Fuse or circuit breaker for transformer prot
Fuse or circuit breaker for transformer prot
(OP)
we have a new 175 KVA, 460/230volts, 3ph dry type transformer. following NEC art. 450 the primary side protection would be 600Amps while the secondary side would be 500Amps. Am I correct in my interpretation of art. 450? Also, would you rather recommend a fuse than a CB. Can someone pls advise? thanks
capuchi
capuchi






RE: Fuse or circuit breaker for transformer prot
At this voltage and size, the only reason to use a fuse instead of a molded case breaker would be cost.
RE: Fuse or circuit breaker for transformer prot
Other than that, I completely agree that the NEC should not be used as a primary engineering tool, but rather as a secondary check and adjustment against what is permissible, AFTER you determine the right way to do it from an engineering standpoint.
Now, all that said capuci, you have apparently made some sort of error in either your calculations or your typing. The primary side protection, in a scheme where you have both sides protected, should be 250% MAX. of the primary current. On that 175kVA transformer it will be around 210A, so the breaker should be NO MORE THAN 500A. Notice that when considering the primary protection, Note 1 that allows for up-sizing to the next available non-adjustable trip rating does NOT apply, so you stop at the next lowest, in this case 500A, not 600A.
On the secondary side, the breaker should be 125% MAX of the secondary current, which in this case will be 439A, so the breaker should be no more than 549A; but based on Note 1, COULD be as high as 600A.
RE: Fuse or circuit breaker for transformer prot
RE: Fuse or circuit breaker for transformer prot
RE: Fuse or circuit breaker for transformer prot
RE: Fuse or circuit breaker for transformer prot
RE: Fuse or circuit breaker for transformer prot
Also how can I be sure that the circuit breaker selected can handle the inruch current?
RE: Fuse or circuit breaker for transformer prot
RE: Fuse or circuit breaker for transformer prot
thanks. Can you direct me to the IEEE document that specifies 12 times the nominal current of the transformer.
RE: Fuse or circuit breaker for transformer prot