Secondary OCP required?

[JFletcher]

Was looking over a set of prints today for a future job. Setup is 480V CT cabinet feeds 600A fused disco (future use upgrades) feeds 100A fused disco feeds 480D/208Y stepdown xfmr feeds 225A 3ph panel. Plans didnt specify this last panel. My question is this: doesnt there have to be OCP on the secondary side of the xfmr, or is it allowed to be protected by the primary side under certain conditions?

eta: if it matters, 225A 3ph panel is at a separate pole, but appears to be LoS to both discos.

 

[jumper]

The secondary side of the transformer, the secondary tranny conductors, and the panel it self all require protection.

Primary transformer protection can protect the transformer secondary under certain conditions, but you still gotta deal with the secondary conductors and panel protection.

You install seems to be outside so using 240.21(C)(4) your conductors can be of any length and installing a properly sized main breaker you can satisfy 240.21(C)(4) and provide protection to all three items mentioned above.

Using Table 450.3(B) if you provide primary and secondary protection for the tranny, the primary OCPD can be 250% of the Transformer-Rated Current and the secondary OCPD can 125%. The next size up rule would only apply to the secondaryOCPD.

David Lucini will probably be along shortly, read my reply and say I am completely off my rocker.

 

[JFletcher]

The secondary side of the transformer, the secondary tranny conductors, and the panel it self all require protection.

Primary transformer protection can protect the transformer secondary under certain conditions, but you still gotta deal with the secondary conductors and panel protection.

You install seems to be outside so using 240.21(C)(4) your conductors can be of any length and installing a properly sized main breaker you can satisfy 240.21(C)(4) and provide protection to all three items mentioned above.

Using Table 450.3(B) if you provide primary and secondary protection for the tranny, the primary OCPD can be 250% of the Transformer-Rated Current and the secondary OCPD can 125%. The next size up rule would only apply to the secondaryOCPD.

David Lucini will probably be along shortly, read my reply and say I am completely off my rocker.

ty Derek for the code sections. yes, it is outdoors. Primary xfmr current was ~50A (45kva), so the 100A fused disco is gtg. Would I need a disco after the tranny before the 225A panel, or would a main breaker panel suffice, or would I need both? (I'll look at the sections you cited after dinner... if they answer my questions, plz disregard the above queries).

The prints have been reviewed by an EE but did not seem quite right to me. Mainly, a secondary fault in the 3/0 feeders could/would lunch the xfmr before the primary fuses blew. and no disconnect means at what is a separate structure.

 

[Smart $]

ty Derek for the code sections. yes, it is outdoors. Primary xfmr current was ~50A (45kva), so the 100A fused disco is gtg. Would I need a disco after the tranny before the 225A panel, or would a main breaker panel suffice, or would I need both? (I'll look at the sections you cited after dinner... if they answer my questions, plz disregard the above queries).

The prints have been reviewed by an EE but did not seem quite right to me. Mainly, a secondary fault in the 3/0 feeders could/would lunch the xfmr before the primary fuses blew. and no disconnect means at what is a separate structure.

If the 100A disconnect has 100A fuses, transformer secondary protection is required because the primary protection is over 125% rated current.

You do not need a fusible disconnect before an MCB panel as long as you meet the requirements of 240.21(C)(4) and the MCB is not rated greater than 175A (125% secondary rated current, upsized to next standard rating) [450.3(B)].

 

[Jraef]

Does the 225A CB panel have a main CB?

 

[JFletcher]

Does the 225A CB panel have a main CB?

It does not specify on the prints if MB or MLO, nor show any secondary side fused discos or mains breakers.

Smart, the 100A fused disco does specify 100A breakers.

It also spec'd 3 ground rods connected by 1/0 to the 600A disco. :huh: There is no CEE, no water pipes, no building steel.

 

[david luchini]

David Lucini will probably be along shortly, read my reply and say I am completely off my rocker.

Sorry, Derek...I missed my cue.:weeping: