30kva secondary breaker sizing

[pushbutton]

On a 30kva Three phase 480 delta ,120/ 208 Y. why would you calculate 83 amps at 208 if most of the circuits being used will be at 120v?

 

[Rock86]

because, P = V_L*I*sqrt(3)

 

[augie47]

It may help if you look at a 208Y/120 30kva 3 phase transformer as (3) 10kva 120v transformers. Same answer 83 amps.

 

[pushbutton]

Thanks gentlemen, I was over thinking You have to calculate 120 x 1.73 to get 208. So I will get 83 amps per phase.

 

[Rock86]

Thanks gentlemen, I was over thinking You have to calculate 120 x 1.73 to get 208. So I will get 83 amps per phase.

That is incorrect. V_L = 208, V_P= 120. Like Augie said, you could look it at as (3) 10kVA 120V transformers --> 10kVA / 120V = 83A, or if you look at it as 3-phase, 30kVA divided by (208V * 1.73) that will give you the 83A.

 

[pushbutton]

That is incorrect. V_L = 208, V_P= 120. Like Augie said, you could look it at as (3) 10kVA 120V transformers --> 10kVA / 120V = 83A, or if you look at it as 3-phase, 30kVA divided by (208V * 1.73) that will give you the 83A.

My math is wrong But Per phase is still 83 amps correct? so in a three phase panel My main will be 100 amp with capability of pulling 83 amps per phase which will give me a panel compacity of 249 if equally distributed between phases.

 

[augie47]

249 x 120 = 30,000

 

[Rock86]

It doesn't work like that exactly. Plain and simply... if you have a 3-phase system with a 30kVA transformer supplying a 120/208 3-phase system, your capacity would be 83A.

 

[Rock86]

249 x 120 = 30,000

They are not taking into account phase vectors.

 

[augie47]

They are not taking into account phase vectors.

Understood but I thought I might help answer the initial question in a simplified form.
I'll let you perform the vector analysis

 

[pushbutton]

They are not taking into account phase vectors.

30 amps per phase?

 

[Rock86]

30 amps per phase?

Rounding up to 30, sure. In simple 100% balance, take 83A from the 3-phase calculations and divide it by 3. that gives you 28A per phase.