800A & 400A Transformer and PDC & OCPD

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Kannox

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CA
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PM
To pull the 800A 3PH 208/120A loadcenter from 3PH 480/277V service with 3000A MSB. Currently, we have 1 x 350A and 400A PDC as spares for future use and plan to use one of them. What size transformer should we ask the contractor to install to use? If not suitable due to NEC code, what size PCD, and transformer if we have to add as new to have a panel with a secondary 800A breaker for the equipment? The total expected load on 208V side is a little less than 500A according to MFG but we want to keep the rest available as a reserve.
 
For 800 amps of 208 you would likely use a 300kva transformer (common size)
 
Can you provide a one line drawing? Need to see wire sizes, primary and secondary OCPD. Transformer protection rules are somewhat different than branch ckt and feeders
 
Kannox said:
Thank you. Can we use existing 350 or 400A circuit breakers that are not in use?

Someone said from Matintainance that PCD is the primary circuit disconnect and MSB is the main service breaker. Not sure if they are the correct terms.
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A 400 amp would be compliant for a 300kav but it wold be best to provide a one line as tom suggests (post #4) in case I( am misinterpreting the scenario.
 
tom baker said:
Can you provide a one line drawing? Need to see wire sizes, primary and secondary OCPD. Transformer protection rules are somewhat different than branch ckt and feeders
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The contractor is working on the drawing with his engineering. What was told is:
On Primary, 3PH4W 400A OCPD w. 4/0 Cu THHN with 2 parallel => 300KVA transformer => 250 mcm Cu THHN with 3 parallel run to a load center with 800A main OCPD. Total estimated total load on secondary is less than 180kVA, 500A.
 
Per 240.21(C) if you are terminating on a 800 amp Over-current device the transformer secondary conductors must be rated 800 amps or greater.
The "next size up" rule (240.4) can not be applied,
255 X 3 = 765 = < 800
 
augie47 said:
Per 240.21(C) if you are terminating on a 800 amp Over-current device the transformer secondary conductors must be rated 800 amps or greater.
The "next size up" rule (240.4) can not be applied,
255 X 3 = 765 = < 800
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I see. So I need to check the size of the conductors on their final drawing and make sure that they are equal to or larger than 800A OCPD despite the contractor's statement. Big thank you for the clear explanation.
 
And a gentle reminder the EGC if a wire type for your secondary conductors must be full size in each raceway, minimum size is 1/0.
 
tom baker said:
And a gentle reminder the EGC if a wire type for your secondary conductors must be full size in each raceway, minimum size is 1/0.
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That would only apply on the load side of the secondary OCPD. On the line side they would be supply side bonding jumpers and sized based on the ungrounded conductor size in the raceway and Table 250.102(C)(1).
 
don_resqcapt19 said:
That would only apply on the load side of the secondary OCPD. On the line side they would be supply side bonding jumpers and sized based on the ungrounded conductor size in the raceway and Table 250.102(C)(1).
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Good point to clarify the difference between transformer secondary conductors and feeder conductors
 
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