Primary feeders to transformers

[mshields]

To my knowledge, transformer primary conductors need to be sized based on the upstream OCPD. So for example, if you size the primary of a 500kVA 4160 to 480/277 unit with a FLA at 4160V of 69A, you may have up to a 300% fused primary [per Table 450.3(A)]. So lets say you use a 200A fused switch, do the conductors need to be rated for 200A or could the conductors be rated for something less than that?

i.e. For a motor your OCPD can be up to 250% with conductors rated for 125%. I don't think there is a similar rule for transformers but that's my question.

Thanks,

Mike

 

[iwire]

You got it,the conductors must be sized to the OCPD not the transformer input current.

 

[lauraj]

Transformer primary conductors must be protected by the primary OCPD, therefore as per your example, you would need 3/0 copper conductors for the 200A OCPD.

 

[jumper]

Sample pic to add to what iwire said.

View attachment 14822

 

[mshields]

thanks

thanks

thought so - thanks

 

[mayanees]

240.101

240.101

Rating or Setting of OCPDs over 1000 Volts gives the flexibility to size the primary cable at 125 % and protect it much higher.

 

[mshields]

Ah! well that changes things!

Ah! well that changes things!

very interesting. And that gybes with Table 450.3(A); i.e. allowing for 3 times (for fuses) and 6 times (for CB's).

 

[Sahib]

Interestingly, the primary OC device is meant to provide protection to the transformer proper. Often it also provides protection to the transformer conductors. That seems to imply that in some cases it may not provide protection to the transformer conductors even if it is sized per code as the OCPD is designed for protecting the transformer proper.

 

[topgone]

The operative word here is "bottom-up" approach.

• You size your conductor based on the load it carries;
• Once you have chosen a conductor, you choose an OCPD based on the conductor size, . . .
• all the way up to the source.

 

[Sahib]

Only if it were possible for the TCC of an OCPD to accommodate the damage curves of both the transformer and the transformer conductors in all cases.........

 

[iwire]

The operative word here is "bottom-up" approach.

• You size your conductor based on the load it carries;
• Once you have chosen a conductor, you choose an OCPD based on the conductor size, . . .
• all the way up to the source.

But once you are supplying a transformer things need to adjust for it.

The primary feeders will often be much larger than the load they supply due to the OCP size needed to energize the transformer.