ECN Forum Forums General Topics of the Trade General Discussion Area How to meter 360 amps?

I'm torn between answering your questions and agreeing with Active1 so I'll do both, sort of.
CT's don't FEED anything. CT means Current Transformer. They look like a big donut and they are basically just an iron coil. As current passes through the conductor that they are installed on they measure the amount of current by interpreting the induction field created by the flow of current. This information goes to the meter and is displayed there.
I have seen them installed on grouped parallel conductors but usually a CT is installed on each conductor in a parallel run. In my area the field guys from the POCO then connect #10 solid wires from each CT to the meter.
Having said all that, if you've never done this, you can't learn it from the internet. Get someone to do it with you who can teach you.

Big question: Is there 3 phase available?

It's a dumb question but... After that, why leave half of it single phase, and un-balanced? Then there's a roll of the voltage dice. Was the original service from a 3 phase transformer, a 120/208Y, or 120/240Delta? Or 120/240 straight single phase?

Adams,
The first time you do anything "new", you feel over your head, don't let it bother you.

The 400 amp 3 phase service you are looking to do, may sound overwhelming the first time you do it, and requires alot of pre-thinking and planning, but they are not hard.

Call the utility, and find out if that option is available, and request a site survey if possible. They should tell you what you need. Around me, the POCO provides the CT's and meter cans. The CT cabinet is usually a supply house item. My POCO provides mounting hieghts requirements, distance requirements and are all spelled out for you in their paperwork they provide to you, at no charge.

It will take some research and phone calls on your part, but after you do one once, you'll have them down.

Another avenue, call your local inspector, he may provide some answers to your questions.


Dnk.......

I'll also ask, is there 3 phase available???? Are you thinking that CT's provide phase conversion or phase creation? Sorry for the question but your use of terms is very confusing, please respond so we all are clear.

Good Lord!,
Why is this such a problem to Electricians that should know the Theory of a CT.
A CT (Current Transformer) like Greg said above, merely steps the current down, say from 400A down to say 3A, a level that the metering gear can use.
The big thing is here, is the Primary winding is a solid Busbar.
Forget about PT's (Potential Transformers) in this case this will only confuse the issue.
However CT's are normally a PoCo thing here because they form part of the PoCo metering equipment and are locked and sealed.
Oh and by the way, never open the Secondary side of a CT, when there is current flowing in the Primary

I had to install some metering at and industrial facility where they wanted to meter just the office building off a seperate service (it had previously been fed from the plant). I think it was a 400A service.

I ran into the same thing you did at the supply houses....they don't normally deal with that stuff. The power company usually does.

I was able to contact TECO and they actually sold me CT's suitable for the installation. I also got a "test switch" (a switch assembly where you connect the voltage taps and CT taps but can disconnect them). I bought an enclosure for all this and did a little drawing on it. It worked great once I found the right CT's and what the Power company was used to using. The CT's are BIG. Bigger than I'm using to seeing on motor amps. They are much more sensitive and accurate CT's than you are prob use to.

I think for a commercial service like that, you can get the power company to do the install...but I'm not sure.

[This message has been edited by Amazingmg (edited 05-19-2005).]

Trumpy, I think most of us here know the theory of a CT, just wasn't sure if the person asking the question understood the theory by the wording of his questions.

As Trumpy indicated, CTs are special animals in that if not treated properly, they can deliver dangerous voltages to the point of exploding in your face.
Please research the implications of dealing with CTs before proceeding with this service.

quote
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..never open the secondary side of a CT when there is current flowing in the primary...
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Trumpy, as a casual observer, I was following this nicely, using the current drawn to give a lower, but proportional, current for POCO metering purposes, but the above quote implies something nasty about to happen if you open circuit the secondary windings when the primary windings are energised. I don't get this at all.
Alan

Alan, from my understanding, the CT tries to induce a proportional current in the secondary circuit, say 400/5 amps, meaning that when 200 amps are flowing in the primary, the CT tries to produce 2.5 amps in the secondary. When you open the secondary, the impedance of the CT rises due to the open secondary, thus causing higher volt drop in the conductor passing through. This causes the voltage from the secondary to rise to surprisingly high levels, sometimes causing flashover, burns, shocks, and other bad things. I was always taught that the correct way to deal with a CT is a shorting link or switch on the secondary side.

I found some info here . Unfortunately I couldn't find any actual figures as to the potential voltage, but many sites mention 'flashover' so I imagine it's quite high. I await Trumpy's response on this one with anticipation

[This message has been edited by chipmunk (edited 05-20-2005).]