At what Amp level is the CT cabinet for meter required

designer82

Member
Location
Boston
I'm pretty sure at 600 amps + there are no standard meter sockets available and you need a CT Cabinet for utility metering

However, at 400 amps I am not so sure. Can you get a standard meter socket/meter at 400amps or do you need the CT cabinet?

I've seen different project drawings show this both ways.

208V-3PH service by the way

Thanks
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
They do make 400 amp and 800 CT and meter cabinets in one enclosure with a 10 point meter. It would be up to the local POCO to decide what they will allow you the use.
The one on the left is 800 amps the others 400 amps.
400 amp Meter Enclosures.jpg
 

Carultch

Senior Member
Location
Massachusetts
I'm pretty sure at 600 amps + there are no standard meter sockets available and you need a CT Cabinet for utility metering

However, at 400 amps I am not so sure. Can you get a standard meter socket/meter at 400amps or do you need the CT cabinet?

I've seen different project drawings show this both ways.

208V-3PH service by the way

Thanks
The reason why you see so many 400A services with self-contained metering, is that it is really 320A continuous duty rated self-contained metering. The meter socket and globe are rated for up to 320A continuous, which usually corresponds to a 400A OCPD and 400A service. The meter socket is built with 400A circuits in mind, with the underlying expectation that the typical load current will generally be 320A and less. It is most common that this is the largest size service that a utility will allow you to do with self-contained metering. For this reason, the market for metering equipment will make it hard to find self-contained metering equipment in any larger ampere rating.

If you have a rare situation where you had more than 320A of non-continuous load that sized you 400A service, and it is realistic to exceed 320A, that would be a case where a 400A service might need to be metered with a separate CT cabinet.

Meter socket manufacturers commonly make the same equipment for both service voltages, in an effort to standardize on parts. The meter globe might be voltage-specific, but the meter socket and CT cabinet are usually the same. Voltage will determine whether hot-sequence or cold-sequence metering is required to meet your utility's standards.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
It's up to the POCOs around here. But as was mentioned, usually anything over 400A will require a CT. I know of one that requires it on just a 400A, others mostly 600A.
 

Carultch

Senior Member
Location
Massachusetts
Thanks for this. Do you mean "rare situation where I have more than 320A continuous"?

I think I fall in this situation
That's not what I mean. If your continuous load exceeds 320A, you would need more than a 400A OCPD anyway. You'd then be looking at a service rating of 450A to 600A (most likely 600A, because it is common to round up to more common sizes), for which most utilities would require instrument metering, and self-contained metering. Even for customer-owned metering, you'd likely be using CT's on a circuit that size, instead of self-contained metering. You could get a socket/CT cabinet as a combo device as shown in Post #2, but I still wouldn't consider that self-contained.

The rare situation I meant, is as follows:
1. Your load adds up to between 320A and 400A
2. You would still be using a 400A OCPD. Ordinary OCPD, as opposed to continuous-duty OCPD, for which you need the 125% factor on continuous loads, when sizing it.
3. You would have to have non-continuous loads for this to be a possibility. Either only non-continuous loads, or a mix of both.
4. This would mean that C+NC > 320A, and 1.25*C+NC <= 400A. C = total continuous load, and NC = total non-continuous load.

I've tried to ask Milbank for clarification about this, and couldn't get the rep to understand the question. What I don't understand is why meter socket manufacturers don't simply rate their product according to the corresponding maximum OCPD with which they expect you to use it, and call it a 400A meter socket. This would avoid the ambiguity of the situation I described above, and avoid the possibility of a skeptic to challenge your selection of a 320A meter base used with a 400A breaker.
 
What I don't understand is why meter socket manufacturers don't simply rate their product according to the corresponding maximum OCPD with which they expect you to use it, and call it a 400A meter socket. This would avoid the ambiguity of the situation I described above, and avoid the possibility of a skeptic to challenge your selection of a 320A meter base used with a 400A breaker.
Perhaps this is being a little nitpicky, but I blame electricians for emphasizing class 320 sockets are only rated 320 continuous, which they do NOT do for most other equipment. I am not sure why so many people do this, it kind of drives me crazy. You never hear someone talk about a "160 amp continuous main breaker panel." Or refer to a typical 200 amp service as a 160 amp service.

In regards to the original question, my experience is most utilities do not allow class 320 meter sockets for 3-phase and/or commercial services.
 

kwired

Electron manager
Location
NE Nebraska
POCO is generally who will have policies on what can be used and when.

I know some that have no problem with 320 single phase or even 208/240 three phase but want anything over 200 to be CT on 480 volts.

There is K base meters/sockets, but I mostly only see them on existing that is at least 20-25 years old around here. New installs they seem to want CT instead of K base anymore.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
POCO is generally who will have policies on what can be used and when.

I know some that have no problem with 320 single phase or even 208/240 three phase but want anything over 200 to be CT on 480 volts.

There is K base meters/sockets, but I mostly only see them on existing that is at least 20-25 years old around here. New installs they seem to want CT instead of K base anymore.
Generally speaking it’s because meter maintenance is so much easier with the “S” base meters than a K base.
As of yet I haven’t seen a bypass on a K base can. There could be a manufacturer making one like that, I don’t know
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I believe 320a meters use bypass-equipped plug-in bases, while fully-rated 400a meters are bolt-in based.
 

kwired

Electron manager
Location
NE Nebraska
Generally speaking it’s because meter maintenance is so much easier with the “S” base meters than a K base.
As of yet I haven’t seen a bypass on a K base can. There could be a manufacturer making one like that, I don’t know
I think I have installed only two or three new K base meter sockets ever. maybe encountered only a dozen at the most existing. Just took one out of service about a year ago and was on a dwelling believe it or not. One that was built in 1970's. Funny thing is it supplied two 150 amp FPE panels (the reason I was doing anything at all was insurance wanted the FPE's gone) but the supply conductor was only 4/0 aluminum. It did get replaced with a 320 socket.

Last K base I did install was on a 400 amp 480/277 service was probably around 12-15 years ago. POCO insisted it must be a K base so that is what I installed. They did come connect it. Not much time afterward I was doing some work at same location I seen there was no meter inside the enclosure, the opening was blanked and a smaller meter socket installed off to the side. Someone at POCO decided it needed converted to CT metering. Be interesting to see how tight they fit things into that cabinet as they basically converted it to a CT cabinet.
 
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