the delicate metering chip. |
| Internal Sub-Metering (Tenant billing/trending) | 180A continuous | < 40 feet (12 AWG) | 200:5, Class 0.6, 2.0 VA | Class 0.6 is highly cost-effective for internal tracking where utility-grade precision is not legally mandated. |
| Long-Distance Metering | 380A continuous | 150 feet (12 AWG) | 400:1, Class 0.3, 2.0 VA | 1A secondary reduces I²R wire losses by 25x compared to 5A, preventing burden overload over long runs. |
| Overcurrent Protection (Relay tripping) | 400A load / 15kA fault | 50 feet (10 AWG) | 400:5, Class C200 | 'C' class ensures the CT will NOT saturate during a 15kA fault, allowing the relay to see the true fault current and trip the breaker. |
What the Table Cannot Tell You (Derating & Burden Limits)
A ratio table gives you the baseline electrical parameters, but it cannot account for your specific physical installation. You must manually verify these three edge cases before purchasing:
- Physical Window Size vs. Conductor: A 400:5 CT might come in a 1.5-inch, 3-inch, or 5-inch window. If you are pulling 500 kcmil THHN through a 3-inch window, it physically will not fit. Always cross-reference the CT's inner diameter (I.D.) with the outer diameter of your cable or busbar.
- Thermal Rating Factor (TRF) Derating: The table lists standard TRFs based on a 30°C (86°F) ambient temperature. If you are installing CTs inside a sealed outdoor enclosure in Arizona where ambient temps reach 50°C (122°F), the CT must be derated. A 400A CT with a 1.0 TRF at 55°C might only safely handle 320A continuously in a hot environment. Check the manufacturer's derating curve.
- Calculated Burden vs. Rated Burden: The table assumes your total secondary burden is within the VA rating. You must calculate your actual burden: Burden (VA) = I_s² × (R_wire + R_meter + R_connections). If your 5A secondary circuit has 1.5 ohms of total resistance, your burden is 37.5 VA. If your CT is only rated for 2.0 VA, it will saturate prematurely, and your meter will read low. (For deep-dive burden calculations, refer to this guide on CT accuracy and burden limits).
Quick-Jump Reference for Common Panel Upgrades
For electricians and controls integrators doing standard commercial retrofits, here are the most frequently queried quick-pick specifications. Bookmark these rows for your next panel upgrade:
- 200A Residential/Light Commercial Main: Pick 200:5A, Class 0.6, Split-Core (for easy retrofit without disconnecting main lugs). Window size: 1.5" minimum.
- 400A Commercial Switchboard: Pick 400:5A, Class 0.3, Solid Core (if installing during rough-in) or Split-Core (if retrofitting). Ensure 1.0 VA burden if using a modern solid-state power meter.
- 800A Main Distribution Panel: Pick 800:5A, Class 0.3. If running secondary wires to a switchgear lineup over 50 feet away, switch to 800:1A to eliminate wire burden errors.
- 1200A Utility Service Entrance: Pick 1200:5A, Class 0.15 or 0.3 (Revenue grade). Must be utility-approved solid-core ring type, typically mounted directly on the utility meter socket or main breaker bus.
By matching your primary load to the exact ratio, selecting the correct accuracy class for your application (metering vs. protection), and verifying your physical and burden constraints, you ensure accurate data and reliable fault clearing. Always default to a 1A secondary when wire runs exceed 40 feet, and never use a metering-class CT for a protection relay circuit.
