Interpreting 250.122(B)

[NOV]

When it comes to interpreting 250.122(B), when you increase parallel conductors to account for voltage drop, is the calculation for increasing the grounding conductor based on an "apples to apples" quantity comparison of the ungrounded conductors, or just the amperage capacity of the ungrounded conductor?
For example:
An 800A feeder is upsized for voltage drop from (2) sets of 600KCMIL to (3) sets of 600KCMIL.
Example #1: Using (3) sets (apples to apples) of 300KCMIL, (855A)-grounding conductor increases by 200% from #1/0 to #4/0
Example #2: Using (2) sets of 600KCMIL, (840A)-grounding conductor increases by 150% from #1/0 to #3/0
In this scenario, the resulting grounding conductor is only 1 conductor size difference but depending on the feeders, there could be quite a difference in the conductor size.
What would be the correct way to calculate this, or is it subject to interpretation?

Thanks in advance!

 

[infinity]

Start with the minimum size, let's use your example of 2 sets of 600's. That's 1200 kcmil, if you use 3 sets that is 1800 kcmil you've increased the ungrounded conductors by 50%. You would then increase the EGC size by 50% also.

 

[NOV]

Hopefully I phrased my question above correctly.
You're saying, the method to use is the one used in Example #2?
Select the conductor required for the intended installation based on the conductors ampacities and not by matching the amount of parallel feeders and then finding the conductor required for the intended installation based on that. Correct?

 

[infinity]

Since your two examples use multiple sets of parallel conductors you would start with the smallest conductors allowed for either 2 sets or 3 sets.
So for 2 sets you need a minimum of 600 kcmil for 800 amps. For 3 sets you need (as you stated) 300 kcmil. With each of those parallel sets (2X600's or 3X300's) the EGC would be #1/0. If you added a set of 600's you're ungrounded conductors increased by 50% so the EGC would also need to increase by that percentage. Your example shows no increase in size for the 300 kcmil conductors.

 

[jaggedben]

It's based on the proportional size of the wire as compared to the minimum required, not any putative ampacity of the size you're increasing to.

 

[infinity]

It's based on the proportional size of the wire, not any putative ampacity.

That's correct although you do need a ampacity starting point to actually know whether or not the ungrounded conductors have been increased in size.

 

[jaggedben]

Yeah, I edited to clarify that.

 

[infinity]

Yeah, I edited to clarify that.

I figured that's what you meant.