Ampacity of C.T.

[binwork91]

If I connect 1230A load to the 1200A MES298 CT with (1) 1/4"x5" copper bus. Is this C.T. undersize?
I mean it is 1200A C.T, connect to 1230A load, it sounds like undersize.
However, the bus ampacity is 1250A, it is good for 1230A load.

What about 800A trans-s feed 830A load? I do not know the bus size in the trans-s. Do I need to measure the bus size to determine if it is good or not? I never see a trans-s provide the bus size in the cutsheet.

The reason I ask if per NYC local code.

If I have 1200A C.T to feed (1) 1200A and (1) 30A service switch. Or I have 800A trans-s to feed (1) 800A and (1) 30A.

I want to hear your opinion.

 

[petersonra]

My opinion is the 30 A switch does not indicate a 30 Amp load anymore than the 1200 A switch is a 1200 Amp load.

The load is what the load calculations comes up with, not the size of the service disconnect(s).

 

[binwork91]

My opinion is the 30 A switch does not indicate a 30 Amp load anymore than the 1200 A switch is a 1200 Amp load.

The load is what the load calculations comes up with, not the size of the service disconnect(s).

You are correct. However, base on NYC local code, service conductor need to size base on the switch size.

That why at the beginning I only talk about the load. Since my problem is base on NYC code.

 

[infinity]

Okay I wanted to look this up. What you've posted meets the exception in the NYC Amendments where you can use the calculated load to size the conductors up to 4000 amps.

(2) Ampacity of the service-entrance conductors for services 1000 kVA and larger shall not be less than
the sum of the maximum ampere ratings of the service disconnecting means. When including fire pump
disconnects in the calculation, 125 percent of the fire pump full load amperes shall be added.
Exception: The ampacity of service-entrance conductors need not exceed the maximum demand
calculated in accordance with Article 220, up to a maximum of 4000 amps per service

Here's a drawing of a 4000 amp service from one of our buildings. You can see that the total of the switch ratings (3000+3000+3000+2000+4000) well exceeds 4000 amps.

 

[binwork91]

Here's a drawing of a 4000 amp service from one of our buildings. You can see that the total of the switch ratings (3000+3000+3000+2000+4000) well exceeds 4000 amps.

I understand this. The conductor into your switchboard does not need to exceed 4000A since sum of service switch is more than 4000A. Per exception.

The ampacity of service conductor that feed the service switch need to size base on the switch size.

In my question: If I have 1200A C.T to feed (1) 1200A and (1) 30A service switch. Then the service conductor to 1200A C.T. need to be more than 1230A. Also, the bus in the C.T. need to be more than 1230A. But we call the C.T as "1200A C.T.". Why not call it 1250A C.T. since bus ampacity is 1250A. If I said I use 1200A C.T. to feed 1200 and 30A service switches, will you consider it as a code violation.

If I have 800A trans-s to feed 800A service switch and 30A service switch, can you tell the 800A trans-s has 830A ampacity?

 

[infinity]

If I have 1200A C.T to feed (1) 1200A and (1) 30A service switch. Then the service conductor to 1200A C.T. need to be more than 1230A. Also, the bus in the C.T. need to be more than 1230A. But we call the C.T as "1200A C.T.". Why not call it 1250A C.T. since bus ampacity is 1250A. If I said I use 1200A C.T. to feed 1200 and 30A service switches, will you consider it as a code violation.

The 1200 amps is just the physical size of equipment it is not the actual calculated load or is it? What is the calculated load for this service?

 

[binwork91]

The 1200 amps is just the physical size of equipment it is not the actual calculated load or is it? What is the calculated load for this service?

In my scenario, it’s a NYC job with a 1000 KVA service and large. The service switch sizes are 1200 A and 30 A. The load does not matter in this scenario. Per code, service conductor is size base on service switch size.
And 1200A C.T. feeds this 1200A and 30A service switch.

 

[Elect117]

Most can exceed their rating a little but it depends. The issue is also whether or not they are revenue grade. Exceeding the rating can cause errors in metering and eventually issues with the insulation or wiring from excessive heat.

I think you will be okay.

 

[infinity]

The load does not matter in this scenario. Per code, service conductor is size base on service switch size.
And 1200A C.T. feeds this 1200A and 30A service switch.

Where are you getting this information? Did you see the exception from the NYC amendments that I posted in post #4?

 

[binwork91]

Where are you getting this information? Did you see the exception from the NYC amendments that I posted in post #4?

I see your post #4. I kinda do not get why you think my question is under exception. I only mention 2 service switch, 1600A and 30A. Sum of them are not large than 4000A.

If i am wrong, please let me know.
The conductor ahead of service switch is service entrance conductor.
(2) Ampacity of the service-entrance conductors for services 1000 kVA and larger shall not be less than
the sum of the maximum ampere ratings of the service disconnecting means.
So no matter what load do I have, I need to size service-entrance conductors
not be less than the sum of the maximum ampere ratings of the service disconnecting means.

 

[infinity]

I see your post #4. I kinda do not get why you think my question is under exception. I only mention 2 service switch, 1600A and 30A. Sum of them are not large than 4000A.

The exception in the NYC amendments allows you to omit using the sum of the two switches when the calculated load is 4000 amps or less.

(2) Ampacity of the service-entrance conductors for services 1000 kVA and larger shall not be less than the sum of the maximum ampere ratings of the service disconnecting means. When including fire pump disconnects in the calculation, 125 percent of the fire pump full load amperes shall be added.
Exception: The ampacity of service-entrance conductors need not exceed the maximum demand calculated in accordance with Article 220, up to a maximum of 4000 amps per service

 

[binwork91]

The exception in the NYC amendments allows you to omit using the sum of the two switches when the calculated load is 4000 amps or less.

Thank you for point this out. I think you are right. I can use exception.

However, for some reason, NYC DOB does not like to use this exception. I don't know if any other people have issue when they use exception for EPR (Advisory board drawing).

Anyway, let's get back to my question. Let's consider the demand load instead of switch size.
If I connect 1230A demand load to the 1200A MES298 CT with (1) 1/4"x5" copper bus. Is this C.T. undersize?
I mean it is 1200A C.T, connect to 1230A load, it sounds like undersize.
However, the bus ampacity is 1250A, it is good for 1230A load.
What about 800A trans-s feed 830A load?