Sizing XFMR secondary conductors according to MCB AT?

I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
We have a 4000A MCB with a 3000AT on a 480V SWGR, 4000A bus, 3PH 3W. Our lead EE wants to size the incoming transformer secondary conductors according to the 3000AT instead of the 4000A.
Is this correct?


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The conductors need to be the same size or larger than the OCPD where they terminate. They will terminate at the CB with the 3000 amp trip?
 
You should size them based on the trip setting, not the frame size. However, this is assuming nobody in the field is going to walk up to the breaker and change the LTPU to something greater than 3000A.
 
lielec11 said:
You should size them based on the trip setting, not the frame size. However, this is assuming nobody in the field is going to walk up to the breaker and change the LTPU to something greater than 3000A.
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That’s my only concern. And precisely how is this conveyed to the field? Should there be signage or some other electro/mechanical means to prevent this from happening ?


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Isaiah said:
That’s my only concern. And precisely how is this conveyed to the field? Should there be signage or some other electro/mechanical means to prevent this from happening ?


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See 240.6(B)&(C). Note the per (B) if the trip setting is accessible, the conductors must have an ampacity equal to or greater than the maximum setting.
 
don_resqcapt19 said:
See 240.6(B)&(C). Note the per (B) if the trip setting is accessible, the conductors must have an ampacity equal to or greater than the maximum setting.
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Most, if not all, modern static trip breakers have this clear protective cover in my experience. I've never looked at the code section you mentioned, I guess because when designing we always assume the CBs were "restricted access"? Anyway, thanks for sharing I guess I always glazed over this section.
 
lielec11 said:
Most, if not all, modern static trip breakers have this clear protective cover in my experience. I've never looked at the code section you mentioned, I guess because when designing we always assume the CBs were "restricted access"? Anyway, thanks for sharing I guess I always glazed over this section.
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In my opinion the code language in (C) would require this cover to have a tamper seal. If not (B) would apply.
 
don_resqcapt19 said:
In my opinion the code language in (C) would require this cover to have a tamper seal. If not (B) would apply.
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I don't agree, it just says "removeable and sealable" cover, it does not specify tamper proof. Also, number (3) makes me think as long as the electrical closet/room is locked, you are good. Thoughts?
 
lielec11 said:
Most, if not all, modern static trip breakers have this clear protective cover...
Click to expand...
And that is all that is required.
The NEC does not require the cover to be sealed at all, it simply needs the provisons to be. But sealing is more about tamper indicating rather than tamper preventing. Companies like Schneider Electric sell plastic sealing items that are not much more than a wire tie.
 
lielec11 said:
I don't agree, it just says "removeable and sealable" cover, it does not specify tamper proof. Also, number (3) makes me think as long as the electrical closet/room is locked, you are good. Thoughts?
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Sealabe equals tamperproof in my opinion...you have to break the seal access the settings
Have never seen gear with adjustable trip breakers in locked electrical rooms...but my experience is mostly adjustable.
 
In my understanding AT only refers to maximum permissible setting, aka the rating, of the OCPD. According to the coordination study, the LTPU can be set even lower, and the conductors are only required to coordinate with the final LTPU setting. Whether this is wise for future proofing, is another question.
 
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