Breaker Sizing UL508A

[Mehdi]

I have control transformer 1000VA, 208/120. I want to know if my breaker sizes are correct as per UL508A. Primary side, 1000/208 *250/100=12A. What should be the size of breaker 10A or 15A on primary side? On secondary side, 1000/120 * 125/100=10A. Is 10A breaker ok on secondary side? Thank you,

 

[petersonra]

Have you looked closely at the paragraph on sizing ocpd for transformers? What does it require?

 

[Mehdi]

Have you looked closely at the paragraph on sizing ocpd for transformers? What does it require?

What I found primary CB should not be round up, it will stay 10Amp. But I still 2nd opinion. Thank you.

 

[topgone]

Please refer to Table 450.3(B). The factor to use for primary only protection of transformers with primary currents 9 amperes or less @ 1000V or less is 167%. In you case that would be 1000/208 = 4.8 X 1.67 =8 amperes. Since available fuse ratings are 2, 4. 6. 10. 16, etc., choose 10A and be done with it.

 

[Mehdi]

Please refer to Table 450.3(B). The factor to use for primary only protection of transformers with primary currents 9 amperes or less @ 1000V or less is 167%. In you case that would be 1000/208 = 4.8 X 1.67 =8 amperes. Since available fuse ratings are 2, 4. 6. 10. 16, etc., choose 10A and be done with it.

Thank you!

 

[petersonra]

Please refer to Table 450.3(B). The factor to use for primary only protection of transformers with primary currents 9 amperes or less @ 1000V or less is 167%. In you case that would be 1000/208 = 4.8 X 1.67 =8 amperes. Since available fuse ratings are 2, 4. 6. 10. 16, etc., choose 10A and be done with it.

he asked about UL508a, not NEC.

according to the OP, it is a control transformer.

control transformers are dealt with in paragraph 42.

the rated primary current is 1000/208 = 4.8 Amps. the rated secondary current is 1000/120 = 8.33 Amps.

Since he indicated he plans to use both primary and secondary protection, he needs to use table 42.2.



the rating of the primary protection can not exceed 250% of 4.8 Amps or 12 Amps.

the rating of the secondary protection cannot exceed 167% of 8.33 Amps, or 13.91 Amps. You cannot go up to the next standard size. Note "a" only applies when the rated secondary current is 9 Amps or more.

I am pretty sure this is the same as is found in the NEC.