Sizing current transformer for monitoring motor current. 2

Suggestion: The CTs must have enough iron to transfer the current proportionally at least to the locked-rotor current. Therefore, 600/5 appears to be more appropriate than 100/5 CT. This rule is usually applied when CTs are used with overload relays, NEMA starter 5 and up. When you monitor current, it depends on the monitoring intent, and the full load current. Perhaps, one may select a smaller ratio than 600/5; however a larger than 100/5.

Off the top of my head I can only remember the data for one motor circuit in our plant (the one I worked on last week). 4KV 700hp motors with approximately 100A FLA. They used 600:5 CT's in that application... I assume they designed it right. Those CT's drove both protection and indication.

It seems like the general considerations for selecting CT should include the following:
- I believe that higher-ratio CT will be more expensive. Therefore select lowest ratio that meets your requirements.
- Continuous current (at the level protected by the circuit) should not exceed the CT rating. From this requirement, if motor is overload protected at 125%, then CT ratio needs to be 125:5 or higher (I think 150:5 is the next higher standard ratio).
- CT saturation should not interfere with protective relay operation or cause misoperation. If CT is for indication only, then you probably don't care. If the CT drives an instantaneous relay for short circuit protection, then you would want to check the available fault current and verify that your CT will perform reasonably. For example a C100 with 1 ohm burden max, you don't want to use a relay with rating of less than 1/20 of the available fault current.
- CT should coordinate with the relay. If you have a specific relay in mind, that might limit your choice of CT's from a standpoint of the current ranges that the relay can accept.

There is a recent post in the electric-power-engineering forum entitled "ct calculation" which covers similar topic with lots of links.

If you are driving an analog current meter (5A input full scale), you'll peg out the meter unless you select approx 600/5 or higher depending on your expected inrush current. See my noncommercial website of motor monitoring/maintenance links at:

Whoops. I added that blurb about my website as a signature. But it gets merged in with the text in a way that I don't like. My website has nothing that addresses
your question.

ps - does anyone know how to delete or modify a post?

Cool web page electricpete!

Suggestion: Visit
for info on CT testing in case that something happen