Transformer math, is breaker oversized??

[brother]

Just double checking my math, I saw this transformer hookup and it appears the breakers are too large to protect it.

this is a 112.5 KVA 480 primary 208 secondary 3 phase. The primary is 175 amp breaker and the secondary is 400 amp.

Even though you can go to the next size up on breaker ocd on the primary, I thought if you went over 1.25 (regardless of next size up) that you had to put the secondary at 1.25 to ensure proper protection.

Am I correct on this?? Is this transformer installed wrong??

 

[david luchini]

This looks OK to me.

The secondary OCPD is sized at 125% (using the next higher standard size.)
The primary OCPD is sized at not more than 250%.

This meets the requirements of T450.3(B) for primary and secondary protection.

 

[charlie b]

I agree that this is OK. Take a look at note 1 under table 450.3(B).

 

[brother]

I agree that this is OK. Take a look at note 1 under table 450.3(B).

Ok, so im to interpet that to mean since you are over sizing above 125%, you are still OK with the protection of the transformer?? My understanding was since you technically did go above the 125% because of the next higher size, then you had to down size on the secondary. Thats language that needs to be made clearer in my opinion.

primary,

112.5 X 1000/(480 X 1.73)=135 x 1.25 =169 next up 175. so you can do this even though its over 125% no secondary protection needed.

 

[charlie b]

primary, 112.5 X 1000/(480 X 1.73)=135 x 1.25 =169 next up 175. so you can do this even though its over 125% no secondary protection needed.

The question of whether secondary protection is needed is separate from the rule about upsizing to the next higher standard OCPD rating. I think the part of this rule that is most confusing is the question of whether secondary protection is needed. They don't say when it is needed and when it is not.

But I think I see what it is that you are asking. The fact that you are upsizing from 169 to 175 will not take you into the requirement for adding secondary protection.

Please note that protecting the secondary conductors (i.e., from the point of attachment to secondary windings to the downstream panel) is another thing entirely.

 

[david luchini]

Ok, so im to interpet that to mean since you are over sizing above 125%, you are still OK with the protection of the transformer?? My understanding was since you technically did go above the 125% because of the next higher size, then you had to down size on the secondary. Thats language that needs to be made clearer in my opinion.

primary,

112.5 X 1000/(480 X 1.73)=135 x 1.25 =169 next up 175. so you can do this even though its over 125% no secondary protection needed.

Per 450.3, if you protect the primary at 125% (using the next standard size up) then the secondary doesn't need protection (though as Charlie points out, the secondary conductors still require protection.)

If you protect the primary at not more than 250% (using the next size down) then the secondary needs to be protected at not more than 125% (using the next size up.)

In your example, you could look at the OCPDs in two ways. First, since the primary doesn't exceed 125% (next size up) then the secondary can be any size you want. A 600A secondary OCPD would be OK - but the secondary conductors would have to have an ampacity of at least 600A.

Or secondly, since the secondary doesn't exceed 125% (next size up) the primary OCPD can be as large as 300A.

 

[iwire]

To the NEC extreme.

This trans could have 300 amp primary side protection with 400 amp secondary side protection.

But to do that the conductors selected would have to have an ampacity exceeding those breaker ratings.