I have a 480/277Y, 3 phase, 4 wire panel and I'm looking at installing a 1kva 480v - 120v step-down transformer to feed a specific piece of equipment. I come up with 1.2 amps for the primary current. Table 450.3(B) sets my primary only protection at 300% and I don't see anything that says if this does not correspond with a standard breaker rating, I can use the next size breaker. I was hoping to use a 2-pole 15a or 20a breaker to feed the transformer but it doesn't look like I can. Don't have a lot of experience with transformers, so any help would be appreciated. Thank you.
Primary protection of xfmr?
- Thread starter SPierce
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Unfortunately the answer is still a definite MAYBE. Check out 240.21 (C) for conditions that permit primary-only protection and see if one of them applies.
Frankly in our facility I insist on secondary fusing even where we meet a valid condition. I never know when someone wants to add to the circuit. For example, you build this thing under (2) and then a year later someone adds a 50ft extension for a cooling fan down the way. The new install does not grant a grandfather exclusion to the original circuit. A 50ft line required a secondary when the original circuit was installed therefore it requires a secondary when you exceed the 10ft limit afterward. And refits can be a ...
I have always considered transformer protection as a correctly sized secondary breaker of fuse will protect the transformer form overload. Then the primary protection ends up to be simply short circuit protect should the transformer fail taking the transformer off line and isolate the transformer. As such you may find that primary protection values which may be higher than one would expect when based upon the primary FLA.
As a former applications engineer we would use a 10A 600v KTK-R primary fuse (20A max) and a 15A 250v FNM secondary fuse assuming that you are refering to a 1kva 1ph 480-120 CPT. The primary amps 2A and you are fusng it a 10A to a Max of 20A which seems to be to high.
But again, remember that the transformer OL protection is provided by a properly sized secondary device and the primary protective device provides short circuit protection even though its rating may seem to be unreasonably high.
As a former applications engineer we would use a 10A 600v KTK-R primary fuse (20A max) and a 15A 250v FNM secondary fuse assuming that you are refering to a 1kva 1ph 480-120 CPT. The primary amps 2A and you are fusng it a 10A to a Max of 20A which seems to be to high.
But again, remember that the transformer OL protection is provided by a properly sized secondary device and the primary protective device provides short circuit protection even though its rating may seem to be unreasonably high.
While I understand your "theory", I don't see how doing so would be compliant with 450.3.
And now that I'm reviewing the OP topic, 2.1A does not qualify for the 300% primary-only protection rating. That heading says, "Currents Less Than 2 Amperes". Under the "less than 9 amperes" column, the max ocp permitted would be 167%... or 250% if secondary protection is added (subject to Note 3).
I agree... but the NEC doesn't apply to control transformers and fusing thereof for listed manufactured equipment.
Good Point Smart$.
Then as long as Sec. OL protection is provided I would go with the highest pri OL protection that is allowed by the NEC as it only provides short circuit protection anyway. You don't want the darned pri fuse blowing when the transformer is energized because of inrush.
Then as long as Sec. OL protection is provided I would go with the highest pri OL protection that is allowed by the NEC as it only provides short circuit protection anyway. You don't want the darned pri fuse blowing when the transformer is energized because of inrush.
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