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Tuesday, April 4, 2023
Question:
Hi CQD, I have a great debate in the office regarding Transformers and the sizing of conductors and OCPD for a 3-phase Delta/Wye 30kVA transformer. Sizing the Primary at 250% [30,000/831=36.10Amps X 2.5] =90.25A, the maximum OCPD will be a 90A breaker. The conductor for this primary is the debate, can you use minimum #4 AWG at 85A, using 240.4B and 75
oC, or do you have to use #3AWG at 100A. Thank you for continuing this forum. Great Job! Jason Reinsch
A
Answer:
Jason, thank you for the kind words, and for participating in the CQD. Based on the information in your question and the calculated ampacity for the primary of the transformer, and without knowing the temperature rating of the connection lugs, Section 110.14(C)(1) requires that the 60o C column of Table 310.16 is used for circuits rated 100A or less. For a rating of 90.25A as calculated, a #2 AWG (95A) Copper conductor would need to be used. Additionally, for transformers rated 9A or more, when the primary OCPD is sized at 250%, Note 1 of Table 450.3(B) does not apply, therefore the next higher standard OCPD cannot be used. The OCPD would have to be rated 90A.
Keep in mind the OCPD sizing provided in Table 450.3(B) is the
maximum rating for transformer protection, meaning you could provide primary protection rated less than 250% of the transformer current rating if the system design permits.