Transformer sizing.

[ElectricMatt]

I am looking for some opinions here. I am working off of the knowledge I have and want to make sure there is something I am not missing before bringing up this discussion to the my engineer.

I am putting in a boiler room, the transformer for the low voltage panel is a 75kva 480v to 208v, 98.6 eff @35% loading. It feeds a 60 circuit panel with a 250a main.

By my calculations a 75kva transformer is only good for 208a. Obviously the connected load as engineered is well below 250amps, however is it legal to size the main breaker so high over the transformers capacity? Please enlighten me and explain why this is/is not acceptable.

Thank you!


Sent from my iPhone using Tapatalk

 

[ElectricMatt]

I attached a picture of the panel schedule with load calculations


Sent from my iPhone using Tapatalk

 

[kingpb]

125% of 208A is 260A. Sizing it 250A is actually being conservative based on the Panel rating. The Main Bus/Panel is rated 250A, but the Main Breaker is supposed to be 225A. Not sure where you are getting the main to be 250A?

 

[JFletcher]

What is the size of the breaker or fuse for the primary side?

 

[ElectricMatt]

What is the size of the breaker or fuse for the primary side?

Primary is OCP is 125amps
Secondary OCP is 250 amps



Sent from my iPhone using Tapatalk

 

[ElectricMatt]

125% of 208A is 260A. Sizing it 250A is actually being conservative based on the Panel rating. The Main Bus/Panel is rated 250A, but the Main Breaker is supposed to be 225A. Not sure where you are getting the main to be 250A?

I am getting he 250amp main breaker from my one line diagram.


Sent from my iPhone using Tapatalk

 

[ElectricMatt]

125% of 208A is 260A. Sizing it 250A is actually being conservative based on the Panel rating. The Main Bus/Panel is rated 250A, but the Main Breaker is supposed to be 225A. Not sure where you are getting the main to be 250A?

So on the secondary side of a transformer you are allowed to add 125% or the rated full load of the transformer?


Sent from my iPhone using Tapatalk

 

[david luchini]

So on the secondary side of a transformer you are allowed to add 125% or the rated full load of the transformer?


Sent from my iPhone using Tapatalk

With a 125A primary circuit breaker, the secondary circuit breaker can be any size, per T450.3(B) "Primary only protection." 250A seems reasonable.

 

[ElectricMatt]

With a 125A primary circuit breaker, the secondary circuit breaker can be any size, per T450.3(B) "Primary only protection." 250A seems reasonable.

I don’t have my code book with me right now, but to make sure I am understanding this .

480 v primary 75kva = 90.3 fla

90.3 x1.25 =112.9 amps

Next size breaker up is allowed which would be 125a.

Is this correct?


Sent from my iPhone using Tapatalk

 

[ElectricMatt]

I am not questioning anyone, I have not done many transformer calculations so I want to make sure I understand the necessary calculations and the theory behind them.

Thank you for he responses so far very helpful!


Sent from my iPhone using Tapatalk

 

[Jraef]

I am getting he 250amp main breaker from my one line diagram.


Sent from my iPhone using Tapatalk

So you have a one-line that shows a 250A breaker that is feeding the 225A breaker that is listed as the Main CB in this panel? Seems redundant...

 

[ElectricMatt]

250a is on the secondary side. I typed main, I meant secondary OCP


Sent from my iPhone using Tapatalk

 

[david luchini]

I don’t have my code book with me right now, but to make sure I am understanding this .

480 v primary 75kva = 90.3 fla

90.3 x1.25 =112.9 amps

Next size breaker up is allowed which would be 125a.

Is this correct?


Sent from my iPhone using Tapatalk

That is correct. If the primary OCPD was larger than 125A then the secondary OCPD would be limited to 300A maximum.

 

[JFletcher]

I don’t have my code book with me right now, but to make sure I am understanding this .

480 v primary 75kva = 90.3 fla

90.3 x1.25 =112.9 amps

Next size breaker up is allowed which would be 125a.

Is this correct?


Sent from my iPhone using Tapatalk

Yes, that is correct. Matter of fact because your primary protection is sized at 125% or less, the secondary protection is not even required. That it is 225 or 250 amps is irrelevant.

edited to add... I see on the diagram that the panel main circuit breaker is 225 amps and that the mains rating is 250 amps. What you have described thus far is a perfectly compliant code installation.

 

[electrofelon]

I am not questioning anyone, I have not done many transformer calculations so I want to make sure I understand the necessary calculations and the theory behind them.

Thank you for he responses so far very helpful!


Sent from my iPhone using Tapatalk

There is more to than the next size up rule. You have to look at table 450.3 (b)

 

[kingpb]

With a 125A primary circuit breaker, the secondary circuit breaker can be any size, per T450.3(B) "Primary only protection." 250A seems reasonable.

I don't think that "Not Required" is the exactly the same as "can be any size"; understanding in the context you are talking about the transformer only but your statement could be misleading when it comes to the panelboard which in this case would be limited to 250A, maximum.