Secondary of a transformer

[dabatman]

Here's my situation: I am looking to feed a 200A load off a 500kVA 480/208V transformer. Primary side is a 800A breaker I believe. Right now there is a 1000A disconnect fused at 600A on the secondary. According to 250.21, with the 10ft tap, I would be at 184.8A. It also says you can't extend the secondary feeders past the device it feeds. I assume then I can't tap off the secondary and then feed a 200A disconnect to feed my load. Maybe I'm reading this wrong.

My new plan then is to have a 200A panelboard in-line with the 600A fuses to step it down and then feed my 200A non-continuous load. Does this work? Also, would I even need a panelboard? Couldn't I just have a box with lugs to land the 600A wire and then step down to a 200A breaker in a box? Is there a better solution that I'm missing.

 

[augie47]

I'm confused, but maybe I can shed some light.
As far as the transformer is concerned, look at 450.3(B). If your primary protection is 125% then you don't need to worry about secondary protection.
If your primary is over 125% then you need to restrict your total secondary protection.
As far as the wiring is concerned, you guide for transformers is 240.21(C).
Remember you can't tap a tap, but you can have multiple tap conductors from the transformer.
As far as your panel is concerned, see 408.36

 

[dana1028]

As far as the transformer is concerned, look at 450.3(B). If your primary protection is 125% then you don't need to worry about secondary protection.
If your primary is over 125% then you need to restrict your total secondary protection.

Maybe I'm confused too, but his XO sure seems like a delta-wye configuration [480/208]; if so, 240.21 requires secondary protection regardless of the size of primary protection. [240.21 "multiphase (other than delta-delta) XO secondary conductors are not considered protected by the primary OCPD."]

 

[augie47]

dana, I'll stick by what I say:

I don't think it differs from your comment as you quote 240.21 which
addressed overcurrent protection for the conductors, not the transformer.

I'm confused, but maybe I can shed some light.
As far as the transformer is concerned, look at 450.3(B). If your primary protection is 125% then you don't need to worry about secondary protection.
If your primary is over 125% then you need to restrict your total secondary protection.
As far as the wiring is concerned, you guide for transformers is 240.21(C).
Remember you can't tap a tap, but you can have multiple tap conductors from the transformer.
As far as your panel is concerned, see 408.36

 

[Volta]

Gus:
Are you considering 240.4(F) when you suggest not to worry about the secondary protection?

 

[augie47]

Perhaps I should have punctuated differently :grin:
In my statement "As far as the transformer is concerned, look at 450.3(B). If your primary protection is 125% then you don't need to worry about secondary protection." I intended to note that 450 covers transformer protection and NO secondary protection is required FOR TRANSFORMER PROTECTION if you have 125% primary protection per 450.3(B).
240.4(F) and 240.21(C) cover conductor protection and 408.36 covers panels and determine whether oveercurent protection is needed in regard to conductors & panels.

 

[dana1028]

dana, I'll stick by what I say:

I don't think it differs from your comment as you quote 240.21 which
addressed overcurrent protection for the conductors, not the transformer.

Augie - I agree with you 100% - but I don't think the OP was concerned with the XO, he was asking about the secondary wiring. That said, after rereading his post 3x, I don't think we've answered his question.

?? He now has an existing 600A fused disco on the secondary, and he only wants to supply a 200A rated panelboard? Is that what he's asking?

 

[augie47]

Augie - I agree with you 100% - but I don't think the OP was concerned with the XO, he was asking about the secondary wiring. That said, after rereading his post 3x, I don't think we've answered his question.

?? He now has an existing 600A fused disco on the secondary, and he only wants to supply a 200A rated panelboard? Is that what he's asking?

I'm not sure. As I started "confused". I was trying to give some basics that might answer his question. I doubt I did.