How do I protect this xformer?

[electrofelon]

I have a 1.5 KVA 480 to 120/240 transformer. There is no secondary protection since the sum of the breakers on the panel off the secondary far exceed the secondary current. So with Primary only protection and current between 2 and 9 amps, table states I need about 5 amps max primary protection. Do I get to use ye old "next standard size up ocpd" rule here? It appears not from the table, so what does one do?

 

[don_resqcapt19]

You are correct that you cannot use the next larger OCPD in this application. Just install a couple of fuses for the required secondary protection. The rated current of the secondary is 6.25 amps and the table permits up to 167% so you could use a 10 amp OCPD.
Don

 

[electrofelon]

Thanks Don,


But I would still need primary protection right?
Having secondary protection would bump me up to 250% for the primary which is 7.8 amps. I would have to use a 5 amp primary fuse?

 

[coulter]

I have a 1.5 KVA 480 to 120/240 transformer. ... So with Primary only protection and current between 2 and 9 amps, table states I need about 5 amps max primary protection. ...

What is the installed method of primary protection?

carl

 

[don_resqcapt19]

But I would still need primary protection right?

Yes, primary protection is always required. Sometimes secondary protection of the transformer is not required, but watch out....secondary conductor protection is required almost always.

Having secondary protection would bump me up to 250% for the primary which is 7.8 amps. I would have to use a 5 amp primary fuse?

There are 7.5 amp fuses available.
Don

 

[electrofelon]

There are 7.5 amp fuses available

Ahh love those small increments in fuse sizes.

Sometimes secondary protection of the transformer is not required, but watch out....secondary conductor protection is required almost always.

I have heard that satement before, so what little ditbit am I missing? Table 450.3(B) seems to state that secondary protection is optional - assuming of course your ocpd comnplies with the values in the table.

 

[don_resqcapt19]

so what little ditbit am I missing? Table 450.3(B) seems to state that secondary protection is optional

Just one thing...Article 450 only covers the overcurrent protection of the transformer itself. You need to go to 240.21(C) for the overcurrent protection of the secondary conductors.
Don