Xmer overcurrent protection

[LUIS FERNANDEZ]

I'm not to clear on the code for this type of xmer We have a job feeding a 1 kva 277/120/240 serving a power supply fot cctv work. We are feeding the primary with a 20A CB and #12 wire and the secondary is also #12. Do we need overcurrent protection at the xmer secondary. The current at the primary is about 4 a. and the secondary is less than 9a.

 

[templdl]

You have a 1ph 3w secondary and as such you need secondary protection.
An overload or a fault will not be seen as such if primary OCP only were to be provided.
If were 1ph2w, then pri. OCP would work just fine.
But take a look at NEC art450.
Remember that the secondary protection also provides transformer overload protection as well as protecting the wire when properly sized. A UL 489 breaker may not be available below 15A. If the secondary FLA is 9a and you are allowed to sice the sec. OCP at 125% that?s 112.5a. The you?re allowed to go to the next higher standard rating which is 15a.
There is also the UL1077 supplementary protective device, which is allowed as long as a UL489 device protects the circuit first. The use of a UL1077 device simply enhances the coordination of circuit protection but is not to be used to replace that of a UL489 device.
In addition, the primary protection really doesn't protect the transformer at all. It protects the distribution system should the transformer decide to take a dump.