CT Cabinet Rating

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solar2021

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Boston, MA
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Engineer
I am currently trying to procure a CT cabinet for a building service meter relocation. The building is currently serviced by a 750kVA 3P 12.74kV primary-480V secondary transformer that feeds into a 1600A switchgear. Based on my calculations the maximum full load secondary current of this transformer is around 900A. We will be installing the CT cabinet in between the transformer and building switchgear.

In my past experience, we have always sized the rating of the meter/CT cab to the maximum continuous rating of the service. For example for a 400A service, you can install a Class 320 meter socket since the maximum continuous current of the service is under 320 amps. Would a 1200A rated CT cabinet work in our case or do I need to size the CT Cab to match the service rating of the building of 1600A?
 

tom baker

First Chief Moderator
Staff member
Same here. Our POCO has very detailed specs on CT metering, including exact type of cabinet. The POCO installs and wires the CTs. Sometimes we have to provide a submittal on CT metering.
 
I am currently trying to procure a CT cabinet for a building service meter relocation. The building is currently serviced by a 750kVA 3P 12.74kV primary-480V secondary transformer that feeds into a 1600A switchgear. Based on my calculations the maximum full load secondary current of this transformer is around 900A. We will be installing the CT cabinet in between the transformer and building switchgear.

In my past experience, we have always sized the rating of the meter/CT cab to the maximum continuous rating of the service. For example for a 400A service, you can install a Class 320 meter socket since the maximum continuous current of the service is under 320 amps. Would a 1200A rated CT cabinet work in our case or do I need to size the CT Cab to match the service rating of the building of 1600A?
Where is the service currently metered? I assume there is either a metering section in the switchboard or is metered at the transformer spades. Why do you need to relocate the metering? Is this metering for personal purposes or for the utility? If the former, we have often found it better to not use a proper CT mounting base and cabinet, rather just an appropriately sized box (which might actually be just the cabinet part of the CT cabinet), and split core CTS. If this is for power company metering, then as others have said you would have to discuss it with them as it's really none of your business 😉
 
As far as your specific question, the only thing I see the NEC saying about a CT mounting base and CT rating is 110.3 which is not specific. I would lean toward you can size them to the NEC load calc and not necessarily the service disconnect rating.
 

Hv&Lv

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Engineer/Technician
As far as your specific question, the only thing I see the NEC saying about a CT mounting base and CT rating is 110.3 which is not specific. I would lean toward you can size them to the NEC load calc and not necessarily the service disconnect rating.
I’m just asking here, because I’m curious..
say the calculations are 1000 amps. Would you use a 1000:5, or something like a 400:5 and use the rating factor If it’s say a 3.0 @ 55C
 

Carultch

Senior Member
Location
Massachusetts
I’m just asking here, because I’m curious..
say the calculations are 1000 amps. Would you use a 1000:5, or something like a 400:5 and use the rating factor If it’s say a 3.0 @ 55C
The CT can "survive" an overload like that, but you would generally size your CT's so that the primary rating meets or exceeds the total operating current of the circuit being measured. The 1.25 factor need not apply to CT sizing, and it can be selected directly from the total full load amps. If you used 400:5 CT's on a 1000A circuit, you would subject your metering equipment to 12.5A, where it either cannot accurately measure, or where it might be unsafe for the metering electronics.

You will also find that most CT's with a 400A primary, are built with the wire window size on the expectation that you will use them on a 400A circuit, so you may not be able to fit a 1000A circuit through their wire window.

If a product specifically built as a CT cabinet is part of a design, it usually is for a metering setup that involves the utility in some form or another, and I've usually seen that the CT's are for the utility to specify and provide. CT's of customer-owned metering that doesn't concern the utility, can typically be installed directly in a panelboard, disconnect, or general enclosure around the feeder they are measuring.
 
I’m just asking here, because I’m curious..
say the calculations are 1000 amps. Would you use a 1000:5, or something like a 400:5 and use the rating factor If it’s say a 3.0 @ 55C
I don't really know. I don't really have any experience sizing and spec'ing CTS. I'll defer to Carultch's comments 😉. But I will say my previous response was based on the presumption of bar type CT's and mounting bases, where the NEC would presumably have more interest in their selection because they are a "conductor"
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
The CT can "survive" an overload like that, but you would generally size your CT's so that the primary rating meets or exceeds the total operating current of the circuit being measured. The 1.25 factor need not apply to CT sizing, and it can be selected directly from the total full load amps. If you used 400:5 CT's on a 1000A circuit, you would subject your metering equipment to 12.5A, where it either cannot accurately measure, or where it might be unsafe for the metering electronics.

You will also find that most CT's with a 400A primary, are built with the wire window size on the expectation that you will use them on a 400A circuit, so you may not be able to fit a 1000A circuit through their wire window.

If a product specifically built as a CT cabinet is part of a design, it usually is for a metering setup that involves the utility in some form or another, and I've usually seen that the CT's are for the utility to specify and provide. CT's of customer-owned metering that doesn't concern the utility, can typically be installed directly in a panelboard, disconnect, or general enclosure around the feeder they are measuring.
I know what your saying, and I know how to size them.

I was just asking for an electricians POV.

Todays CTs and metering are designed to take these “overloads”.
The old mechanical meters couldn’t handle more than 5 amps, and to make the selection process simple manufacturers simply rated their CTs at xxx:5. We haven’t moved away from that nomenclature. Same as we still use “time dial” in relays even though there isn’t a “time dial” in them.

The meters of today are designed and we are encouraged to go upwards of 20 amps. Hence the reason for “class 20” meters.
With the variable load swings in commercial buildings now it’s also advisable to use the rating factor to meter more accurately, rather than less accurately as stated.
The older CTs weren’t accurate below 10%. So any loads on a 1000:5 CT that are at 100 amps or less wil not measure accurately. The newer extended range CTs are far superior to yesterdays CTs and are what we use exclusively now.
If I remember correctly (off the top of my head) the extended range 500:5 are more accurate at lower amperage’s than a 200:5 sitting next to it.

I have on the shelf old 200:5, 600:5, up to 1500:5. Same size window for padmounts. And I wouldn’t size my wire at 1000 amps for a 1000 amp service anyway. You send me this request, your getting about a 400, maybe 600 amp from us. Your 1000 amp service isn’t going to use 1000 amps anyway.😉
 
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