Transformer Secondary Protection

[dy11]

I have a 3 phase 112.5 kva 480-208/120 transformer being protected by a 200A primary breaker where the secondary is being tapped into 3 single phase 208V/120 panels each being protected by 200A fused disconnects. Looking at table 450.3(B) note 2, in order for this to be considered secondary protection, the sum of all OCPD's must not exceed 125% of 312A.

Since these are 3 single phase 208/120 circuits, do they collectively provide 200A of protection as a 3 phase fused disconnect would?

 

[don_resqcapt19]

I see the 3 sets of fuses as providing 400 amps of transformer secondary protection. 125% of 312 is 390, so the 400 amps pf protection is compliant.

 

[charlie b]

I think this is OK. If all three circuits were loaded right at 200 amps, the total load would be about 125 KVA, or about 11% over the transformer's rating. The current in each leg would be about 347 amps, or (here again) about 11 % above the rated 312 amps.

 

[infinity]

If I'm understandly Don correctly A-B phase would have 400 amps of protection so would B-C and A-C.

 

[dy11]

Thank you, I had to sketch it to see it but the 400A on each phase makes sense. If a fourth panel were connected similarly, it would exceed 6 OCPDs and could not be considered secondary protection, correct?

 

[texie]

A little out of the ordinary but seems compliant to me. One of those cases where you would want to do your bond at the xformer just save a little work.

 

[don_resqcapt19]

If I'm understandly Don correctly A-B phase would have 400 amps of protection so would B-C and A-C.

Yes, that is what I was trying to say, but you said it better.

 

[don_resqcapt19]

Thank you, I had to sketch it to see it but the 400A on each phase makes sense. If a fourth panel were connected similarly, it would exceed 6 OCPDs and could not be considered secondary protection, correct?

No. Note 2 to Table 450.3(B) limits you to six sets of OCPD. You only have 3 sets now. However, the forth set would take you over the maximum permitted total of 400 amps for this installation.

 

[don_resqcapt19]

I just read Note 2 closer. Where are the panels in relationship to each other? It appears that they must be grouped in a single location to use the provisions of the note.