# Checking my work



## icdubois (Nov 16, 2013)

Yes this is a homework problem but I want to double check my answers.

I have a 50KVA transformer 480 Delta to 120/208Y. I need to find the primary OCPD the Secondary OCPD the primary and secondary wire size, the GEC, EGC, supply side bonding jumper, System bonding jumper, and conduit size for the primary and secondary.

Here's what I came up with
Primary: 
OCPD 80A 
50KVA*1000/480*1.73=60 60*125%=75A then round up to find common size OCPD per 450.3(B)
75A = #4cu Hots per 310.15
#8 ground per 250.122
1" FMC per Table C.3

Secondary:
175A OCPD
50KVA*1000/208*1.73=138 138*125%=173A round up to 175A OCPD 
173A= 2/0cu Hots
#4 Neutral
#6 ground
2" FMC

#4 Supply Side bonding jumper 250.102
#4 System Bonding Jumper 250.102
#4 GEC 250.66

#4 GEC Ground bar to building steel
#4 SBJ XO to ground bar
#6 EGC ground bar to ground bar in panel
#4 Neutral XO to Neutral Bar in panel

Does this look correct, anything wrong with my findings?


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## telsa (May 22, 2015)

icdubois said:


> Yes this is a homework problem but I want to double check my answers.
> 
> I have a 50KVA transformer 480 Delta to 120/208Y. I need to find the primary OCPD the Secondary OCPD the primary and secondary wire size, the GEC, EGC, supply side bonding jumper, System bonding jumper, and conduit size for the primary and secondary.
> 
> ...


Typical homework... as you'll find it essentially impossible to purchase a 50kVA 3 phase dry-type transformer.

45kVA is the NEMA standard.

50kVA is available in SINGLE PHASE, though.
*
See Ugly's tables.*

BTW, it's COMMON for the Secondary GEC System run to be integrated into the MAIN Service GEC System.

Re-stated, the Xo is bonded every which way... and then the conductor is routed all the way up and back to the Service where it will be bonded to the MAIN GEC System.

This path will entail bonding bushings the whole way... wherever ferrous raceways are involved. 

This real world scheme is just not found in homework questions, BTW. I've never seen it.

Back out in the real world, the standard is 75kVA so that one can drive a 225A three-phase panel.

45kVA is a most uncommon size, as no-one usually wants to set merely a 125A panel... though you'll find this size used for isolation circuits.


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## B-Nabs (Jun 4, 2014)

45 kVA is admittedly not as common as 75, but I wouldn't go so far as to call them uncommon. I have personally set several of them, and my career is not yet particularly long.

Sent from my SM-G920W8 using Tapatalk


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## telsa (May 22, 2015)

B-Nabs said:


> 45 kVA is admittedly not as common as 75, but I wouldn't go so far as to call them uncommon. I have personally set several of them, and my career is not yet particularly long.
> 
> Sent from my SM-G920W8 using Tapatalk


They're not in the NEMA catalogs.

They're not in the Ugly's tables.

They don't mate with NEMA standard panel sizes. ( 125A and 225A totally dominate 3-phase panel frame sizes. These can be easily cut back to 100A and 200A via OCPD. )

50kVA does exist in single-phase designs, though.

The moment ANYONE moves away from NEMA standard designs -- let me assure you -- prices EXPLODE.

I was quoted $2,300 for a transformer that penciled out for $300 if I went with a NEMA standard design. The size did not change. It was merely the wiring pattern. (!!!)

That was twenty-five years ago.


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