current transformers?

[journeyman0217]

wondering if someone could tell me when current transformers are used on a service? I read in a COMMERCIAL electrical book that low voltage metering is required for services over 200 amps? I know that residential services of 400 amps do not require ct's, so why are they required in commercial services over 200amps? Is this something that the local utility company would say when they want ct's installed? thanks!

 

[iwire]

wondering if someone could tell me when current transformers are used on a service? I read in a COMMERCIAL electrical book that low voltage metering is required for services over 200 amps? I know that residential services of 400 amps do not require ct's, so why are they required in commercial services over 200amps? Is this something that the local utility company would say when they want ct's installed? thanks!

The power company decides how they will meter their services, each power company has different procedures.

Some will allow plug in meters up to 600 amps and others may expect CT metering on a 100 amp service.

 

[journeyman0217]

ah, that answers my question! thanks!

 

[kwired]

Most will supply 200 amp plug in type meters, especially for 120/240 single phase, but there are some on 480 volt services as well.

320 amp plug in meters are often acceptable to most POCO on dwellings maybe light commercial again at 120/240 single phase, but I have run into some three phase 320 meters at times.

Larger capacity self contained is usually a bolt in style meter, but that is where you seem to see more preference from POCO to use CT metering instead. I don't know all the particular reasons why but it is up to each individual POCO what they choose to allow and in what circumstances.

 

[Iron_Ben]

wondering if someone could tell me when current transformers are used on a service? I read in a COMMERCIAL electrical book that low voltage metering is required for services over 200 amps? I know that residential services of 400 amps do not require ct's, so why are they required in commercial services over 200amps? Is this something that the local utility company would say when they want ct's installed? thanks!

We went to significant lengths to *avoid* CT metering wherever we could. Our meter department was unhappy with CT metered services that didn't need to be. They were (Public Service Commission) mandated to check such installations for accuracy every five years, and they rightly considered it a waste to send a man out to do an hour or two of testing on a service that should have had a self contained meter from the start.

It was common for us to see plans showing a "600 amp" service and associated CT metering. Unless we felt the service might on occasion draw over 400 amps, we routinely specified a 400/320 amp meter base. The electrician would get duplex lugs to land his parallel 350's and all were happy. There are an awful lot of 600 amp services out there that never draw even 200 amps. We had thousands of them.

 

[kwired]

We went to significant lengths to *avoid* CT metering wherever we could. Our meter department was unhappy with CT metered services that didn't need to be. They were (Public Service Commission) mandated to check such installations for accuracy every five years, and they rightly considered it a waste to send a man out to do an hour or two of testing on a service that should have had a self contained meter from the start.

It was common for us to see plans showing a "600 amp" service and associated CT metering. Unless we felt the service might on occasion draw over 400 amps, we routinely specified a 400/320 amp meter base. The electrician would get duplex lugs to land his parallel 350's and all were happy. There are an awful lot of 600 amp services out there that never draw even 200 amps. We had thousands of them.

Did those 320 amp meter cans have sufficient room to land 600 amp worth of conductor in them?

 

[Iron_Ben]

Absolutely. They were/are roomy. A little pre planning to make sure our incoming feed (usually one run one of 350 aluminum quad) didn't get in the way of the exiting parallel copper 350's was enough to get a quality installation with no bending radius or other issues.

 

[meternerd]

As a POCO meter guy both in Alaska and northern California, I can tell you what we required. Any commercial service over 200A got CT's. Any single phase residential over 400A (Class 320 meter) got CT's. Any three phase, residential or commercial over 200A got CT's. Has a lot to do with meter inventory. With a 200A, thermal rating factor 4.0 CT, that same Class 20 meter/CT setup can be used on service up to 800A. CT meters can be removed for testing and CT's can be ratio and burden tested in place without interrupting service because they are fed from test switches. We do scheduled meter audits on CT rated services on a regular basis since these services tend to be the BIG money users. Just the way we do it, though. I don't think there's a general rule anywhere.

 

[Steve Sink]

I worked for two different utilities for over ten years as a meter man and I can tell you that it is definitely up to the utility to decide. I was a supervisor over the meter department at my second utility and I required anything that was three phase and over 200 amps to be a CT installation. I also required anything over 320 amps that was single phase to be a CT installation. Also, anything that was above 240v got CT'S and PT's as well. I have seen too many 480v metering installations without PT's explode for no reason. Fortunately nobody was hurt.

My preference is to use CT's when possible because it has been my experience that the bolt in type meters were prone to failure and we're not very accurate. Not to mention the fact that when the bolt in meters are changed out bypasses are need and I have heard more than one story of someone blowing the bypasses up. I have a website that is all about metering if you are interested in learning more. Just go to my about me on my profile.

 

[meternerd]

I worked for two different utilities for over ten years as a meter man and I can tell you that it is definitely up to the utility to decide. I was a supervisor over the meter department at my second utility and I required anything that was three phase and over 200 amps to be a CT installation. I also required anything over 320 amps that was single phase to be a CT installation. Also, anything that was above 240v got CT'S and PT's as well. I have seen too many 480v metering installations without PT's explode for no reason. Fortunately nobody was hurt.

My preference is to use CT's when possible because it has been my experience that the bolt in type meters were prone to failure and we're not very accurate. Not to mention the fact that when the bolt in meters are changed out bypasses are need and I have heard more than one story of someone blowing the bypasses up. I have a website that is all about metering if you are interested in learning more. Just go to my about me on my profile.

No reason? Can't say I have the same experience. I have seen more PT failures than 480 explosions. Plus, some utilities use fuses in the PT circuit as well, for safety. I never fuse meter circuits except on primary metering. Too much possibility of losing a phase voltage due to a blown fuse and not noticing. Big loss of revenue. Not so much of a problem now, with solid state meters that alarm when they lose a phase, but with remote reading, it still may go unnoticed unless you set it up to stop transmitting when it has an alarm. Problem with that is that any time you have a momentary single phase situation on the distribution system, all of the 3 ph meters on that circuit go into alarm and require a meter reader or tech to go reset 'em. Way off the subject, though......:happyyes: It'll never be a perfect world.

 

[Steve Sink]

I never said that I had seen more explosions than PT failures. I agree with you on that. I have definitely replaced more PT'S in my career than meters that have exploded. My point was that when the 480v meters fail it is catastrophic and I have seen it result in a fire more than once. And typically the entire meter base and wiring harness needs to be replaced. Most of the time when at PT fails it is just a quick swap and you can be on your way.

Also, some utilities are implementing policies now that do not allow 480v metering without PT's. I never fused my PT's either.