Yes I get that.. what I was saying is that in the case above going from a #8 wire on a 40 amp cir with 10 equipment grounding conductor to a #6 wire on a 40 amp circuit would require a #6 equipment grounding conductor. It doesn't make sense.The #12 part is correct but the #8 to #6 is not for a 40 amp circuit. Since you start with a 40 amp circuit and a #8 phase conductor by using a #6 for the same 40 amp circuit you have increased the size of the phase conductor therefore you must also proportionately increase the size of the EGC. So if the #8 AWG cable has a #10 EGC and the #6 AWG cable has the same #10 EGC you cannot use the #6 cable for the 40 amp circuit because you have not increased the size of the EGC (both cables have a #10 EGC). The proportionate increase in the EGC would be the same ration as the increase from #8 to #6.
As I said previously DUMB...
I didn't do the math but if you had a 40 amp circuit with a #8 ungrounded conductor and went to a #6 ungrounded conductor the EGC would increase from a #10 to a # 8 not a #6.Yes I get that.. what I was saying is that in the case above going from a #8 wire on a 40 amp cir with 10 equipment grounding conductor to a #6 wire on a 40 amp circuit would require a #6 equipment grounding conductor. It doesn't make sense.
VD is the reason for the rule. It is worded as one size fits all type of thing and makes for unnecessary required increase in some instances where same size EGC covers a range of OCPD's in T250.122 and voltage drop is not a concern, espcially when using cable wiring methods where you only have on size EGC commonly found in certain cables as related to the insulated conductors.10 thru 6 insulated cables will typically have 10 AWG grounding conductor.I think the upsizing was put in where the conductor is upsized for voltage drop, then the egc would require upsizing for the same reason. That would not be the case here, but then code reads like a law book! LOL!
Wait, what are we talking about? You only have to up-size if for voltage drop.True, but irrelevant. :happyno: You would still need to upsize the EGC. The rule says that if you increase the size of the ungrounded conductors (with "increased" having been explained in the 2017 edition as meaning beyond the minimum ampacity requirements), then you increase the EGC proportionally.
There was a time when the NEC did limit the need for upsizing the EGC to instances when the ungrounded conductors were increased because of VD considerations. But I think there was a bit of abuse going on, as in, "I didn't use the larger wire because of VD. It was just what I had in the truck at the time." The rule was changed to require upsizing the EGC regardless of the reason that the ungrounded conductors were upsized. The oldest code book I have handy is 2008, and this change happened before then.Wait, what are we talking about? You only have to up-size if for voltage drop.
I may have had some influence in getting that clarification into the NEC. For several cycles, I sent in update recommendations that pointed out that the statement, "increased in size" means nothing, unless you clarify by stating "increased from what?" It now says "increased in size from the minimum size that has sufficient ampacity for the intended installation." So your example fits perfectly into the current language.That means that a #8 conductor is no longer adequate for a 40A load, meaning that #6 is the smallest size allowable. In which case the ungrounded conductors have not been increased in size, and there is no problem with EGC size.