I am working on a machine circuit design and am trying to determine if my secondary transformer protection is to code. I have 45 KVA transformer 480 volt primary 208y/120v secondary. The secondary will feed 6 circuits with 20amp CB's on each. Is a single large CB also required between the transformer secondary and the six 20amp Circuit Breakers?
Transformer Protection
- Thread starter AlanT
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Re: Transformer Protection
What is the rating of the primary OCPD? What size are the conductors between the secondary terminals and the 20A breakers? Hpw long are these conductors?
Don
What is the rating of the primary OCPD? What size are the conductors between the secondary terminals and the 20A breakers? Hpw long are these conductors?
Don
Re: Transformer Protection
taking don's questions into account and applying 408.34 which would require a main breaker is important under an NEC application, however, if this is machine design internal circuits NFPA 70, NEC, is not the applicable code. It would appear the breakers you describe would protect the transformer from overload on the secondary side, but the arrangememt might or might not meet the applicable control cabinet code.
taking don's questions into account and applying 408.34 which would require a main breaker is important under an NEC application, however, if this is machine design internal circuits NFPA 70, NEC, is not the applicable code. It would appear the breakers you describe would protect the transformer from overload on the secondary side, but the arrangememt might or might not meet the applicable control cabinet code.
- Location
- Illinois
- Occupation
- retired electrician
Re: Transformer Protection
Alen,
Don
Alen,
I know nothing of that standard, so I can't comment. Assuming that the six twenty amp breakers are not in breaker panel, that installation would be ok with the NEC.
Don
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: Transformer Protection
UL508A 35.2
Primary protection only for 9 or more amps - 125%.
Primary and secondary protection for 9 or more amps - 250% and 125% respectively.
35.2.2.2 allows mutiple secondary devices where the sum of the ratings does not exceed 125%.
I see nothing in UL508A that addresses (for or against) tapping conductors. It appears this, #0 down to #12, would be between you and your UL inspector.
UL508A 35.2
Primary protection only for 9 or more amps - 125%.
Primary and secondary protection for 9 or more amps - 250% and 125% respectively.
35.2.2.2 allows mutiple secondary devices where the sum of the ratings does not exceed 125%.
I see nothing in UL508A that addresses (for or against) tapping conductors. It appears this, #0 down to #12, would be between you and your UL inspector.
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