Sizing Equipment Grounding Conductor for Voltage Drop

[speck1]

I have a 600A overcurrent device on a long feeder run. Per 250.0122 it would require a #1cu ground before any adjustment for voltage drop. I am running 3 sets of 350kcmill to accomidate for voltage drop. When I calculate what the revised ground should be using the example from the handbook 250.122, B the answer does not seen correct to me.

Example 1
600A conductor per 310.16 = 1500 kcmill
3*350kcmill conductors = 1050 kcmill

1050/1500 = .7

I take this to mean that even tho I up sized the current carring conductiors the ground does not need to upsized?

Example 2
But if you use 2 set of 350's as your starting point the calculation becomes

1050/700= 1.5 which directs you to a 2/0 ground

Example 3
Or if you use 4 sets of 1/0 you get

1050000/422400 = 2.4 which directs you to a 4/0 ground

Can this be correct that by the tables in the code (using 1500kcmill) the ground does not need to be upsized in this case?

And what about is changing depending on how you parallel it?

 

[charlie b]

Truly an excellent question! This code article has been a non-favorite of mine for a long time, and now you have added a wonderful twist to the story.

My answer is that your example 2 has the right approach. My reasoning is that you start with a set of conductors that has sufficient ampacity for the application. If you add more conductors, or use a larger conductor, then you upsize the EGC. In example 2, a pair of 350?s has enough ampacity, and you choose to use three 350?s. That is a 150% increase in cross sectional area. I will not double check your math, but your approach is good.

As to example 1, a single 1500 has sufficient ampacity. If you use that size, then you do not increase the EGC. Yes, a set of three 350?s has more ampacity. But you are not upsizing from some number of 350?s to a greater number of 350?s, so that math problem does not apply.

Similarly for example 3, you are not upsizing from some number of 1/0?s to a greater number of 1/0?s, so that math problem does not apply.

It?s not a great answer. But example 2 is the only one that makes sense.

Welcome to the forum.

 

[Dennis Alwon]

This is a very interesting enigma.