Routing of monitoring cables

Chamuit

Senior Member
Location
Texas
Let's say you need to install a monitoring setup like SiteSage or Neurio on a recessed panel, sheetrocked wall, with no space for the brain. What would be wrong with installing the brain outside on the wall and running the CT leads out a KO in the panel into the wall then out a neat hole in the drywall to the brain?

What am I missing?
 

paulengr

Senior Member
On a short run nothing. In one particularly big differential scheme at a chemical plant the CT leads are over 2000 feet long.

Only three things to watch out for.

There are two kinds of CTs. On the 4-20 mA kind if you are going longer than a few feet it is best to use shielded signal cable and ground the shield on one end only. Disconnecting is a non issue. Treat like any other instrument. Usually you can tell these because it will have more than two terminals. On the normal type do not EVER allow the CT circuit to be broken. Shorting is OK. Some of these have protection but most do not. Shielding not needed.

Second, both types do have a maximum burden. Don’t expect to go 2000 feet normally. That was a special situation.

Third on CT wiring especially with the typical 2 wire kind, use damage resistant wiring methods where it can never accidentally get loose. No fork lugs and certainly not push in’s or wire nuts. All connections and extensions need to be crimped on ring terminals and barrel connectors.

I realize with a lot of these power monitors they sell them with CTs and they purposely try to hide the specs so you can’t buy cheaper ones but a little sleuthing can figure it out.

The thing with “normal” CTs is they are a type of transformer. If it is rated say 500:5 then 500 A in gives 5 A out. The secondary side is almost a short circuit, maybe a few ohms at most. The transformer runs shorted so the output is only a few Volts. But if it ever gets loose that ratio works in reverse on voltage. So a 240 V input on a 500:5 CT becomes 24,000 V theoretically. The CT usually destroys itself but the wiring arcs and becomes very dangerous. Hence everything stays shorted all the time!
 

Chamuit

Senior Member
Location
Texas
These were 360:1 CTs. We were installing a test product in a residential panel. I was really worried that there would not be enough room in the panel for the whole setup of the monitoring equipment, but it worked. Thought I might have to install a PVC j-box below the panel to house the brain then route the leads through the bottom of the panel into that box.
 
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