75 KVA 480 Volt Service

[Dwayne Cooper]

Hi all,
I have an existing service (480V 75KVA) service.
The MDP has a 400 amp breaker (Cat # LAP36400MB).
My concern; is the breaker is over sized for the transformers (3 transformers Delta)?
The math says it is.

 

[electrofelon]

Are the transformers the utility's? If so they don't really care about the NEC.

 

[Dwayne Cooper]

They are, but I'm looking to add a 200 amp feeder to the MDP. I'm concerned about the additional load.
Drawing attached.

 

[Jraef]

Something is not right here. If that is truly a 75kVA transformer, it is only good for 90A at 480V...

I'm thinking that you are reading 75kVA on EACH of those transformer cans, in which case this is a 225kVA 3 phase transformer BANK, good for 271A at 480V.

Either way your transformer setup is too small to be adding another 200A load to it. Have you checked in with the PoCo on this idea yet?

 

[Dwayne Cooper]

It is a transformer bank and I agree with the math. I have not checked because I wanted to make sure I was right before I pursued re-designing the service to handle the load.
Did you look at the drawings? I think the MB is also too large.

 

[MattS87]

It is a transformer bank and I agree with the math. I have not checked because I wanted to make sure I was right before I pursued re-designing the service to handle the load.
Did you look at the drawings? I think the MB is also too large.

As jraef said 225 Kva = 271A. Multiply by 125% you get 338A which would round up to a 350A main breaker so your 400 is over-sized. From your drawing you can't do a load calc, but just the 3 motor loads shown have an FLA of 288A so your service appears to be undersized

 

[topgone]

It is a transformer bank and I agree with the math. I have not checked because I wanted to make sure I was right before I pursued re-designing the service to handle the load.
Did you look at the drawings? I think the MB is also too large.

I did some quick calculations using 225 kVA as the transformer bank rating and your single-line diagram and tells me your transformer bank is just enough for the existing load! You can't possibly add 200A load to the system without overloading your transformer.

 

[Dwayne Cooper]

That's what I thought. My first concern was the 400 amp MB (that will never clear a fault before the transformer's fuses). Thanks to everyone that replied.
Many thanks!

 

[electrofelon]

As jraef said 225 Kva = 271A. Multiply by 125% you get 338A which would round up to a 350A main breaker so your 400 is over-sized. From your drawing you can't do a load calc, but just the 3 motor loads shown have an FLA of 288A so your service appears to be undersized

I did some quick calculations using 225 kVA as the transformer bank rating and your single-line diagram and tells me your transformer bank is just enough for the existing load! You can't possibly add 200A load to the system without overloading your transformer.

Again, the utility doesn't care about the nec. Pretty standard to have utilit size their stuff 1/2 to 1/3 of what nec would be. OP just has to give the utility his loads and they will decide.

 

[winnie]

The NEC has us do load calculations. Under the NEC we are required to install things like feeders and circuit breakers sized per those load calculations.

As others have said, the utility doesn't care about the NEC. In particular, they use totally different load calculations, and are perfectly happy to let their transformers operate in overload for a significant amount of time.

What is your current actual load? Does your meter provide peak demand information? When you add the new feeder, how much actual load will you be adding?

Which is not to say you shouldn't be concerned, because if the transformers are undersized for your actual load, you will have power quality and reliability issues.

What this means is that you don't need to worry about the 400A main disconnect being too large for the transformers, but you do need to talk to the utility about the additional load you will be adding, so that they can upgrade the transformers and service drop as necessary.

-Jon