single phase transformer overcurrent protection less than 600v

[LUIS FERNANDEZ]

What will be the primary and secondary overcurrent protection for a 7.5kva single phase transformer 480v-120/240v

 

[david luchini]

What will be the primary and secondary overcurrent protection for a 7.5kva single phase transformer 480v-120/240v

That depends on whether you want to protect it as "primary only" or "primary and secondary" per T450.3(B).

 

[LUIS FERNANDEZ]

That depends on whether you want to protect it as "primary only" or "primary and secondary" per T450.3(B).

I want protection on both

 

[don_resqcapt19]

I want protection on both

Remember that you find the rules for protecting the transformer itself in Article 450 and the rules for protecting the secondary conductors in 240.21(C). You must satisfy the rules in both Articles.

 

[petersonra]

That depends on whether you want to protect it as "primary only" or "primary and secondary" per T450.3(B).

with a 120/240 xfmr you have to have secondary conductor protection, so it is kind of a moot point.

 

[david luchini]

with a 120/240 xfmr you have to have secondary conductor protection, so it is kind of a moot point.

:? How does that make it moot? You can still protect the Transformer as "primary only" or "primary and secondary."

 

[petersonra]

:? How does that make it moot? You can still protect the Transformer as "primary only" or "primary and secondary."

Granted that in a legalistic sense you end up having to do both, but most times when you end up being required to have OCPD on the secondary, it is a moot point in a practical matter whether the secondary OCPD protects the xfmr or the conductors. It is going to do both unless the secondary conductors and its corresponding OCPD are way oversized for the actual xfmr secondary current.

 

[david luchini]

It is going to do both unless the secondary conductors and its corresponding OCPD are way oversized for the actual xfmr secondary current.

Not necessarily way oversized. Certainly not way oversized with the OPs transformer.

The secondary conductors could be #8awg with a 40A secondary OCPD (not more than 125%) with a "primary and secondary" protected transformer.

Or they could be #8awg with a 50A secondary OCPD (more than 125%) with a "primary only" protected transformer. The secondary conductor size doesn't need to change at all between the two methods.

 

[petersonra]

Not necessarily way oversized. Certainly not way oversized with the OPs transformer.

The secondary conductors could be #8awg with a 40A secondary OCPD (not more than 125%) with a "primary and secondary" protected transformer.

Or they could be #8awg with a 50A secondary OCPD (more than 125%) with a "primary only" protected transformer. The secondary conductor size doesn't need to change at all between the two methods.

In both cases you would end up with OCPD on both sides of the xfmr.

However, as I pointed out, he cannot use the primary OCPD to protect the secondary conductors in this case because it is not a 2 wire secondary.

 

[david luchini]

In both cases you would end up with OCPD on both sides of the xfmr.

However, as I pointed out, he cannot use the primary OCPD to protect the secondary conductors in this case because it is not a 2 wire secondary.

Of course there is an OCPD on both sides in both cases. But with a 50A secondary OCPD, you would have to protect the transformer as "primary only."