Thiết bị & Hệ thốngCâu 262 / 521
A 30 kVA, three-phase transformer has a 480 V primary (primary current ~36 A). With primary-only protection and no supervised condition, what is the maximum primary overcurrent device using the 125% factor of Table 450.3(B)?
a.36 A
b.45 A
c.60 A
d.72 A
Giải thích
For a transformer with primary current of 9 A or more, Table 450.3(B) limits primary-only protection to 125%: 36 x 1.25 = 45 A (with the note allowing the next standard size where 125% does not correspond). 36 A omits the factor, 60 A is ~167%, and 72 A is 200%.
Trích dẫn luật: 2023 NEC §450.3Luyện miễn phí toàn bộ 521 câu hỏi — không cần đăng ký.
Câu hỏi liên quan cùng chủ đề
- An A/C condensing unit has a compressor rated-load current of 18 A and a fan motor of 2 A. Using the 125%-of-largest-motor-plus-others method, what is the minimum circuit ampacity (MCA)?
- For a hermetic motor-compressor with a rated-load current of 18 A, what is the maximum branch-circuit short-circuit/ground-fault protective device, based on the 175% value of NEC 440.22(A)?
- If a motor-compressor (rated-load 18 A) will not start on the 175% device, NEC 440.22(A) permits increasing the protection to a maximum of what value?
- NEC 450.3(B) overcurrent protection percentages for transformers 1000 V or less are based on what?
- What is the rated primary current of a 25 kVA, single-phase transformer with a 240 V primary?
- A 45 kVA, three-phase transformer has a 208 V secondary. What is the rated secondary current?
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