Sizing of 10ft. from XFMR secondary

[Geraldselectric]

I am having a problem understanding this exam question. Here is what is known and verified:

1. 75 degree terminations and use THWN
2. Feeder fed to 70Kva 480/208v transformer
3. Secondary supplies panelboard.

I calculated the following:
1. FLA to be 194A secondary output
2. Panel OCPD to be 175A to protect secondary output of 194a

The problem i run into is that I am told I should use a 2/0 (175a) to protect the secondary output. But, I came up with 3/0 (200a) when using the 10 ft. tap rule. Does anyone know if I only have to size this wire to the terminating OCPD in the panelboard and NOT size the wire for the larger 194A output on the secondary? I understand that the circuit should trip at 175A before overloading the secondary.

 

[cpal]

is this a single or three phase example ???Actually if it is 3P it probably should be a 75kVA Trans.

The secondary conductors only need be sized for the connected load. Is it a 70kVA load??

 

[don_resqcapt19]

Gerald,
As long as the load is less than 175 amps and the OCPD is 175 amps, you are permitted to use the 2/0 copper. See 240.21(C)(2).
Don

 

[Geraldselectric]

Test question gave 3 phase, 70Kva. This 2/0 is running from the secondary output to the sub panel 175A OCPD. I was concerned with having a conductor that also protect the transformer secondary output which is 194A.

 

[Geraldselectric]

The OR statement in 240.21c2(1b) clarified why I can use the 2/0 instead of 3/0. Thanks.

 

[Dexie123]

194*125%=242.5 amps.

Wouldn't your wire be required to be 4/0? With a breaker of 250amps?