Parallel Grounded Conductor size v EGC/GEC

[akpdx]

Hello! A bit of a rabbit-hole question about sizing grounded conductor run in parallel...I'm hoping someone can help me understand something. I'm looking at a problem with a calculated load for a multifamily dwelling just over 900 amps.

Service OCPD is 1000A
Paralleled service conductors - 3 sets of 400 kcmil Cu
Paralleled neutral minimum size as calculated is 152 amps - 2/0 Cu
Minimum grounded conductor according to Table 250.102 (C)(1) is 3/0 Cu

I have seen it said several times that a grounded conductor cannot be smaller than the grounding electrode conductor or an equipment ground.

Where in the Code does it say that? 250.24 (C) doesn't tell us anything about comparing it to any grounding conductor. Nor does 250.102.

What am I missing?

 

[don_resqcapt19]

See 250.24(D)(2). That tells you use Table 250.102(C)(1) based on the size of the ungrounded conductors in each raceway, with a minimum size 1/0 because of the parallel conductor rules in 310.10(H). Since the minimum size grounded conductor in each raceway is smaller than the required size based on the calculated load, you will need a 2/0 in each raceway.

 

[akpdx]

Sorry, I don't follow. There's no .24(D)(2) in 2020. To illustrate my original info, unbalanced load was calculated in this problem to be 455 amps, so parallel 2/0 to handle 152A each. But the grounded conductor rules in 250.102 say run a 3/0 since 1200 x 12.5% = 150,000 cmil, or 3/0.

I was inquiring about the statement I see sometimes that reads basically "grounded conductor is not allowed to be smaller than the grounding electrode conductor" or "equipment ground conductor", (depending on the quote).

 

[augie47]

250.24(C)(2) (in 2020) advises IF the parallel conductorsd are in separate raceways you can size the grounded conductor based on the phase conductor in each raceway, 1/0 minimum due to parallel conductor rule.

 

[akpdx]

250.24(C)(2) (in 2020) advises IF the parallel conductorsd are in separate raceways you can size the grounded conductor based on the phase conductor in each raceway, 1/0 minimum due to parallel conductor rule.

Actually I'm glad you brought attention to that because it says "shall" so you can't just do a normal unbalanced load calc.

 

[don_resqcapt19]

Sorry, I don't follow. There's no .24(D)(2) in 2020. To illustrate my original info, unbalanced load was calculated in this problem to be 455 amps, so parallel 2/0 to handle 152A each. But the grounded conductor rules in 250.102 say run a 3/0 since 1200 x 12.5% = 150,000 cmil, or 3/0.

I was inquiring about the statement I see sometimes that reads basically "grounded conductor is not allowed to be smaller than the grounding electrode conductor" or "equipment ground conductor", (depending on the quote).

It was moved and revised in the 2023 code, but no real change in the meaning.

(2) Conductors Connected in Parallel in Two or More Raceways or Cables.
If the ungrounded service-entrance conductors are connected in parallel in two or more raceways or cables, the grounded conductors shall also be installed in each raceway or cable and shall be connected in parallel. The size of each grounded conductor(s) in each raceway or cable shall not be smaller than 1/0 AWG and shall be sized in accordance with 250.24(D)(2)(a) or (D)(2)(b) in accordance with 250.24(D)(1).

• (a)Shall be based on the largest ungrounded conductor in each raceway or cable.
• (b)Shall be based on the sum of the circular mil areas of the largest ungrounded conductors from each set connected in parallel in each raceway or cable.

 

[don_resqcapt19]

Actually I'm glad you brought attention to that because it says "shall" so you can't just do a normal unbalanced load calc.

You have to do both and use the larger of the sizing for the unbalanced load or the sizing from 250.24.