Engineer Please Explain

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infinity

infinity

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This is a 200 amp panel with a 200 amp main so why would anyone specify 300 kcmil from the transformer to the main CB? The conductors are less than 10' long. IMO this is a dumb design and a waste of money.

200 Amp Panel.jpg
 
75 kVA @ 208V is 208 A. The conductor should be sized 125% of the design amps, therefore the ampacity of the wire required must be greater than 208 A x 1.25 = 260 A. Isn't 4/0 good for that 260 amps?
 
topgone said:
75 kVA @ 208V is 208 A. The conductor should be sized 125% of the design amps, therefore the ampacity of the wire required must be greater than 208 A x 1.25 = 260 A.
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That all sounds like a design criterion, not an NEC requirement. I would think the only NEC requirement is that the conductor is of sufficient size for the calculated load per Article 215/220, and that it is properly protected per Article 240.

Maybe in the OP the designer doesn't like 240.21(C)(2) or (6), and figured out the worst possible secondary current without overloading the primary protection (which is more than just 480/208 * the primary OCPD, because of the secondary neutral conductor, IIRC) and chose to size it accordingly. Not sure how to do that, so I can't check if that comes out to 300 kcmil.

Cheers, Wayne
 
infinity said:
This is a 200 amp panel with a 200 amp main so why would anyone specify 300 kcmil from the transformer to the main CB? The conductors are less than 10' long. IMO this is a dumb design and a waste of money.

View attachment 2562157
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Don’t assume MEPS do the calculations. They use charts with grossly overly conservative (incorrect) values. Most have no idea how to use the Nether-McGrath tables.
 
Interesting that this is a feed thru setup and 3/0 is spec for the 2nd panel. So why would 300 be on the supply to the first panel? If it was on my job I would certainly question it.
 
Since the OCPD is 200 amps is there any reason why the secondary conductors couldn't be #3/0? Looks like the 300 kcmil was chosen intentionally since the SSBJ (#2) is the correct size for 300 kcmil. We haven't recived the panelboard yet but I'm thinking that a 200 amp CB terminals might not even accept the 300 kcmil.
 
I just done a gate operator at a natural gas facility that requires 1-20amp circuit.

It was probably 150ft from the MCC.

Got there and they had 3c#4AWG 🫣
 
Kwired and Dennis what you may be referring to as the EGC is the SSBJ and may be why infinity said where.
The #2 for the 300 is based off of 250.102 for SSBJ.
However I could be not seeing something.
1663446440773.png
If your referring to the #6 I don't see where it list the OCD for the feeder. Infinity may know what it is and not shown in pic.
 
My unpopular (on this forum) engineer's take: We don't know exactly why it was slightly oversized. As I used to tell contractors - the NEC is not a design guide. It's not outrageously oversized and I doubt it was a typo (maybe a copy/paste error). It's very common to size xfmr secondary feeders at 125% of the transformer rating. Perhaps there was some thought that the panel MB size might be increased at some point. If this was on a set of plans that was bid to, it doesn't really matter what the reason is. It meets the NEC, so that is what should be installed. I would see nothing wrong with submitting an RFI asking for confirmation that this size is what was intended, but assuming that everything should be built to bare minimum NEC sizing is not a good way to look at things. No matter what the design is, someone can always find a way to do it more cheaply.
 
Tulsa Electrician said:
Kwired and Dennis what you may be referring to as the EGC is the SSBJ and may be why infinity said where.
The #2 for the 300 is based off of 250.102 for SSBJ.
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That's correct the conductor in question is a SSBJ and sized according to the 300 kcmil secondary conductors so #2 is the ocrrect size. It is not an EGC.
 
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