Transformer secondary

[hhsting]

Attached sketch shows 480V to 208V/120V three phase 150kva transformer secondary conductor 2 sets of 5#4/0 + 2/0 fed going into trough. The conductors are tapped to three panelboards P1, P2, P3as shown. Each tap is 4#1/0 + #2 GRD to each panelboard as shown in sketch. Each secondary panelboard has 250A MCB as shown in sketch. The transformer primary is fed from 300A breaker.

Questions:

1. Is the attached sketch code complaint in terms of secondary conductors being tapped as shown? If not then which code section. I don’t see can’t tap a tap NEC 2014 for transformer secondary conductor.

2. How would one size equipment grounding conductor from xfmr secondary conductor to trough and from trough to panelboard? Based off of what?

 

[electrofelon]

So much wrong with that. Perhaps send the plans back with a note that just says, "LOL, try again"

I don't have the book in front of me, but see the beginning of 240.21 where it says something like "conductors supplied under these provisions shall not serve another conductor except thru an OCPD." that means you can't tap a tap. It could be made compliant by running a dedicated set to each PB, complying with 450.3(b) if necessary.

 

[david luchini]

Attached sketch shows 480V to 208V/120V three phase 150kva transformer secondary conductor 2 sets of 5#4/0 + 2/0 fed going into trough. The conductors are tapped to three panelboards P1, P2, P3as shown. Each tap is 4#1/0 + #2 GRD to each panelboard as shown in sketch. Each secondary panelboard has 250A MCB as shown in sketch. The transformer primary is fed from 300A breaker.

Questions:

1. Is the attached sketch code complaint in terms of secondary conductors being tapped as shown? If not then which code section. I don’t see can’t tap a tap NEC 2014 for transformer secondary conductor.

2. How would one size equipment grounding conductor from xfmr secondary conductor to trough and from trough to panelboard? Based off of what?

In addition to not being able to tap the secondary conductors, you don't have proper transformer protection.

There is no EGC from the transformer secondary, there would be a SSBJ.