Grounding Electrode conductor sizing

[arnettda]

I have a 320 meter socket whos service mast/ riser is supplied by parallel 4/0 wire. I come off the load side of the meter with 250 mcm AL. to two service disconnects. Do I size my Grounding electrode conductor by the parallel 4/0 or the 250's? Art 100 makes me think the service mast are service entrance conductors so I should use the parallel 4/0 but reading the code it says conductors supplying the disconects?
Thanks

 

[augie47]

In your particular case I think would end up with the same GEC in either event.

 

[suemarkp]

You have to look at each section to see which is the largest. The first column of table 250.66 says "Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel Conductors (AWG/kcmil)". Note 1 to table 250.66 says "If multiple sets of service-entrance conductors connect directly to a service drop, set of overhead service conductors, set of underground service conductors, or service lateral, the equivalent size of the largest service-entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set."

So your two 4/0's come to around 420 Kcmil (table 8 says the area of 4/0 AL is 211,600 mils, two of them would be 423.2 KCmil)
The set of 250's is 500 KCmil.

As Augie mentioned, both of these put you in the same row of the table, which is a #2 cu or 1/0 AL GEC per table 250.66. Table 250.102 works the same way and may be a bit clearer in its notes.

You also have some options for GECs. You can run one large one, or smaller taps from the 250's feeding each disconnect.

 

[arnettda]

Thank you for the replys. 4/0 to 250 is #4 CU over 250 is #2CU