Transformer disconnect & overcurrent

[jasonmorgan]

Here is the situation,

We have 3 750 VA transformers 277 to 120 with secondary fuse, mounted on poles for cameras. They are mounted in weather proof boxes at the base of each pole. All 3 are feed from a single 277V branch circuit.

1. Is a disconnect means required at each transformer ?
2. How would you size the primary overcurrent protection ?
3. How would you size the secondary overcurrent protection ?

Thanks for any help.
Jason

 

[charlie b]

Item 1:
No. A local disconnect is never is needed for a transformer.

Item 2:
Primary current is 750/277, or 2.7 amps. Per Table 450.3(B), you can go as high as 250%. That gives you 6.8 amps. Per that table's Note 3, you can use the next higher standard size OCPD. That is 15 amps. So I would use a 15 amp breaker at the panel to serve all three transformers.

Item 3:
Secondary current is 750/120, or 6.25 amps. That is less than 9, so Table 450.3(B) allows you to go as high as 167%. That gives you 10.4. Again, the next higher standard size is 15 amps. So I would use 15 amp secondary fuse on each transformer.

Welcome to the forum.

 

[don_resqcapt19]

Item 3:
Secondary current is 750/120, or 6.25 amps. That is less than 9, so Table 450.3(B) allows you to go as high as 167%. That gives you 10.4. Again, the next higher standard size is 15 amps. So I would use 15 amp secondary fuse on each transformer.

Welcome to the forum.

Charlie,
I don't see a "note 1" tag after the 167% value in the table. There is no provision to let you round up the next standard size in this case.

 

[Smart $]

Item 1:
No. A local disconnect is never is needed for a transformer.

Item 2:
Primary current is 750/277, or 2.7 amps. Per Table 450.3(B), you can go as high as 250%. That gives you 6.8 amps. Per that table's Note 3, you can use the next higher standard size OCPD. That is 15 amps. So I would use a 15 amp breaker at the panel to serve all three transformers.

Item 3:
Secondary current is 750/120, or 6.25 amps. That is less than 9, so Table 450.3(B) allows you to go as high as 167%. That gives you 10.4. Again, the next higher standard size is 15 amps. So I would use 15 amp secondary fuse on each transformer.

Welcome to the forum.

My thoughts are to put them all on a typical 15A or 20A circuit then get the 3-slot xfmr-mount fuse blocks and use low current fuses for winding protection.

 

[charlie b]

I don't see a "note 1" tag after the 167% value in the table. There is no provision to let you round up the next standard size in this case.

You are right. That makes a 10 amp secondary fuse the appropriate choice. Mea culpa.