transformres ocpd

[kilobee]

hey guys i need your help on this im installing a step up xmr (120/208 - 480 )to take 1200 amps at 480 to another xmr and stepped down to 120/208 with another xmr at the first xmr and protecting the primary by a 1500 amps disconnect base on the 125 % on table 450.3 (b) .Now do i need any protection on any or the other sides of the second XMR or the CB's in the distribution panel on the secondary are going to protect that side

 

[coulter]

kb -

I understand jt's pain about a mildly convoluted system explanation. A sketch would help.

If I am understanding you, the answer is yes.

 

[jim dungar]

Transformers always require some type of primary protection under article 450.
Secondary conductors are always subject to article 240.

 

[templdl]

That's an interesting point.
Regarding the step-up transformer I would agree that providing a pri. OCPD as you have described is allowed as the secondary is simple 3ph 3w.
When considering the second transformer as a step down feeding 3ph4w you would have to provided secondary protection which actually protects the transformer form overload if sized correctly.
The question is what about the pri OCPD? If the transformer is already protected from overload would the pri. OCPD be redundant?
Or, would one consider that the pri. OCPD may be simply short circuit protecting in the event of a transformer failure should their be a winding failure.
If one were to agree that the pri. OCPD on the first transformer also protects that transformers as well as the secondary conductors that feed the second transformer will that pri. OCPD located on the line side of the step-up transformer see and respond to a failure (short circuit) that may occur in the second transformer?
This may not answer your question but I personally believe it would be allowed as that pri. OCPD does provide short circuit protection for 3ph3w secondaries, which should also provide short circuit protection for that second transformer.

 

[don_resqcapt19]

The first transformer requires primary protection. Assuming that it is 208 delta to 480 delta and that the primary protection does not exceed 125% of the primary rating, secondary protection is not required and the conductors between the two transformers are protected by the primary protection of the first transfomer. At the second transformer protection is required for the secondary conductors per the rules in 240.21(C).
Don

 

[templdl]

Don,
As I understand from your reply an OCPD is not required at the Pri. of the second transformer as I had assumed.

 

[don_resqcapt19]

templdl,
I don't think that one is needed assuming that the primary rating of the second transformer is eqaul to or greater than the ampacity of the feeder conductors between the two transformers. If the conductors are protected then the transformer primary must also be protected.
Don