Transformer Question

[knot stable]

I have a 30KVA 3 phase transformer. 480 Pri 208 sec. Going to feed the 208 side cause I need 480 for a couple of machines (load is 24 amps). How would I size my OCDP. Using the load I am suppling? I'm thinking to much here and now I'm confused.

 

[augie47]

Follow 450.3(B) to select your transformer protection...(most likely 100 or 110 amps)
Select your secondary protection based on the correct protection for the individual load.
Don't forget 250.30 for grounding (and possibly 250.21(B) )

 

[knot stable]

Thanks Augie.

 

[kingpb]

Your considering a step-up situation which means the LV side is the primary and the HV side is the secondary.

I point this out because when describing transformers, the preferred nomenclature is to use simply the LV and HV designations. Primary and Secondary can be confusing, as seen here since it is a step-up.

The protection should be based on what will adequately protect the equipment. Slapping in a breaker willy-nilly based on some erroneous chart in the NEC is not necessarily going to work.

 

[texie]

I have a 30KVA 3 phase transformer. 480 Pri 208 sec. Going to feed the 208 side cause I need 480 for a couple of machines (load is 24 amps). How would I size my OCDP. Using the load I am suppling? I'm thinking to much here and now I'm confused.

While this can be compliant, I would not recommend it. I know it is done by some, but there are some real drawbacks to this as you now have a delta secondary. You will have no neutral and will have to be corner grounded or left ungrounded. If ungrounded, that triggers GF detection requirements. If you corner ground it, the equipment must be rated for this. Much modern equipment may not be suitable for connection to an ungrounded or corner grounded 480 system.

The better choice IMHO would be to get a 208 delta X 480Y transformer. They are a little more expensive but you get a lot of benefits doing it this way.

 

[Nom Deplume]

Do NOT connect the XO terminal on the LV side of the transformer.
You will need to ground one of the HV legs though. It will give you a corner grounded delta system.

 

[knot stable]

Do NOT connect the XO terminal on the LV side of the transformer.
You will need to ground one of the HV legs though. It will give you a corner grounded delta system.

Ok, which leg do you ground? But if I carry an equipment ground thru the system why is there a need to ground one leg, I do not need a neutral. I know not to connect XO.

 

[GoldDigger]

Because if you do not ground some reference point on the secondary side the NEC requires you to install a ground detector to alert you to the first fault to ground in the system.
And because not as many people are currently familiar with ungrounded systems and will get into trouble later.
And because some equipment is not suitable to be powered by an ungrounded system.
Just for starters....

Tapatalk!

 

[knot stable]

Because if you do not ground some reference point on the secondary side the NEC requires you to install a ground detector to alert you to the first fault to ground in the system.
And because not as many people are currently familiar with ungrounded systems and will get into trouble later.
And because some equipment is not suitable to be powered by an ungrounded system.
Just for starters....

Tapatalk!

Ok, But that still did not answer my question. "some reference point" so where is that reference point?

 

[Nom Deplume]

The reference point is the grounding electrode.

 

[Smart $]

Ok, But that still did not answer my question. "some reference point" so where is that reference point?

For corner-grounded delta, the grounded leg is considered Line B (i.e. grounded-B phase). NEC requires busses to be arranged left to right, top to bottom, or front to back. As such, the correlation would be Line A to Line C as the ungrounded in these bus arrangements.