Calculate CT ratio, primary current, or secondary current from any two inputs and find the missing value, with results shown in mA, A, or kA units.
CT Ratio to Current Formula
The CT ratio relates the current on the primary side of a current transformer to the current on the secondary side. The calculator uses amperes as the base unit, then converts the result back to the unit you select.
CT = I_p / I_s
I_p = CT * I_s
I_s = I_p / CT
- CT = CT ratio as a numeric value, such as 100 for a 500:5 CT
- Ip = primary current
- Is = secondary current
If you leave the CT ratio blank, the calculator divides primary current by secondary current. If you leave primary current blank, it multiplies the CT ratio by the secondary current. If you leave secondary current blank, it divides the primary current by the CT ratio.
Common CT Ratios and Secondary Currents
Many current transformers use a 5 A or 1 A secondary rating. The CT ratio entered in the calculator should be the primary current divided by the secondary current.
| Nameplate CT Ratio | Numeric CT Ratio | Primary Current at Rated Secondary | Rated Secondary Current |
|---|---|---|---|
| 100:5 | 20 | 100 A | 5 A |
| 400:5 | 80 | 400 A | 5 A |
| 500:5 | 100 | 500 A | 5 A |
| 1000:5 | 200 | 1000 A | 5 A |
| 1000:1 | 1000 | 1000 A | 1 A |
Current Unit Conversions
| Unit | Equivalent in Amperes | Example |
|---|---|---|
| mA | 0.001 A | 500 mA = 0.5 A |
| A | 1 A | 5 A = 5 A |
| kA | 1000 A | 2 kA = 2000 A |
Example
Example 1: Find primary current
You have a CT ratio of 100 and a secondary current of 4.2 A.
I_p = CT * I_s
I_p = 100 * 4.2 = 420 A
The primary current is 420 A.
Example 2: Find secondary current
You have a primary current of 600 A and a CT ratio of 120.
I_s = I_p / CT
I_s = 600 / 120 = 5 A
The secondary current is 5 A.
FAQ
How do I enter a CT ratio like 500:5?
Enter the numeric ratio, not the full nameplate expression. For a 500:5 CT, divide 500 by 5. The value to enter is 100.
What does primary current from CT ratio mean?
It means the estimated current flowing through the main conductor, based on the CT ratio and the measured secondary current. For example, if a CT has a ratio of 100 and the secondary current is 3 A, the primary current is 300 A.
Can secondary current be zero?
Secondary current cannot be zero when calculating CT ratio, because the formula divides by secondary current. A zero secondary current may mean no load current, an open circuit, a measurement issue, or that the CT is not producing output.
