NEC 240.21(C) Transformer Secondary OCPD

[Shujinko]

Installing a 75KVA transformer in a building. 480V delta pri - 480Y/277V sec, This will feed a panel with a 250A MCB that is more than 25' from the transformer secondary. My interpretation on 240.21(C) and 240.21 is that the Overcurrent protection device needs to be within 25' or 10' of the transformer secondary. It was brought to my attention that 240.21(C) is only talking about rules for not providing an OCPD on the transformer secondary. That once you provide an OCPD on the secondary you can put it any distance away from the transformer secondary. I was noting that 240.21, requires the OCPD to be located at a point where their conductors receive their supply except as specified in 240.21 A-H. So which one would be correct, OCPD within 25' (max) of secondary or no limit of OCPD distance from transformer secondary? Please provide applicable code citations.

My state in under NEC 2017.

 

[augie47]

240.21(C) addresses the conductors from a transformer in general noting that those conductors should meet the specifics in 24021(C)(1)-(C)(6)
(C)(1) specifies where secondary protection is not needed in certain situations which are not relevant to your installation therefore you must follow (c)(2) thru (C)(6) the mosty common being the 25ft rule.

 

[david luchini]

250A MCB panel seems a bit large for a 75kVA transformer.

 

[Engser18]

you are required to complied both Art 240 and 450. 240 for protection of primary and secondary conductors, 450 for protection of transformer. if your primary CB can protect your primary conductors then 25ft rule only apply on secondary only. if it is not, then 25 ft rule apply for both pri and sec (total can't exceed 25 ft). if it is outside you can apply unlimited rules. The 125% secondary of 75kva (480/277) is 112.76A, so 250MCB is so excessive. Keep in mind next standard sized CB is not allowed for protection of secondary conductors (the secondary conductors ampacity shall be equal or higher than CB)