Transformer Protection

Transformer must be protected according to Table 450.3(B) that states for a primary current of less than 2 amperes the primary protection must be less than or equal to 300 per cent of the pri-mary current.

For transformer protection, fuses must be rated not over 3.0 x .73 Amperes or 2.0 Amperes. shall be placed in EACH ungrounded conductor per 240.20(A).
 
 

Grounding of Secondary Circuit

Secondary circuit shall  be Grounded per 250.20(A).   If the transformer supply exceeds 150 volts to ground then the secondary circuit shall be grounded (as shown).

Secondary Conductor Protection

Secondary control circuit Wires shall have ampacity of at least (480 / 24) x I amperes, or 20 amperes.  A No. 12 wire rated at 20 amperes would be ok here or else secondary conductors must be protected.

The secondary conductors are protected according to Section 725.24(D):

"Class 1 circuit conductors supplied by the secondary of a single-phase transformer having only a 2-wire (single-voltage) secondary shall be permitted to be protected by overcurrent protection provided on the primary 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. Transformer secondary conductors other than 2 wire shall not be considered to be protected by the primary overcurrent protection. ."

For the above problem this converts to: (24/480) x (20) <= 1 ampere