UL 508A branch circuit protection calculation

[David Charles]

I wanted to make sure i had this correct.

what is the maximum branch circuit protection allowed for both the primary and secondary sides of a 1500 VA 480/120 volt single-phase power transformer.

Primary:
1500/480 = 3.125
using table 35.2 in UL 508A standard
3.125 * 2.5 = 7.8125

Secondary:
1500/120 = 12.5
12.5*1.25 = 15.625

so the answer would be Primary = 8amp, and secondary = 20amp.

 

[petersonra]

Look closely at the notes under the table for the primary side. It doesn't say you can go to the next size standard circuit breaker like it does on the secondary side. On the other hand there might be some dispensation for rounding up. Don't remember off hand.

 

[David Charles]

Look closely at the notes under the table for the primary side. It doesn't say you can go to the next size standard circuit breaker like it does on the secondary side. On the other hand there might be some dispensation for rounding up. Don't remember off hand.

"
Where the calculated size of the branch circuit protection does not correspond to a standard size fuse or nonadjustable
inverse-time circuit breaker, the next larger size is able to be used."

I am a bit confused about this because if the primary calculated was 7.8125 it doesn't correspond to any of the fuse sizes listed in 31.3.8