CT Questions

[Cliff Brady]

I am sorry for this question, but I am a Mechanical Engineer that has been thrown into an Electrical Engineering position.

The previous employee designed a project which has a 600 amp service with a dedicated transformer. The contractor is saying we need a c/t cabinet. I can not figure out what is the purpose of a c/t cabinet and when do you need it? Are there any codes on this issue or who's specifications do we use?

 

[bphgravity]

Perhaps they should hire a real EE?

A CT is used to meter the electrical usage from a remote location. That is, instead of bringing the service conductors into the meter enclosure and back out, you simply install leads to the CT cabinet and the service conductors directly to the service equipment. See 110.23 of the NEC for some important information.

 

[charlie b]

The question of whether you do or do not need a c/t cabinet is one for the utility that is serving your transformer. The c/t will provide information; the question is whether the utility (or the owner, I suppose) needs to know that information.

A c/t ("current transformer") is a coil of wire that is wrapped around each of the three phases of the incoming service. OK, so it's not actually wrapped around, but instead the phase conductors are inserted through the coil before being connected to their terminals, but the effect is the same. Current flowing through the phase conductors will create a magnetic field around the conductors. The coil (the c/t) will sense that field, and will create within itself a current that is proportional to the field. So essentially the current in the coil is a measure of the current in the phase conductors, and that is the information that you get from the c/t.

The design of this type of equipment is beyond what may be reasonably expected from an ME. You really should get yourself out of the position of having to make design decisions for this installation.

 

[Cliff Brady]

Well I am trying to learn how to do Electrical Engineering. I purchased and watched the Mike Holt Videos. They only cover so much and there is a lot of material in the NEC. I have quickly figured out that you definitely can't learn electrical engineering overnight. The good thing is though, I am already putting together drawings that rival my predecessor. Just some of the issues, I don't understand.

 

[charlie b]

I have quickly figured out that you definitely can't learn electrical engineering overnight.

Well I'm glad you figured that out. It took me four years to learn the basics, and a good part of my career to learn the specifics. But I don't understand why you would even try. There is a limit to what an ME can be reasonably expected to do in an electrical design project, and I think you are beyond that limit.

Who, for example, will be formally issuing these drawings that you are developing? Will they be issued under a PE seal, and if so are you going to be the person applying that seal? Does you state allow a person without an EE degree to seal electrical drawings, or to prepare electrical drawings under the supervision of an electrical PE?

 

[charlie b]

Well I am trying to learn how to do Electrical Engineering. I purchased and watched the Mike Holt Videos. They only cover so much and there is a lot of material in the NEC.

Mike Holt is not an electrical engineer, and he does not teach electrical engineering. In addition, the NEC is not about electrical engineering.

Mike has been an electrician, an inspector, and a teacher. He is a well-know and respected expert in the NEC. But even the NEC itself declares in no uncertain terms that it is not a design manual. You must first know how to do the design, and then you go to the NEC to determine what is or is not acceptable as a minimum, in order to achieve a safe installation. The NEC is about safety, not about what works well or what will make the owner happy.

 

[dcl34769]

In FL, progress energy doesn't CT anything less than 800A. We are required to use a 7 jaw, lever by-pass meter can. Contact your POCO and speak with the engineer. They usually also have a handbook that is free, listing metering requirements ranging from meter height to transformer pad specifications. We have 4 different POCOs in my area and I carry a spec book for each on my service truck.