Transformer Overcurrent Protection

[mike0w]

Confused? Not even the right word. Just wondering the opinion of forum members on this topic. Going through the code (2005) and familiarizing myself with different sections is something I have been working on but have come across a stumbling block. I have looked at some previous projects within the office and spoke with different engineers and never seem to get the same response. So with all that being said here is the question:

Transformer Protection:

When the conductors are connected to the secondary of the transformer
(ex: 45kva with #1 secondary conductors) what is the length limitation of
The secondary conductor?? Primary sized at 125% of xfmr fla.
What if I then ran the conductors into a wireway tapped the conductors
Three times with #3s (not exceeding 10?-0?) to feed three mcb panels @ 100a mcb. Could the secondary transformer conductors be run 50?, 100?, ?? where can I find this limitation.
From what I have gathered the secondary conductors do not have a length limitation. If the primary ocpd is sized at no more than 125% I can tap the conductors and have up to six disconnect as long as they are grouped together with any size mcb?s as long as the tap conductor matches the associated mcb size?.

#2
When the conductors are connected to the secondary of the transformer
(ex: 112.5kva with 500kcmil secondary conductors)
The primary ocpd is set at 350A (250% of primary fla)
Secondary conductor?s length 30? into a wireway
Conductors tapped with three times with #1 into three
Separate panels containing 125A m.c.b.
The sum of the panel mcb?s does not exceed the associated secondary transformer ocpd size (note#2 of the xfmr chart in 450.3) and the #1 taps do not exceed 10?-0?.

This is what I have concluded (below) is this correct?

If the primary ocpd is no more than 125% the panels fed from taps on the secondary side need to be grouped together, with the conductor taps matching the size of the ocpd it is terminating in.

If the primary ocpd is sized at 250% the panels fed from taps on the secondary side need to be grouped together, with the conductor taps matching the size of the ocpd it is terminating in, and the sum of the ocpd?s cannot be greater than the associated secondary xfmr size if there was one.

So if this is true there is no limitation on the length of transformer secondaries? Unless each set of conductors (multiple conductors on the xfmr secondary) feed separate loads, then the tap rules apply? Am I on track or way off base?
Thanks in advance.

 

[jim dungar]

First off, you are dealing with three different areas of concern. The easiest method i have found is to start at the laod and work your way back to the source.

First you have a panelboard, so you must protect it per 408.36(A) or (B). Remember any required OCPD may be mounted remote from the panel.

Second you have conductors feeding that panelboard, so now you are in 240.20. But you have a choice on the location of any required OCPD which takes you to 240.21 and because a transformer is involved maybe 240.21(C).

At this point you have some type of protection on the secondary of the transformer, either at the end of your cables under 240.21 or at the transformer under 240.20.

Now you can go to 450.3(B) and see if your secondary OCPD complies with Table 450.3(B) Note 2. If your secondary OCPD is sized correctly then your primary OCPD may be increased above 125%.