CT Ratio Calculator
Calculate the CT secondary current from the primary current and the CT ratio, or find the correct CT ratio for your system. Includes formulas, tables, and examp...
ct secondary current from primary
div The CT ratio (Primary/Secondary) shows how high primary current can be safely cut to a normal secondary (1 A or 5 A). Calculate with the following formula: \[ I s = I p X S sec /P ct \] where I p is the current of primary, P ct is the primary rating of the CT, and S sec is the secondary rating of the CT. The right CT ratio is essential to maintain use of correct measurements and shielding of electrical systems.
Formula & Table Summary:
- Secondary Current: I_s = I_p × (S_sec / P_ct)
- Required CT Primary: P_ct = I_p × (S_sec / I_s)
- Primary-Equivalent from Measured Secondary: I_p_eq = I_s × (P_ct / S_sec)
Converter Table (Input → Output):
| Input | Formula | Output |
|---|---|---|
| CT ratio & Primary Current | I_s = I_p × (S_sec / P_ct) | Secondary Current |
| Primary & Desired Secondary | P_ct = I_p × (S_sec / I_s) | Required CT Primary |
| Measured Secondary & CT Ratio | I_p_eq = I_s × (P_ct / S_sec) | Primary Equivalent |
current transformer ratio formula
High primary current Current Transformer (CT) ratios specify the safe scaling of high primary current to standard secondary current (typically 1 A or 5 A) to use as a meter and protection. CT ratios are expressed as primary / secondary e.g. 400/5. To compute secondary current: product of the primary current and secondary rating/CT primary rating. The key part in selecting the correct CT ratio is to make sure that the primary rating is at least the maximum expected primary current and that the secondary has the standard ratings. It has formulae and converter tables, sample calculations, keywords that are low competition, image metadata, frequently asked questions as a JSON file, ready-to-use TailwindCSS calculator snippet to make instant evaluations. These resources simplify computation of CT ratios, make them accurate and accessible to both electrical practitioners as well as to students.
ct primary secondary conversion
| Scenario | Input | Calculation | Result |
|---|---|---|---|
| Example A | CT 400/5, I_p = 200A | I_s = 200 × 5 / 400 | 2.5 A |
| Example B | CT 800/5, I_p = 300A | I_s = 300 × 5 / 800 | 1.875 A |
| Choose CT | I_p = 120A, desired I_s = 5A | P_ct = 120 × 5 / 5 = 120 → next standard | 150/5 recommended |
| Back-Calc | Measured I_s = 4A, CT 400/5 | I_p_eq = 4 × 400 / 5 | 320 A |
Frequently Asked Questions - CT Ratio Calculator:
What does a CT ratio mean?
CT ratio Primary/Secondary shows how primary current is stepped down to a standardized secondary (e.g., 400/5).
How to get CT secondary current?
I_s = I_p × (S_sec / P_ct). Example: CT 400/5, I_p 200A → I_s = 2.5A.
What are common CT secondary ratings?
1 A and 5 A.
How to choose CT primary rating?
Pick CT primary ≥ expected max primary current and standard secondary (1/5A). Round up to next standard.
How to compute primary-equivalent from measured secondary?
I_p = I_s × (P_ct / S_sec).
Why choose 1A vs 5A secondary?
1A reduces burden current and wiring size; 5A is common for protection circuits.
What happens if CT is overloaded?
CT can saturate and give inaccurate readings; never open-circuit secondary under load.
Can CT ratio be fractional?
CT ratios are typically whole numbers; fractional secondary currents arise if primary current isn’t an integer multiple.
How to handle multiple primary turns?
Effective primary = N×current; formula applies using effective primary current.
Where to find CT standard sizes?
Refer to CT manufacturer catalogs; common primaries: 5,10,20,50,100,150,200,400,800 A etc.
