# NEC 250.122B EGC Size

Question asked by rafis on Jan 5, 2020
Latest reply on Jan 15, 2020 by dmatern

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
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?