Transformers and Power Supplies
Current Transformer Ratio Calculator
Derive the nameplate-style CT ratio from primary and secondary current ratings; this page finds the ratio rather than converting a live meter reading.
Enter both current ratings
Use current values stated on the same rating basis. A 1000 A primary and 5 A secondary produce a 200:1 ratio.
Ratio calculation
The sample calculation is 1,000 A ÷ 5 A = 200. The equivalent ratio is therefore 200:1, also commonly written as 1000:5.
Both expressions describe the same proportional relationship. The equivalent-to-one form is useful for arithmetic, while the rated-current form preserves the nameplate values.
What this page calculates—and what it does not
This calculator answers a rating question: how many primary amperes correspond to one secondary ampere? It does not accept a meter reading and return primary current.
When the CT ratio is already known and a secondary reading must be converted, use the Current Transformer Reading Calculator. Keeping the two directions separate avoids treating the ratio as though it were another current measurement.
Rated secondary current matters
A CT marked 1000:5 has a 200:1 ratio. A CT marked 1000:1 has a 1000:1 ratio. Those devices may represent the same primary rating, but they do not deliver the same secondary current and should not be substituted in a calculation without changing the ratio.
Use the rating associated with the actual secondary circuit. Do not infer 5 A merely because it is common; low-power and electronic systems may use a 1 A rating or another specified output.
Accuracy is more than the nominal ratio
The arithmetic ratio does not describe ratio error, phase error, accuracy class, saturation, burden, polarity, frequency range, or transient performance. These characteristics determine whether the CT is suitable for metering or protection.
Connected lead and instrument load can be examined separately with the Current Transformer Burden Calculator. A correct nominal ratio does not compensate for excessive burden or operation outside the rated class.
Check a changed rating
If the primary rating changes from 1000 A to 800 A while the secondary rating remains 5 A, the equivalent ratio becomes 160:1. If instead the secondary rating changes from 5 A to 1 A, the ratio becomes 1000:1.
Change one rating at a time and record the original nameplate expression. That makes it clear whether a different result came from the primary rating, secondary rating, or an incorrect unit conversion.
Installation and safety boundary
Never open-circuit the secondary of an energized current transformer. This calculator cannot verify wiring, polarity, grounding, shorting arrangements, insulation, or safe work procedures.
Selection and installation must follow the equipment documentation and applicable electrical requirements. The calculated ratio is only one specification among several.
Questions about CT ratios
Is 1000:5 the same as 200:1?
Yes. Dividing both sides of 1000:5 by 5 gives the equivalent ratio 200:1.
Can I enter milliamperes?
Yes, if both entries use the same current unit. Mixing amperes and milliamperes without conversion produces the wrong ratio.
Does this calculate the actual primary current?
No. It calculates a nominal ratio from two ratings. Use the reading calculator when secondary current is measured and primary current is the unknown.