Under 450.3, when is a transformer permitted to be protected by primary-side overcurrent protection alone, with no secondary protection?
Not quite. The answer is D: When the primary overcurrent device is rated no more than the percentage of primary current allowed by Table 450.3(B) — 125% where the primary current is 9 A or more
For transformers rated 1,000 V or less, Table 450.3(B) permits protection on the primary side alone if the primary overcurrent device is rated no higher than 125% of primary current when that current is 9 A or more (higher percentages apply at lower primary currents, with next-higher standard size allowed where 125% is not a standard rating). Otherwise, protection is needed on the secondary as well. Supervised-installation percentages belong to the over-1,000 V table, not this one.
Reference: NEC 2023 450.3(B) (Transformer Overcurrent Protection — 1,000 V or Less)
Checked against multiple published sources
Two trade-press articles agree that primary-only protection under 450.3(B) is limited to 125% of primary current at 9 A or more, with the next higher standard size permitted. One is based on the 2020 NEC and the other names no edition. Neither is the code text.
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