CT's
- Thread starter raven
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Should not be an issue. Either will read 400a correctly.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
That depends upon your equipment requirements.
In general, a CT simply produces an output current proportional to the sensed current. This output current is fed to your metering equipment. If you need to measure a 400A circuit but have an 800A CT, then the output current will be half of full scale. If your metering equipment is happy with this, then the setup is fine. I would expect that using an 800A CT for a 400A circuit will result is reduced accuracy of measurement, but this may not be significant for your application.
-Jon
In general, a CT simply produces an output current proportional to the sensed current. This output current is fed to your metering equipment. If you need to measure a 400A circuit but have an 800A CT, then the output current will be half of full scale. If your metering equipment is happy with this, then the setup is fine. I would expect that using an 800A CT for a 400A circuit will result is reduced accuracy of measurement, but this may not be significant for your application.
-Jon
- Location
- New Jersey
- Occupation
- Journeyman Electrician
If the 800 amp CT's are already installed then there is no problem. They can read up to 800 amps.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I think CTs are generally applicable down to about 6-10%. So as long as you don't care much about light loads, you should read okay in the range of 80A and higher.
It matters on some relays. Often you can only set the pickup setting down to about halfpenny 2.5 A on a 5 A CT. So metering is one thing but for protection you may want to size closer. I’ve been burned by this more than once.
With 4-20 mA style metering CTs don’t forget that the size is the limit of what you can measure. So a 400 A CT reads 20 mA for 400 A on more. It’s generally a good idea on these to go over so 600 or 800 A would be the better choice.
Finally on wide spans sometimes you need two sets. For instance on one of our motor test stands which can do up to around a 4000 HP motor it has a CT rated 200:5 and one rated 2000:5. The panel checks the current on the high range CT and then uses the low range value if it’s under 200’A. This gives it high accuracy from around 10-20 A to probably 4000 A.
I have seen some equipment use multi tap CTs from GE. You can use just one CT with several ranges depending on how you wire it so one CT can go from say 100:5 all the way to 800:5.
With 4-20 mA style metering CTs don’t forget that the size is the limit of what you can measure. So a 400 A CT reads 20 mA for 400 A on more. It’s generally a good idea on these to go over so 600 or 800 A would be the better choice.
Finally on wide spans sometimes you need two sets. For instance on one of our motor test stands which can do up to around a 4000 HP motor it has a CT rated 200:5 and one rated 2000:5. The panel checks the current on the high range CT and then uses the low range value if it’s under 200’A. This gives it high accuracy from around 10-20 A to probably 4000 A.
I have seen some equipment use multi tap CTs from GE. You can use just one CT with several ranges depending on how you wire it so one CT can go from say 100:5 all the way to 800:5.
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