Main Breaker Sizing

[Electricalhelp]

The client gave me the following information
"1500KVA Transformer-13,200V-480Y/277V Delta Primary - Wye Secondary"

I am trying to size the main breaker after the transformer

I'm not sure if I use 480V or 277V in my calculation for the Main Breaker

Since the voltage at the Main breaker should be 277/480V/3P

 

[infinity]

Use 480 volts in your calculation. 1,500,000/480/1.73=1806 amps

 

[Electricalhelp]

That's what I thinking but the current main breaker installed is 2500/3P

 

[infinity]

Take a look at T450.3(A). For a 480 volt secondary you would typically use 125%. {1806*125%=2257 amps} next standard size 2500 amps.

 

[Electricalhelp]

Thank you for the help infinity

If there is another step down transformer after the main breaker (2500A/3P) at 750kVA 480-208/120V
Can I use T450.3(A) to size the breaker after the transformer again

 

[david luchini]

If there is another step down transformer after the main breaker (2500A/3P) at 750kVA 480-208/120V
Can I use T450.3(A) to size the breaker after the transformer again

Use T450.3(B) for the second transformer.

 

[Electricalhelp]

If I am sizing the Primary and Secondary protection for the 750kVA 480V-208/120V transformer
For the Primary: 750kVA/√3*480=902*1.25 = 1128A. Which I rounded up to 1200A
For the Secondary: 750kVA/√3*208=2082*1.25 = 2603A. Which I rounded up to 3000A.

I don't think the size of the secondary is correct. Since it has a 1600A/3P breaker installed for 208/120V/3P.

 

[Electricalhelp]

FYI...The 1600 AMP is than feeding 5 Distribution Panels

 

[electrofelon]

You don't have to use the max ocpd size. For the secondary it's all about the load. For the primary you typically would want to stay somewhat near the max for inrush when energizing the transformer.

 

[Electricalhelp]

For the 750kVA on the secondary, so I could max it out to 3000A if the secondary load requires it.

I was assuming there is a safety factor of 80% in the code I was missing somewhere. Hence why the current breaker installed was 1600A.
For the Secondary: 750kVA/√3*208=2082*.8= 1665.6 A. So the rounded down to 1600A.

 

[infinity]

For the 750kVA on the secondary, so I could max it out to 3000A if the secondary load requires it.

I was assuming there is a safety factor of 80% in the code I was missing somewhere. Hence why the current breaker installed was 1600A.
For the Secondary: 750kVA/√3*208=2082*.8= 1665.6 A. So the rounded down to 1600A.

Not sure why they oversized the transformer so much but using only 1600 amps on the secondary could be a question of economics. If you went with the 3000 amps then your conductors and OCPD would need to be 3000 amps also. Big difference in cost between 1600 and 3000 amps.

 

[Electricalhelp]

I agree, but they are going to be an additional 20 more apartments and everything is going to be Electric.

I just want to make sure the new load can be accommodated by the current transformer