Transformer Protection

[Brandon Loyd]

I found a 50 KVA single phase transformer with only #6 wire leaving on the secondary. The primary is protected by a 125 amp breaker with proper wire size, but dont see any protection on the secondary conductors. If my calcs are right, primary amps are 60 and secondary amps are 120. Can the secondary wires be protected by the primary OD (if it was 60 amps instead of 125)? Or does the secondary need to be protected also, always.

 

[david luchini]

You don't mention the transformer primary and secondary voltage, but I don't see how your calculations can be correct. A primary voltage of 833V would give a primary rated current of 60A, and a secondary voltage of 417V would give a secondary rated current of 120A.

The secondary conductors can be protected by the primary OCPD if it complies with 240.21(C)(1).

 

[bphgravity]

Not that this answers your question, but keep in mind that protection of the transformer itself is a section 450.3 issue. Protection of the conductors supplying and supplied by the transformer is an Article 240 matter... Calculations are based on the prescribed requirements within those sections...

 

[Brandon Loyd]

You don't mention the transformer primary and secondary voltage, but I don't see how your calculations can be correct. A primary voltage of 833V would give a primary rated current of 60A, and a secondary voltage of 417V would give a secondary rated current of 120A.

The secondary conductors can be protected by the primary OCPD if it complies with 240.21(C)(1).

480 primary, 120/240 secondary.

 

[david luchini]

480 primary, 120/240 secondary.

Your primary rated current would be 104A, secondary rated current 208A.

Because the secondary is 3-wire, the secondary conductors cannot be protected by the primary OCPD (240.12(C)(1)). The #6 secondary conductors should be protected by a 60A (or smaller OCPD) within 10' per 240.21(C)(2) (assuming its not an outdoor installation per (C)(4)).