250.122(B) with parallel conductors

[NOV]

I'm looking at a set of drawings which has an 800A distribution panel fed with (5) parallel feeders of 500KCMIL. The feeder has a Key Note stating it was upsized due to voltage drop. The Feeder Schedule lists the feeder with a #1/0 grounding conductor which doesn't meet 250.122(B). In order to size the grounding conductor for this, am I correct in thinking I have to divide 800 by 5 (160) and use that corresponding conductor (#2/0) circular mil area to calculate the percentage the ungrounded conductor was increased? Then of course increase the grounding conductor by that percentage.

 

[augie47]

I may be viewing it incorrectly but it appears to me you have increased the standard (2) 500s by a multiplier of 2.5 thus you would need to increase the standard EGC (1/0) by 2.5 also meaning your EGC would be a 300 kcmil.

 

[wwhitney]

There are multiple analyses possible when you have multiple sets, and 250.122(B) isn't clear on which approach to use.

E.g. 800A is too large for one conductor, so it requires at least 2 sets. Those two sets could have consisted of 900 kcmil Cu (520A 75C ampacity) all in one conduit (0.8 ampacity adjustment, for a total of 1800 kcmil. But 2500 kcmil was used, so the factor is 25/18.

Or you could say that 2 sets would normally be done in 2 conduits, so each set would only have to be 600 kcmil, and the ratio is 25/12.

Or if the load is in fact under 760A you could say as Augie suggested that it could have been 2 sets of 500 kcmil, so the ratio is 5/2.

Or if you have 5 sets in one conduit (no idea if that would actually be practical), the ampacity adjustment would be 0.5, so each set would need to have a base ampacity of 320A, and hence 400 kcmil, so the ratio is 5/4.

Or if you have 5 sets in 5 conduits, you have your suggestion, where each set needs to be 2/0 (133.1 kcmil) and the ratio would be 500/133.1

Or for the biggest answer, if the load is 750A or under, you could say that it could be 5 sets of 1/0 (105.6 kcmil) in 5 conduits, and the ratio would be 500/105.6.

Cheers, Wayne

 

[NOV]

I came up with 3.75%. I used 500000/133100 which I got from Chapter 9 Table 8. Did I use the wrong numbers somewhere?

 

[d0nut]

In my opinion, 250.122(B) leaves quite a bit of room for interpretation. It requires the EGC to be increased proportionally where the ungrounded conductors are increased in size from the minimum size that has sufficient capacity for the intended installation. What the minimum size actually is presents more of an interpretation issue. Is it 5 sets of #2/0 (875A, 665.5 kcmil), 4 sets of #3/0 (800A, 671.2 kcmil), 3 sets of 300 kcmil (855A, 900 kcmil) or 2 sets of 600 kcmil (840A, 1200 kcmil)? Each one of those options would be the minimum size for that number of sets of conductors, but would give vastly different multipliers depending on which one was chosen.

 

[NOV]

I made an assumption of (2) 500 KCMIL for the intended because he has 400A feeders that use 500 KCMIL, no 600 on the schedule corresponding to 400A or 800A.
I never gave it a thought (or did the math) for the different scenarios you all have laid out. I appreciate your responses, thank you all!

 

[Tulsa Electrician]

Here is a pic from a simple speed sheet I use.
I used 2- sets of 500 as the selected conductor. I based this from a 800 amp feeder, with a connected load at or below 760 amps(2 x 380) 75 c copper conductor and round up rule.

Note, based on what code version your on this can vary. An example is the exception for 2020 sized by qualified person.
The sizing can also be changed based on feeder connected load, and selected conductor. 3-400 MCM (3 x 270 =810)Al @75 c as an example. This would yield a different result.

Another issue would be if a copper EGC is used or AL. This would also yield different answers.

The first pic uses copper for EGC and Phase/Grounded
The second pic uses copper EGC and AL phase and Grounded.
The third pic uses Al for EGC and AL phase and Grounded

One can refine by stating what the selected conductor and the new conductor is in way of size based off of connected load and conductor type
( CU or AL) for both EGC and Phase/Grounded.

Dont forget about code year. Verbiage is different and the exception for 2020.

 

[Tulsa Electrician]

I should edit my sheet and use the proper wording as written in the NEC.
Instead of saying phase / grounded. I should use "ungrounded conductors".