Equipment & SystemsQuestion 263 of 521
NEC 450.3(B) overcurrent protection percentages for transformers 1000 V or less are based on what?
a.The secondary conductor ampacity
b.The connected load kVA
c.The rated primary (and where applicable secondary) current of the transformer
d.The available fault current
Explanation
Table 450.3(B) percentages (e.g., 125%, 167%, 250%, 300%) are applied to the transformer's rated primary or secondary current, not the load or fault current. This protects the transformer itself; separate rules protect the secondary conductors.
Law Reference: 2023 NEC §450.3Practice all 521 questions free — no signup required.
Related questions on this topic
- 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?
- 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)?
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- Why must a motor circuit have both overload protection AND short-circuit/ground-fault protection?
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