Hello, Im bidding a job for a print shop. They have an existing 225 amp single phase panel that pulls 190 amps when they have everything going. They want to hook up 180 amps worth of 3 phase printing equipment. I know how to do everything for the job EXCEPT how to meter the two panels. I called the supply house and asked for a 400 amp 3 phase meter base and they said all they make is a 320 amp meter base. They went on to say that I could hook up my 2 panels to this meter base but if the current goes over 320 amps then the meter will not record the watts properlly. So my question is do I need to use a current transformer? I think thats what they are called. Could someone who deals with these types of service installations please give me some help? Thanks.

I am missing something out of your post, I think.
Do they have a single phase service now?
Are you planning on putting in a 3 phase service?

Around me, a 400a 3 phase service utilizes "ct's".

I have never seen a 3 phase 400 amp meter.

Dnk....

Yes you are correct. They only have single phase and want to add a 3 phase panel. So I will need to use a current transformer? Should most electrical supply houses sell these. I asked the supply house I use if thats what I needed and if they sold CT's and they acted as if I was the only electrical contractor to ever ask for meter equipment for a 400 amp service. They said the highest meter they sell is a 320 amp, 3 phase.

[This message has been edited by AdamsAtoms (edited 05-16-2005).]

A 320 meter base is rated 320 amps continuous or 400 amp max. This is no different from a standard 400 amp panel.

A 320 sockets is the right item to use for a 400 amp service as long as your local power company does not want CTs.

In my area we use plug in meters for services 400 amps and below.

Call the utility co. and ask them what they want.

Around here a 400A 3PH service would require CT metering. The service conductors from the POCO's transformer enter a CT can, typically a 36366 NEMA3R can with CT mounting studs, the CT's, which are provided by the POCO slip over the conductors and they pass through the can and into the service. A 1" conduit goes from the can to a meter base and the wiring in that conduit is done by the POCO.
The real question that seems unclear in your reply to Dnkldorf is whether everyone is clear that the POCO will have to supply 3PH service to this building. I guess that should be obvious, but sometimes.....

Hello Greg, after reading your post several questions came to mind. Im aware now that I need to use a 400 amp CT. I will have 2 panels. 1-225A-sinlge phase and 1-200A-3 phase. 1) Will I need to run my service wires comming off of the load side of the CT into a gutter below my 2 interior panels and feed them from there or do most CT's have provisions for feeding 2 panles? 2) I was going to parallel 2 sets of 3/0 for the line side of the service, are CT's capable of metering paralleled conductors, do they put 1 CT around both conductors? thanks.

Adam,

Don't take this the wrong way but I have a feeling your in over your head. Maybe you would be better off subing out the service to another EC. Or bid out the job with service work to be done by others and only bid the inside job.

Tom

as in with any service you need to do what the power co. in you local wants, called there standards, get ahold of them. have you done any services at all yet? I would think anywhere, any load over 225 amps would have ct's

I will just call the power company tommorow and talk to them in more detail. I talked with another master friend of mine who told me what all I need to do. Thanks for the help.

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
-------------------------
..never open the secondary side of a CT when there is current flowing in the primary...
-------------------------
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).]

I posted this on Mike Holt's forum earlier today:

A typical CT is available as 400:5 (equal to 80:1), meaning that one amp will flow through the meter for each 80 amps in the line (for a given phase, of course).

I've never opened one, but I take it that the CT has 80 turns of wire. Therefore, with an open secondary, the CT would operate as a 1:80 step-up transformer.

With even a lowly 120v primary, that would be (80*120) 9.6kv! That's higher than the average insulation rating, to say the least, as well as quite lethal.

In order to 'get' the way a current transformer is dangerous, you need to think about the concept of a _current_ source.

