- Location
- Illinois
- Occupation
- retired electrician
You can not protect the secondary with primary OCPD when you have a delta/delta 3 wire to 4 wire.
1) Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.
You can't protect the secondary conductors with the primay OCPD in other than two wire to two wire or 3 wire delta-delta installations. You can protect the secondary of the transformer itself with the primary OCPD as long as the rating of that device does not exceed 125% of the rated primary current.
The rules in Article 450 only apply to the protection of the transformer itself and the rules in Article 240 apply to the protection of the conductors.