I am trying to understand what base size we are supposed to use when we need to proportionally increase the size of the Equipment Grounding Conductor (EGC) per NEC 250.122B. Let’s say the following scenario:

Breaker=800A

Load=600A

Voltage=480V

Circuit Length=1,000ft

I will need a conductor to exceed 600*1.25=750A

Possible Circuits that could be considered as Base

1. 2 sets of 500KCMIL. Total CIR=1,000,000

2. 3 sets of 250KCMIL. Total CIR=750,000

3. 4 sets of 3/0. Total CIR= 671,200

4. 5 sets of 1/0. Total CIR=528,000

To compensate for voltage drop we decided to increase the conductor size to:

5 sets of 500 KCMIL Total CIR=2,500,000.

Now to adjust the EGC we will need to proportionally increase its size:

Since 800A breaker, I will need 1/0 EGC Total CIR=105,600

If Base 4 is considered:

(2,500,000/528,000)*105,600=500,000

I will need to increase the size to 500KCMIL EGC

If Base 1 is considered

(2,500,000/1,000,000)*105,600=264,000

I will need to increase the size to 300KCMIL EGC

To minimally increase the EGC, I would consider No.1 as Base. Can I proceed with 300KCMIL for EGC or am I doing this incorrectly?

At the bottom (just below) is 'Related Content'. There have been a couple of similar posts in the past few months.

NEC 250.122(B)

Example above demonstrates that each separate Conduit (for each Set) has the it's own EGC sized per 800A. So if you do (5) Conduits with each having a Set of 500kcmil, then each of the (5) Conduits would have a 1/0 EGC.

Looking at it from that perspective, I have to ask; at what point do you realize that a Bussbar would be less costly in the long run when including installation?