Calculate primary or secondary current from a CT ratio such as 500:5 or 2000/5, or find the CT ratio from both currents, with results in A, mA or kA.
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 = CT ratio as a numeric value, such as 100 for a 500:5 CT
- Ip = primary current
- Is = secondary current
Choose Current ratio to divide primary by secondary current, Primary current to multiply the ratio by secondary current, or Secondary current to divide primary current by the ratio. The ideal magnitude model does not account for burden, saturation, ratio error, or phase error.
Common CT Ratios and Secondary Currents
Many current transformers use a 5 A or 1 A secondary rating. The CT ratio is the primary current divided by the secondary current; you can enter it as 500:5 or as 100.
| 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.
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.
The secondary current is 5 A.
FAQ
How do I enter a CT ratio like 500:5?
Type it as written, 500:5 or 500/5, or enter the numeric ratio 100 (500 ÷ 5). The calculator divides the nameplate values for you.
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?
A positive secondary current is required to calculate the current ratio. Zero current is valid in either current mode for the ideal no-load case, but zero/zero cannot determine a ratio. A zero reading does not prove safe isolation. Never open an energized current-output CT secondary; dangerous voltage may result. Follow the manufacturer’s safe procedure.
