Sizing transformers

[n240sxy]

How do you size a 277/480V to 120/208V transformer that has a 200A circuit breaker on the primary size? Once the transformer has been sized, how would you size the secondary 120V Panel and it's main circuit breaker?

 

[iwire]

In general you calculate the load you need to supply, that determines the size of the transformer and to some extent the over current protection and conductor sizes you will use.

 

[nakulak]

I think you generally size a transformer based on how much power you need. what are the requirements on the secondary ? x safety factor = size of transformer

 

[n240sxy]

On the secondary side, it's powering a 225A panel

 

[iwire]

On the secondary side, it's powering a 225A panel

So it is allready in place and wired?

What size is the transformer in kVA

 

[n240sxy]

it's not wired. it is in design stage. For a 225A 120/208V, 3ph, 4W panel, what should the transformer and primary side protection size be designed for?

 

[nakulak]

appears to be 112kva, but you are assuming that you have a fully loaded 225A panel which may or may not be the case., knock off a few amps and you could easily get by with 75.

 

[mull982]

appears to be 112kva, but you are assuming that you have a fully loaded 225A panel which may or may not be the case., knock off a few amps and you could easily get by with 75.

I believe you can use a 75kVA transormer and use a 225A panel main breaker as the transformer secondary protection device (within 125% of transformer secondary) and still be o.k.

 

[hillbilly]

appears to be 112kva, but you are assuming that you have a fully loaded 225A panel which may or may not be the case., knock off a few amps and you could easily get by with 75.

How did you calculate 112KVA? :smile:

steve

 

[jim dungar]

In this area, based on 480-208Y/120, many people use:
50A breaker -> 30kVA -> 100A panel
80A breaker -> 45kVA -> 150A panel
100/125A breaker -> 75kVA -> 200/225A panel
150A breaker -> 112.5kVA -> 400A panel
200/225A fuse/breaker -> 150kVA -> 600A panel

 

[charlie b]

How did you calculate 112KVA?

Here's how I get it: 225 amps times 208 volts times 1.732 gives me 81KVA. The next size up is 112.5.

 

[charlie b]

it is in design stage. For a 225A 120/208V, 3ph, 4W panel, what should the transformer and primary side protection size be designed for?

How did you select a 225A panel in the first place? That should not be step 1, as others have already said. First, you calculate the load. Then you pick the equipment ratings (transformer and panel). The last step is picking the overcurrent settings.

There is nothing in the code that would prevent you from feeding a 225 amp panel with a 5 KVA transformer, so long as the calculated load is less than 5 KVA. You have to start by knowing the load.

 

[iwire]

I believe you can use a 75kVA transformer and use a 225A panel main breaker as the transformer secondary protection device (within 125% of transformer secondary) and still be o.k.

Yes you can and it is a popular set up but the load must remain under 208 amps.

If your design for 225 amp panel is so close that the load is greater then 208 amps you would have to use the 112.5 transformer.

 

[kjless]

could you use 3/0?