# CT Secondary Conductor Size



## gpop (May 14, 2018)

some ct output 5 amps others are analog current transducers that output 4-20ma.

18 does sound small for a 5 amp output ct.


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## micromind (Aug 11, 2007)

Most CTs can go 4X their rated current so if the secondary is 5 amps, it's possible to see 20 amps depending on what the primary is rated vs. how much current is flowing. 

The full-load current of this motor will be somewhere around 120 amps (wild guess......lol), if they used 100:5 CTs, the secondary current will be around 6 amps. If they're 200:5, it'll be 3 amps. Either way, #18 is ok. 

I've seen CTs with no shorting bars but not often. Usually, they're not a good idea with motor protection because if they're shorted, then relay sees no current and can't protect the motor. Most Multilin relays need to see current to transition from start to run. If the CTs are shorted, it'll stay in start.


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## joe-nwt (Mar 28, 2019)

#18 will work electrically, but for mechanical strength most utilities spec #10 stranded or occasionally #12. As bad things happen with an open CT circuit, I tend to agree with this philosophy. 

Also, depending on the length of the secondary wiring, a #18 may add a significant burden on the circuit.


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## JRaef (Mar 23, 2009)

So they had a CT circuit on the NGR of the service transformer that was going to a 369 that was in the motor controller, without going through any form of disconnect or shorting terminals? Doesn't sound right to me. Unless of course, as gpop said, it's not a current Current TransFORMER, it's actually a TransDUCER where the output is 4-20mA. But even then, there is no input for an external analog to a 369 relay.

That said, I'm a bit puzzled by the concept here. There is no input on the 369 for an NGR, because it's a MOTOR Protection Relay and the NGR would have nothing to do with the motor. More likely, the 369 has an input for a Core Balanced CT with a 50:0.025A output, or 2 PTs with an open delta through a current limiting resistor to get that level of mA current. This is for Ground Fault sensing on an ungrounded or resistance grounded system where ground current detection is critical. If the wires are going to terminals 101 and 104, that's what it is. Its a 25mA circuit and limited to never more than 207V.


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## paulengr (Oct 8, 2017)

JRaef said:


> So they had a CT circuit on the NGR of the service transformer that was going to a 369 that was in the motor controller, without going through any form of disconnect or shorting terminals? Doesn't sound right to me. Unless of course, as gpop said, it's not a current Current TransFORMER, it's actually a TransDUCER where the output is 4-20mA. But even then, there is no input for an external analog to a 369 relay.
> 
> That said, I'm a bit puzzled by the concept here. There is no input on the 369 for an NGR, because it's a MOTOR Protection Relay and the NGR would have nothing to do with the motor. More likely, the 369 has an input for a Core Balanced CT with a 50:0.025A output, or 2 PTs with an open delta through a current limiting resistor to get that level of mA current. This is for Ground Fault sensing on an ungrounded or resistance grounded system where ground current detection is critical. If the wires are going to terminals 101 and 104, that's what it is. Its a 25mA circuit and limited to never more than 207V.



AB “transformer feeder” where someone soeced Multilins?


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## glen1971 (Oct 10, 2012)

JRaef said:


> So they had a CT circuit on the NGR of the service transformer that was going to a 369 that was in the motor controller, without going through any form of disconnect or shorting terminals? Doesn't sound right to me. Unless of course, as gpop said, it's not a current Current TransFORMER, it's actually a TransDUCER where the output is 4-20mA. But even then, there is no input for an external analog to a 369 relay.
> 
> That said, I'm a bit puzzled by the concept here. There is no input on the 369 for an NGR, because it's a MOTOR Protection Relay and the NGR would have nothing to do with the motor. More likely, the 369 has an input for a Core Balanced CT with a 50:0.025A output, or 2 PTs with an open delta through a current limiting resistor to get that level of mA current. This is for Ground Fault sensing on an ungrounded or resistance grounded system where ground current detection is critical. If the wires are going to terminals 101 and 104, that's what it is. Its a 25mA circuit and limited to never more than 207V.


Yes it is the CT on the NGR that goes to the Core Balanced CT input on the 369, with no way to short it when working on the 369. The set-up is a 4,160 volt single cell from a transformer that feeds a motor. My concern was if there was an existing fault on the feeder from the transformer to the disconnecting switch, and the NGR CT wiring was opened, bad things would happen (from my memory) on opening the CT with a current in it.


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