Transformers

[strand16]

What would the breaker size need to be for a transformer with a 3P 480V primary and a 3P 120/208 secondary. The load is going to be around 50A initially but need to leave room for expansion to 100A? I was going to put in a 60A breaker with #6 but was told this was to big.

 

[jim dungar]

What would the breaker size need to be for a transformer with a 3P 480V primary and a 3P 120/208 secondary. The load is going to be around 50A initially but need to leave room for expansion to 100A? I was going to put in a 60A breaker with #6 but was told this was to big.

First - choose the conductors for your load.
Second - choose the protection for those conductors per 240.20(C).
Third - choose the primary protection for the transformer per 450.3: 125% maximum, unless the device you chose in step 2 is not greater than 125% of the transformer output.

 

[chaterpilar]

The most important parameter is kva rating?

What is the kva rating of your transformer?

Cheers.

 

[strand16]

transformers

transformers

thanks for responding the KVA rating is 30

 

[chaterpilar]

The maximum you can load this transformer @ 208 volts is 96 amps.

S0, secondary breaker would be of 100 amps, you have mentioned that your present load is only 50 amps ( Is that motor load?) if yes, what is the size of the biggest motor?

The primar circuit breaker (@480volts) can be of 40 amps ( with inrush current delay feature).

The primary inrush current of any transformer can go upto 12 times the primary full load current for few milliseconds and hence they have to have the inrush delay in them.

 

[donw]

chaterpilar, I come up with 83.3A. How do you get 96A?

 

[chaterpilar]

yes it is 83.3 A., but i would still use a 100 amps breaker.

 

[kingpb]

What would the breaker size need to be for a transformer with a 3P 480V primary and a 3P 120/208 secondary. The load is going to be around 50A initially but need to leave room for expansion to 100A? I was going to put in a 60A breaker with #6 but was told this was to big.

Answering from a purly technical standpoint, a 3-phase system with 120/208V does not exist and therefore it is meaningless to provide an imaginary breaker size.

Now, if you actually meant a 208Y/120V system, well that is entirely different. (That should get a post outa CharlieB :grin

With a low voltage side breaker at 125% of 83.3 A is the 100A, but if your load is going to grow to 100A then you will need a 45kVa Xfmr. If it is for sure a 30KVA transformer, then look in Table 450.3(B), the 480V protection can be as high as 250%.