Hi There
I am quoting an installation of 2 2000MW Emergency Generators from Cummings.

It is my first time dealing with high AMP feeders.
The system is 480/277 V. with aluminum conductors in raceway which type is not specified because
is out of the my specific project's scope I mean doing by others.
I received the electrical drawings with the information which include the in-line diagram
for the generators, the feeder towards each EMSB ( Emergency Switch board), and from both EMSB to many ATS ( Automatic Transfer switch) down steam for feed different emergency loads.

The drawing include a table with a list of different capacity CB and its corresponding AWG for feeders.
More exactly the table specify Circuit Breaker Amps, # sets , and # of wires for each sett plus the AWG for grounding conductor.
For example let's take one of those listed feeders:
For 700 A CB we have 2 sets of 3-600 Kcmil plus 1 600Kcmil for Neutral. plus 1 #3/0 grounding conductor.

Among the notes included with the feeder table there is the following:
Subscript "V" denotes an oversized feeder due to voltage drop. Increase equipment grounding conductor size per requirements of 250.122 (B)
In total there are 12 feeders towards its respective ATS. Most of those feeders runs are in average 600 feet and have a tag "V" doing reference to the above note.
Here is my first question: Is that table valid for 100' runs or it is already updated for long runs?
Let's take the case of one of the feeders: 1600 A running 810 feet.
They specify 6 sets of 3-400 Kcmil, 1_400Kmil for Neutral and 1 350Kcmil for grounding.

I got that for one set , 480 v , four conductor in parallel (Phases and Neutral), PVC conduit(it's assumed), temperature 102 F deg then the voltage drop is 7.3 %. Is my calculation correct?

Should I conclude that the Feeder table is for 100 feet run?


My 2th question is regarding the equipment grounding conductor and table 250.122 B
Let's take again the example feeder
For 700 A CB we have 2 sets of 3-600 Kcmil plus 1 600Kcmil for Neutral. plus 1 #3/0 grounding conductor.
If I look up on the table 250.122(B) NEC 2011 I found that for Aluminum 3/0 conductor the corresponding CB should be 800A.
This is 100 A more than the 700 A feeder.
How should I interpret this?




I would appreciate so much son help on this and Thank you all.