400A CT cabinet feeding two 400A service switches

[Tainted]

A 400A CT cabinet is feeding two 400A service switches. These service switches have 400A fuses too. One service switch is for the elevator and the other service switch is the house loads. Does this mean the combined load of the two service switches cannot exceed 400A because of the 400A CT? Why would someone install two 400A service switches to be fed by a single 400A CT cabinet?

Contractor said he needs an additional 400A for the electric vehicle charging station but there is no more room for another 400A service switch. I was thinking of combining the house load and elevator load into 1 service switch (800A trans S cabinet) and have it feed an 800A switchboard which will feed the 3 loads (elevator, house, and EV charging stations) , would that give me the extra 400A of capacity? See picture attached

 

[Tainted]

Just to clarify, the 800A switchboard will have (3) 400A switches feeding the 3 loads above

 

[electrofelon]

A 400A CT cabinet is feeding two 400A service switches. These service switches have 400A fuses too. One service switch is for the elevator and the other service switch is the house loads. Does this mean the combined load of the two service switches cannot exceed 400A because of the 400A CT?

Basically yes. I think really the only code reference would be 110.3

Why would someone install two 400A service switches to be fed by a single 400A CT cabinet?

Good question. I could think of a few reasons it may be that way, but it may also just be a bad/inefficient design.

I was thinking of combining the house load and elevator load into 1 service switch (800A trans S cabinet) and have it feed an 800A switchboard which will feed the 3 loads (elevator, house, and EV charging stations) , would that give me the extra 400A of capacity? See picture attached

In general it looks ok to me, but you were a little vague on the actual load calcs. Do you really need 400A for the elevator and house loads? For example, if say you only needed 600A total, would adding a 200A self contained meter and rearranging some stuff be a cheaper option than starting over?

Only other comment is A panelboard would probably be more economical than a switchboard.

 

[topgone]

My take.
If we talk about the protection side of the service, I doubt a designer will choose a 100% CT size. Why? Because CTs have specific accuracy limits and we want our protection relays to be accurate even during faults. And if the fault levels on the installation are great, the installed CTs might saturate and its output may not be an accurate reflection of the actual currents passing the CTs.
Ex. FL = 400A, If = 10kA (available fault at the point). You have a ratio of 10,000/400 =25. A typical CT accuracy limit is 20 and your Installed CT with a possible fault of 10kA (or a factor of 25) will likely saturate the CTs-->the CT secondary current will be lower than the expected fault current transposed to the secondary, hence protections might not activate. Then one will choose a CT with a bigger CT ratio, perhaps 600/5 (ALF=10,000/600 =16.67, which is lower than 20.
Even for metering purposes, we normally choose twice the possible current to register 50% on the meter scale when the system is fully-loaded. Else, you could damage the meter that will always be on full scale when the service is on full load.

 

[electrofelon]

My take.
If we talk about the protection side of the service, I doubt a designer will choose a 100% CT size. Why? Because CTs have specific accuracy limits and we want our protection relays to be accurate even during faults. And if the fault levels on the installation are great, the installed CTs might saturate and its output may not be an accurate reflection of the actual currents passing the CTs.
Ex. FL = 400A, If = 10kA (available fault at the point). You have a ratio of 10,000/400 =25. A typical CT accuracy limit is 20 and your Installed CT with a possible fault of 10kA (or a factor of 25) will likely saturate the CTs-->the CT secondary current will be lower than the expected fault current transposed to the secondary, hence protections might not activate. Then one will choose a CT with a bigger CT ratio, perhaps 600/5 (ALF=10,000/600 =16.67, which is lower than 20.
Even for metering purposes, we normally choose twice the possible current to register 50% on the meter scale when the system is fully-loaded. Else, you could damage the meter that will always be on full scale when the service is on full load.

Topgone,

In the OP's setup, there is no protective relaying going on. Further, in a typical set up like this, the CT's would be spec'd and probably owned and installed by the POCO and not governed by the NEC. So I would say the CT mounting base (assuming bar type CT's) is only governed by NEC 110.3 as I am not aware of anything more specific that would cover a CT mounting base. 110.3 doesnt say how to determine your current, but I assume most AHJ's would go by an article 220 calculated load.

 

[Tainted]

In general it looks ok to me, but you were a little vague on the actual load calcs. Do you really need 400A for the elevator and house loads? For example, if say you only needed 600A total, would adding a 200A self contained meter and rearranging some stuff be a cheaper option than starting over?

Only other comment is A panelboard would probably be more economical than a switchboard.

I do not know the actual existing loads yet. I was thinking of verifying it using 220.87. I am still going to refeed the house and elevator with 400A disconnect switches using a panelboard/ or switchboard because existing conditions had it setup that way previously

 

[jaggedben]

Yes, the load calculation is actually the critical thing. See 230.90 Exception 3.

 

[Tainted]

Yes, the load calculation is actually the critical thing. See 230.90 Exception 3.

I am almost 95% sure that 220.87 loads + new loads will not exceed 800A service switch size. I'm still going with 220.87 before adding new loads

 

[jaggedben]

I am almost 95% sure that 220.87 loads + new loads will not exceed 800A service switch size. I'm still going with 220.87 before adding new loads

They need to not exceed the apparent 400A rating of the service, as judged by the CT cabinet.