Based on the 2020 NEC.
Section 250.122(B) is possibly the most peculiar requirement in the Code. It can be difficult to understand or explain, and it's virtually impossible to enforce. Let’s try to make sense of what it says.
If ungrounded conductors are increased in size for any reason other than for ambient temperature correction or more than three current-carrying conductors, any wire-type equipment grounding conductors (EGCs) installed shall be increased in size proportionately to the increase in circular mil area of the ungrounded circuit conductors. On the surface, this may seem easy enough, but it really does not make sense sometimes either. Here are a few examples of how this requirement can sometimes cause confusion.
A 40A circuit with a 10 AWG EGC and 8 AWG THWN circuit conductors is perfectly Code-compliant. But, that same 10 AWG EGC on a 30A circuit with 8 AWG THWN circuit conductors could be a violation. Yup, that’s correct!
Using that same 10 AWG EGC on a lower-rated circuit breaker is literally a violation if those 8 AWG circuit conductors were increased from 10 AWG by design. That really doesn’t seem to make sense to me.
Okay, let’s try again. A 125A circuit with a 6 AWG EGC and 1 AWG circuit conductors is perfectly Code-compliant. But, that same 6 AWG EGC could be a violation on a 110A circuit with 1 AWG circuit conductors. Is the 110A circuit breaker somehow more dangerous than the 125A breaker? I don’t think so, but that same 6 AWG EGC on a lower-rated circuit breaker is literally a violation if those 1 AWG circuit conductors were increased from 2 AWG. And yet, according to Table 250.122, a 6 AWG EGC would be perfectly fine on a 200A circuit too. I’m having a difficult time reconciling this in my brain.
Let’s review. A 6 AWG EGC is permitted for a 200A circuit, but it could be a violation on a 175A, 150A, 125A, or 110A circuit. But it’s not always a violation on a 175A, 150A, 125A, or 110A circuit. Is your head spinning yet? Mine is.
Thankfully, an exception was added to Sec. 250.122(B), allowing the EGC to be sized by a qualified person if it provides an effective ground-fault current path. I’m thinking a 6 AWG EGC that would otherwise be perfectly fine for a 200A circuit would be perfectly fine on any circuit rated lower than 200A too. But you may want to have a discussion with your AHJ about whether they see it the same way.