We are all familiar with _voltaqe_ sources. An idea voltage source is some apparatus which maintains two terminals at constant voltage, no matter what the circumstances. If you connect a load to the voltage source, current will flow through the load. As the load impedance changes, the output current adjusts up and down to maintain a constant voltage drop across the load. As the load impedance is decreased, more current flows to the load, and more VA is delivered to the load. If the load happens to be resistive, than as you drop the load resistance, the power delivered increases. The result is that if you want to somehow 'turn off' the output of an ideal voltage source, you want to make the load impedance infinite: you want to open the circuit. Fuses work well for voltage sources. Note: _real_ voltage sources are never ideal, and have their own 'internal impedance', but the distribution transformers that we normally deal with are rather good approximations to ideal voltage sources.

But now imagine an ideal _current_ source. This is some apparatus that maintains a constant current flowing between its two output terminals, no matter what the circumstances. The terminal _voltage_ adjusts up and down to maintain the constant current flow through the load. As the load impedance is decreased, the _current_ stays exactly the same, but the _voltage_ drops, and _less_ VA is delivered to the load. If the load is resistive, then as you drop the resistance, the power delivered _decreases_. If you want to 'turn off' a constant current source, then you _short circuit_ the output. The output voltage falls to zero, and the power delivered similarly falls to zero. On no account do you want to _open_ a constant current circuit, because then the output voltage (and power) goes to infinity! Again, a _real_ current source is never ideal, and will have its 'internal conductance'. As the output voltage increases, the output current _will_ go down slightly. But there are a few situations where we see some pretty good approximations to current sources.

A CT is one such beastie. A CT is a _step up_ transformer. Like all step up transformers, as the output voltage is increased, the output current is decreased. Like any transformer, there is a primary coil and a secondary coil...the primary 'coil' is just the bus bar threading through the donut, and the secondary coil is wrapped around the donut. The voltage drop on the primary is related to the voltage drop on the secondary, by the turns ratio of this transformer. What makes the CT behave is unexpected ways is that the voltages on both primary and secondary side are _very_ small, perhaps 1V on the secondary side and 1/80V on the primary side (warning: numbers being pulled out of left but cheek).

The primary side is connected in series with the entire electrical system, and because the voltage drop on the primary side is so very small, it is the load on the electrical system that actually controls the current flowing on the primary side. The total voltage on the primary circuit may be 277V, with 276.99V dropped in the load, and 0.0125V dropped in the primary of the CT.

Now, imagine that the voltage on the secondary side of the CT is multiplied by a factor of 80. The secondary voltage drop has to increase, and the primary voltage drop needs to increase to match this. But the primary voltage drop is so very small, that as far as the over-all primary circuit is concerned, there is no change at all. On the secondary side we see 80V, on the primary side we see 1V, and the total primary circuit sees 276V dropped in the load, and 1V dropped on the primary of the CT. The voltage on the secondary side can go through the roof well before the primary side sees any significant change.

Now, if you actually open circuit CT, the voltage drop on the primary won't go to the full 277V, and the secondary voltage won't go to 22KV steady state, with no current flowing in the primary side; magnetic limitations will come into play to limit the final output voltage; and the inductance of the entire system will influence the transient response. There is potentially quite a bit of energy stored in the magnetic field in the donut, enough to strike an arc if you open the secondary. But the voltage will easily get high enough to cause significant damage. Remember, a CT is _not_ an ideal current source; just a damn fine approximation to a current source.

To protect the secondary of a CT, you don't want a fuse. Instead you want something that is the opposite of a fuse; something that will short circuit in the even that the potential gets too high. I don't know what these are called in the electrical distribution world; in the electronics world, the device is known as a 'crowbar circuit'.

-Jon

In electrical distribution, it's called a shorting bar, a shunt, a bypass, or a jumper.

The CT could be considered to be "impedance protected", since its maximum current is limited by its internal impedance.

In the case of an 80:5 CT, only a primary overload can increase the secondary current beyond the 5-amp maximum design current.

Back to the original question, you have to ask the POCO if 3-phase is available in the area. While it may be possible to have two services to one customer, check with both the POCO and local AHJ.

You might have to either combine the existing and new loads into a center-grounded Delta service, or rewire the existing service into a single combined Y system. This is up to the POCO.