Current Transformer leads/other wires in same conduit

[chad44]

I have two conduits to an outdoor switchgear. One conduit will contain 24 VDC power and control wiring. The other will contain 120 VAC power wiring. Also, in the switchgear there is a set of 100:5 Current transformers. I need to bring the leads from those ct's back in one of the two conduits. Which one?

 

[scottf]

I would vote for the 24VDC conduit. If you're using the CT for revenue metering, I would recommend running shielded, twisted-pair cable. Otherwise, it shouldn't really matter.

 

[dpc]

Probably either one will work. The 24 V dc conduit will be safer for the CT circuit. I suppose there's a chance that a fault that put 15 or 20 amps through the CT secondary could cause your dc control circuits some heartburn, but I've never known it to be a problem.

 

[waross]

Metering connections commonly run CT currents in the same conduit or cable as the 120 volt PT circuits.
I understand the constrictions of using existing conduits, but given the implications of an open circuit on a CT secondary, I would investigate whether it is possible to run another conduit dedicated to CT and PT wiring.
If not possible, I would go with the 120 volt circuits. I would try to physically arrange the CT wires or cable so that it is unlikely that it would be inadvertently opened under load. Remember that depending on the impedence of the load, a CT secondary may show a very low, supposedly safe voltage under load, but if the circuit is accidentally opened, the voltage will tend to increase so as to maintain the current. The voltage can go extremely high until it is limited by saturation of the CT.
My first choice as I said would be with the other AC wiring, all other things being equal. However, I it became apperent that by running with the AC circuits the connections may be somewhat confusing, and by running with the DC circuits the CT wiring could be arranged so as to remove all confusion that may result in a disconnect under load, I would go with the DC wiring.
I would consider any scheme that significantly reduced the possibility of confusion in the future to be the preferred method of connection.
respectfully

 

[EEJaime]

The only restriction would be that code requires that any conductor in a power conduit have the correct insulation. So if you can get your instrumentation conductors with 600V insulation then there would be no problem routing it with the 120VAC.

 

[jghrist]

I agree that either would work. A small advantage to running the CT circuit with the DC would be that loss of either would probably disable the protection. Putting them in separate conduits would increase the chances of losing protection from damage to one conduit.

 

[scottf]

My concern with running the CT wiring with the AC power wiring is the chance for EMI....particularly MI. A comment was made that it is common to run CT leads in the same conduit as VT leads. This is true, but the current in the VT wiring is extremely low <100 mA for a normal electronic meter or relay connection.