Xfmr ?

[Redskinsdb21]

We have a 45 KVA, 3-phase, 480-120/208 V Xfmr, determine secondary conductors size annd secondary overcurrent device?

The solution says:I = VA divided by (208*1.732) = 45kva divided by (208*1.732) = 125 Amps, why do they multiply line to line voltage, by 1.732, shouldnt they divide by 1.732 other than multiply by it to get line to neutral voltage?

 

[Wildsoldier PE]

The solution is correct. If 120V is used you need to divide the total KVA by 3 since is a 3 phase this equal 15 kva per phase.

Now 15kva/120=125 amps

Which give the same answer.

I wish i can help you better understand this but the only way to learn this is going thru the college book and learn about 3-phase systems..let me know if you have a book that teach you this. If not i can recommend one that will help you a lot.

Now if my memory is correct thats the full load amps...so I suggest an over current protection of 150amps since you cant load a breaker more than 80%... thats takes care of breaker size...and the wire shall be 1/0. Now if fuses are used 125 amps is fine and the wire can be #2 per code is allowed, but in reality i suggest to go with #1. Im using copper with 75 d. insulation.

 

[knight1fox3]

so I suggest an over current protection of 150amps since you cant load a breaker more than 80%... thats takes care of breaker size...

Depends on the application and the type of circuit breaker being used.