A 25 kVA single-phase dry-type transformer has a 240 V primary. Applying 450.3(B)'s 125% maximum for primary-only protection (primary current ≥ 9 A), what is the required standard primary overcurrent device rating?
Not quite. The answer is C: 150 A
Primary FLC = 25,000 VA ÷ 240 V ≈ 104.17 A. At 125%, the calculated maximum is 104.17 × 1.25 ≈ 130.2 A — not a standard overcurrent device rating, so 450.3(B) permits rounding up to the next higher standard rating under 240.6(A), which is 150 A (there is no standard 130 A or 140 A size). Stopping at the raw 130.2 A calculation without taking the next step up is the trap.
Reference: NEC 2023 450.3(B) / Table 450.3(B) (Transformer Primary Overcurrent Protection)
Checked against multiple published sources
Two trade-press articles agree that 450.3(B) Note 1 applies at primary currents of 9 A or more and allows the next higher standard rating under 240.6(A); the ECM example uses 125%. One is based on the 2020 NEC and the other names no edition. Neither is the code text.