Hey boys and girls, I am working on installing some control trnasformers and it has been years since I did the calculations. The largest is a 200va with 277 pri/24sec voltage. According to 725.21(D)
Primary Side of Transformer. 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.
If I am reading it right, all I need is a 1 amp fuse for a 200va transformer because:
200/277=0.7 amp load
0.7x1.25=0.9 or 1 amp (next strd size up)per NEC Table 450.3(B)
277/24=11.5 primary/seconday winding ratio
11.5x0.9=10.4 minimum wire ampacity (14 gauge sec. wire spec'd)
Max calculated sec. load 159vA
Am I doing this correct or am I am infected by PVC glue fumes...again?
