Does 250.122(F)(1)(b) make any sense?

[JoeStillman]

250.122(F)(1)(b) requires us to up-size the equipment grounding conductor for parallel feeders, but only when the equipment grounding conductor is a wire-type. If the equipment grounding conductor is RGC, IMC or EMT, no upsizing is required.

So, if we run no ground wire at all, we have adequate grounding, but when we run a ground wire, it needs to be a bigger than the ground wire that would otherwise be more than adequate.

And how about 250.122(F)(2)? Under some circumstances, the jacket of AC and MC cable is permitted to be the grounding conductor. If a cable is listed according to 250.118(8) or (10)(b), does the grounding conductor inside have to be upsized for paralleling?

 

[tortuga]

250.122(F)(1)(b) requires us to up-size the equipment grounding conductor for parallel feeders, but only when the equipment grounding conductor is a wire-type. If the equipment grounding conductor is RGC, IMC or EMT, no upsizing is required.

The origins of our modern 250.122 were submitted into the code in the late 60's by UL when there was no shortages of copper. I have done extensive research on this topic. The substantiation was concern about the mechanical integrity of the grounding conductors and nothing electrical.
I have pretty much concluded the NEC should harmonize with the Canadian grounding table ( I forget its number), as they are not having grounding problems up there and table is quite a few pounds lighter.

 

[don_resqcapt19]

250.122(F)(1)(b) requires us to up-size the equipment grounding conductor for parallel feeders, but only when the equipment grounding conductor is a wire-type. If the equipment grounding conductor is RGC, IMC or EMT, no upsizing is required.

So, if we run no ground wire at all, we have adequate grounding, but when we run a ground wire, it needs to be a bigger than the ground wire that would otherwise be more than adequate.

And how about 250.122(F)(2)? Under some circumstances, the jacket of AC and MC cable is permitted to be the grounding conductor. If a cable is listed according to 250.118(8) or (10)(b), does the grounding conductor inside have to be upsized for paralleling?

If the cables are in single raceway or cable tray, a single external EGC is permitted. If the cables are installed individually, you will need a custom cable with a larger EGC.

Note that for the application of the rule in 250.122(A) that says the EGC is not required to be larger than the ungrounded conductor applies to the sum of the areas of the conductors connected in parallel and not to the ungrounded conductors in an individual raceway or cable. The code language is not completely clear on this, but there are CMP-5 statements that make that idea clear, and the first revision for the 2023 code contains new language on this issue.

 

[jaggedben]

250.122(F)(1)(b) makes very little sense, IMHO.

 

[don_resqcapt19]

250.122(F)(1)(b) makes very little sense, IMHO.

Especially when you compare it to sizing requirements for supply side bonding jumpers which is a worse case as far as fault clearing as the supply side bonding jumper is on the line side of the OCPD. The size of the SSBJ for parallel runs is based on the size of the ungrounded conductors in each raceway. There have been PIs to change the rule in 250/122(F), but they have been rejected for lack of technical substantiation. It is my opinion that the SSBJ rule is all of the technical substantiation that is needed, but CMP 5 does not agree..