Help Conductor and Transformer OCPD

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brother

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"I have learned, in whatsoever STATE I am, therewi
Just brushing up on my calcs. I saw this 500 kw 208/480 volt 3 phase transformer being fed off a 800 amp breaker.
I come up with 500,000/(1.73 X 480)=602.12 amps
602.12 amps per phase right??

The conductors appeared to be ran in parrallel about 4 ft long each (could only see the pipe not the conductors) There were 3 conduit runs(if i recall correctly) from the transformer to the 800 amp. I read table 450.3(B) it looks like you can go as high as 125% of the rating of the transformer for primary protection. 1.25 X 602.12=752.65 . According to 240.6 they dont make a 752 amp breaker so you can go to 800 amps on this, i also read the note #1 on table 450.3(B)

I dont have much experience on some of the transformers. so in each conduit
I assume are 3 4/0 cables (4/0 good at 230 amps table 310.16) and maybe one conduit has a 1/0 ground in one of them table 250.122 (800 amp overcurrent device) 3 X 230 amps= 690 amps.

But would this 800amp also serve as the ocpd for the primary conductors feeding the transformer?? I know the 800 amp can serve as the ocpd for the transformer itself but what about the conductors?? Do they not need overcurrent protection?? then again maybe its 250 mcm in this run thats parrelled.

How would an install like this normally be done??
 
brother,

For parallel feeders the EGC must be run full size in 'each' conduit. 250.122(I)

This transf. is a step-up or step-down ?
 
The primary conductors must have overcurrent protection at or below their rated ampacity. With most transformers you would size the OCPD to protect the transformer and install conductors from the OCPD to the transformer that have a ampacity equal to or greater than that of the OCPD.
 
don_resqcapt19 said:
The primary conductors must have overcurrent protection at or below their rated ampacity. With most transformers you would size the OCPD to protect the transformer and install conductors from the OCPD to the transformer that have a ampacity equal to or greater than that of the OCPD.
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would it be ok to go to the next UP size breaker on parrell (sp?) feeds to the transformer? for example if you had 250 mcm 3 conductors per phase on a 800 amp breaker? 250 is good at 255 amps X 3 = 765 amps, they dont make a 765 amp breaker.

I know you can do it for single conductors under certain conditions, but would it be ok to do it for parralles
 
The secondary of the transformer is a separtely derived system and table 250.66 applies for sizing the grounding electrode conductor.Table 250.122 for primary. Pay attention to Art 240.21 (B)(1) thru (B)(5) for tap rules.
 
Brother, since the primary OC device does not protect the conductors, that would be considered a tap supplying a transformer, and you'd have to meet 240.21(B)(3)(1-5), including limiting the size of the secondary OC protection and overall primary plus secondary length. Also, note when sizing the secondary OC device and conductors, 240.21(B) disallows the use of 240.4(B).
 
In the instance jameselectric sited, I don't see why 240.4(B) wouldn't apply, allowing you to choose the next standard size breaker.
 
I don't know about that, read " Don's " post agian , ' at or below it's rated

ampacity ' that is the OCPD and the transf.'s rated ampacity. Just reread it.
 
benaround, I think Don was talking about the feeder conductors to the transformer needing to be sized at or below the primary OC protection device rating (which can be 125% of the transformer-rated current per Table 450.3(B)). Where in the code does it say that 240.4(B) can not be applied to the OC device protecting the conductors feeding a transformer?
 
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