Current Transformer Ratio Calculator
Calculate the CT ratio factor and convert between ideal primary and secondary current. Enter the nameplate primary and secondary ratings before the measured current.
Written and checked by TheToolNet Editorial Team · Last reviewed: September 20, 2026 · Editorial method
Calculate current transformer ratio
Current transformer ratio example
A 500/5 CT has a ratio factor of 100. With an ideal measured secondary current of 4 A, estimated primary current is 4 × 500/5 = 400 A. In the reverse direction, 400 A primary corresponds to 4 A secondary.
Scope and safety limitation
This tool performs the ideal nameplate-ratio calculation only. It does not account for CT class, burden, phase error, saturation, accuracy limits, polarity, wiring, relay settings, or transient performance. Protection and metering work must follow the CT, relay, and system manufacturer's documentation and be performed by qualified personnel.
Frequently asked questions
What does 500/5 mean on a CT?
It means the rated primary current is 500 A when the rated secondary current is 5 A, giving a 100:1 factor.
Can I use a 1 A secondary rating?
Yes. Enter the actual nameplate rating; common systems may use 1 A or 5 A secondaries.
Does the ratio guarantee measurement accuracy?
No. Accuracy depends on the transformer's class, burden, operating current, connected equipment, and installation.