A CT ratio is the relationship between the electrical current flowing through a power line and the smaller, proportional current a current transformer (CT) produces on its output side. It’s expressed as two numbers separated by a colon, like 300:5, meaning 300 amps on the power line produces 5 amps on the measurement side. This scaled-down current lets meters, monitors, and protective devices safely measure high currents without being directly connected to them.
How the Ratio Works
A current transformer has two sides: the primary, which carries the full load current of the circuit being measured, and the secondary, which outputs a much smaller current to connected instruments. The CT ratio tells you exactly how these two sides relate. A 300:5 CT, for instance, is rated so that when 300 amps flow through the primary, exactly 5 amps come out of the secondary.
The relationship is linear and proportional. If only 150 amps flow through that same 300:5 CT, the secondary output drops to 2.5 amps. If 60 amps flow through, you get 1 amp out. This proportionality is what makes CTs useful: you can always multiply the secondary reading by the ratio to know the actual current on the power line.
The math is straightforward. Divide the primary current by the secondary current, and you get the transformation factor. For a 100:5 CT, that’s 100 divided by 5, giving a factor of 20. Every amp measured on the secondary side represents 20 amps on the primary side.
Why 5A and 1A Are the Standard Outputs
Almost every CT you’ll encounter has a secondary rated at either 5 amps or 1 amp. The 5A secondary is far more common and pairs well with most measurement devices, which tend to have their highest accuracy class at 5A inputs. A 1A secondary is the better choice when the measurement equipment sits far from the CT, because lower current means less energy lost in the wiring over long cable runs. So you’ll see ratios like 200:5, 800:5, 1500:1, or 2000:1 depending on the installation.
Multi-Ratio CTs and Tap Selection
Many current transformers don’t lock you into a single ratio. Multi-ratio CTs include several wiring taps on the secondary side, letting you choose the ratio that fits your application. A single physical CT rated at 600:5 might offer ten different ratio options by connecting different tap combinations.
This works because each tap corresponds to a specific number of wire turns inside the transformer. To get a 150:5 ratio from a 600:5 multi-ratio CT, you divide 150 by 5 to get 30, then find which pair of taps gives you 30 turns. Connect the secondary leads to those two taps, leave the rest open, and the CT now behaves as a 150:5 unit. This flexibility is especially valuable in protective relay applications where the required ratio may change if the system is reconfigured.
Choosing the Right Ratio
Selecting a CT ratio starts with knowing the maximum current you expect on the primary side and what you’re using the CT for. The general guideline is to pick a ratio just above your expected load current. If a circuit normally carries 1,154 amps, you’d select a 1,250:5 CT rather than a 1,000:5, which would be too small, or a 2,000:5, which would waste accuracy at the low end of the range.
The intended purpose also matters. CTs used for metering need to be highly accurate across a wide range of normal operating currents, so they’re designed to saturate quickly above their rated range. This protects delicate measurement instruments from dangerously high currents during a fault. Protection CTs serve the opposite purpose: they need to faithfully reproduce current signals even during fault conditions, when current can spike to many times the normal level. A protection CT is built to avoid saturation so that circuit breakers and relays get accurate information exactly when it matters most.
Some installations need a CT that handles both roles, which requires careful compromise between metering precision and fault-current performance.
Accuracy Classes and What They Mean
No CT reproduces its ratio perfectly. The accuracy class tells you how close the secondary output will be to the ideal value. A CT rated at accuracy class 0.3 is guaranteed to be within 0.3% of its stated ratio at full rated current, as long as the load on the secondary side stays within specified limits.
That secondary load is called the “burden,” and it includes the resistance of the wiring, meters, and any other devices connected to the CT’s output. Every accuracy rating comes with a maximum allowable burden. A CT rated 0.3B0.1, for example, maintains its 0.3% accuracy only if the total secondary burden stays below 0.1 ohms. Exceed that burden, and the ratio error grows beyond the rated class.
Protection CTs have wider error tolerances because their job is to work under extreme conditions rather than provide precise readings. Under IEEE standards, a protection CT with a C100 accuracy class will keep its ratio error under 10% at currents up to 20 times rated secondary current with a standard 1.0 ohm burden. The “C” designation means the CT’s performance can be predicted from its published characteristics, while a “T” designation means accuracy has to be verified by physical testing. IEC standards use a similar but differently labeled system, with classes like 5P and 10P, where the number indicates the maximum percentage error and “P” stands for protection.
Why an Open Secondary Is Dangerous
One critical safety rule with current transformers: never leave the secondary circuit open while current flows through the primary. Under normal operation, the secondary current creates a magnetic field that opposes and balances the primary’s magnetic field. If the secondary circuit is broken, that balancing force disappears. All the primary current’s energy drives the transformer core into heavy magnetic saturation, and the CT responds by generating extremely high voltage spikes on the secondary terminals.
These voltages can reach several thousand volts, arriving as short pulses lasting less than a millisecond, repeating every half cycle of the power frequency. That’s enough to break down insulation, destroy connected instruments, and create a serious shock or arc-flash hazard. In nuclear power plant evaluations, engineers found that open-circuit voltages from large CTs (rated 40,000:5) could exceed the insulation ratings of connected equipment and potentially damage safety-related instrumentation. This is why CTs are always short-circuited across their secondary terminals before any connected device is removed.