transformer grounding and ampacity

[m2oliva]

I came across a transformer that was wired this way.. I belive there are several code violatons . I am just not sure of the sections they violate.First of all the building steel ground went to the outside of the cabinet.then they had a 45kva transformer with two greenfields coming off the load side ,each feeding a 100amp panel.In the same closet they have a 30 kva transformer feedind a panel with a 125 amp main.I would think that according to the code .the secondary calculation would be as follows 30kva/208x1.73. then take the 83x125% which =104amps. the next size breaker according to code is 110amps.are my calculation right?thanks

[ May 28, 2003, 09:08 PM: Message edited by: m2oliva ]

 

[jim dungar]

Re: transformer grounding and ampacity

You do not provide enough information to determine if there are any violations.

Lighting and appliance branch circuit panelboards must have OCPD sized not to exceed their busbar ratings.

The conductors feeding the panels must have properly sized OCPD at the point where they receive their power unless a tap rule is applicable, in which case the OCPD may be located at the load end.

You need to describe the grounding and bonding of the transformer core and case in more detail. In general, if the only ground is to the case only there is a probale violation. The core of each transformer and the X0 terminal (if present) needs to be bonded either at the transformer or at the panelboard.

The transformer secondary OCPD can be any size you want (larger or smaller) than the output of the transformer. The secondary OCPD is restricted to a maximum of 125% secondary current only if the transformer primary OCPD is larger than 125% of the primary current.

In your case:
The 45 KVA transformer is allowed to feed multiple 100A panels if the conductors are rated for 100A and the transformer primary OCPD is not greater than 125% of the FLA(a 60A device at 480V).

The 30 KVA transformer can feed a 110A panel only if the primary OCPD rating is 125%of FLA (or 45A rated at 480V). Of course the conductors to the panel must be rated for 110A.

Is this a good design? The NEC doesn't care.

[ May 29, 2003, 12:04 PM: Message edited by: jim dungar ]

 

[m2oliva]

Re: transformer grounding and ampacity

To explain the grounding that i belive is a violation is the gec is terminated to the outside of the cabinet. Is this not a violation?Also can you bring two greenfields off the secondary?doesnt this have to be done
in a pull box or trough?


gec
++++--9*---

 

[dereckbc]

Re: transformer grounding and ampacity

The GEC should be connected directly to the Xo terminal in the transformer, or the neutral bus of the first diconnecting means..