Over current protection

[Eduardo Maun]

I got 2000 kva transformer, 3 phase delta-delta, 11kv, 433v/220 system. I calculated the primary current and I got 2,666.75 ampere i find in the sample problem there is a factor of 1.67 after multiplying that factor i got 4,453.47 ampere my question(s) are: where the 1.67 came from is it a factor of what? by the way does my calculation is correct? for the secondary i got 5,248.64 ampere and when I multiplied the Is (Vs/Vp) according to NEC the primary can protect the secondary conductors if the Ip does not exceed to value of secondary conductor ampacity by the secondary to primary transformer voltage ratio. I use the voltage ratio as 433/220v. I know you can help me on this.

Thank you!!!

 

[kingpb]

The 1.67 sounds like you have a transformer with 2 stages of cooling, therefore the lowest rating is 2000KVA, but with 1 stage of cooling it becomes 1.33 x 2000KVA = 2660KVA, and for second stage of cooling it becomes 1.67 x 2000KVA = 3340KVA.

At 433V the low voltage current would be the 4453 amps. I don't see what the 5248 amps represents.

 

[Eduardo Maun]

Over current protection...

Over current protection...

Thank you Sir,

The 5,248.34 ampere is the secondary current that I got from my calculations.

NEC says: The primary over current protection can protect the secondary conductors provided the primary protection device does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer ratio.

Ip = 4,453.46 A while Is = 5,248.64 Ip did not exceed 5000 A, is it safe that I will use an over current protection of 5000 A, If i will use 5000A does the secondary will be protected?

Again Thank you much Sir.

 

[bsh]

NEC 240.4 (F) does not allow the secondary cables to be protected if the transformer is delta-delta - only single phase single winding. The secondary cable will be protected by the secondary OC protection if you use one of the tap rules in 240.21.

 

[don_resqcapt19]

bsh,

NEC 240.4 (F) does not allow the secondary cables to be protected if the transformer is delta-delta - only single phase single winding.

It does in my code book.

(F) Transformer Secondary Conductors Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary to primary transformer voltage ratio.

Don