Code recognition for "natural gray" neutral insulation doesn't mean "gray" insulation.
Recently, a large industrial enterprise asked us, after an inspector challenged them, about continuing to specify gray wire for their branch-circuit neutral conductors originating from 480Y/277V distributions. They wanted to distinguish these conductors from similar conductors on 208Y/120V systems, for which they used white wire as they had been doing for some 30 yrs. They also wondered about dangerous confusion that might result from creating such an inconsistency in their established practice.
The EC&M Panel's response. We are forced to agree that the inspector's observations are in accordance with the literal text of the Code. We also agree, however, that the facility's wiring practice agrees with the customary installation practice throughout the country. Most all of us know the identification rules for these conductors (actually, any grounded circuit conductors) use the color "white or natural gray." We say it over and over without really thinking about it.
The problem is "natural gray" and "gray" just aren't the same thing. The word "gray" refers to a controlled color, as stated on wire reels and in manufacturers' catalogs. The phrase "natural gray" refers to the uncolored, grayish, off-white state of natural latex rubber insulation. It isn't a controlled color, and you can't use it synonymously with "gray."
The historical context makes this pretty clear. The phrasing "natural gray" in conjunction with white first appeared in the 1923 Code, essentially as it does today. At that time, however, the only insulation system available for conductors for general use was rubber, often referred to as "code-grade" rubber. It was natural latex rubber (the kind from rubber trees) applied to the wire and vulcanized. It didn't have antioxidants or stabilizers, which is why that old conductor insulation turned brittle over time.
The natural color of latex is a very whitish gray, and so we think it is completely understandable that back in 1923, with technology what it was, the NEC Committee said natural gray insulation and white insulation could be safely used for equivalent purposes. The actual words never changed. However, current standards don't allow a pure "natural" rubber; it has to have additional compounds added so it won't deteriorate. Those compounds add some color (often a reddish hue), which varies by each manufacturer's preferences. That's why all these wires have colors added to counteract the coloring effect of the stabilizing chemicals. The result is that we seriously doubt there are any "natural gray" conductors now in production. If anything, "natural" today might mean "natural pink" or "natural green" (depending on the stabilizing chemicals).
So, forget about "natural gray" as a color choice on today's conductors. What we have is an unfortunate lapse in the process where something that made perfect sense 75 yrs ago simply hasn't been updated. Instead, with no change in language, it has become widely interpreted to mean something (controlled color) quite opposite to its original meaning (uncontrolled color).
Furthermore, if "natural gray" really meant "gray," then Sec. 200-6(d), which covers cases where two voltage systems are in the same enclosure, would allow "white" and "natural gray" to distinguish the systems. It doesn't. Either system (typically either the 208Y/120V or the 480Y/277V system) can be white (or natural gray), and the other (your choice) must then be white with a stripe. Meanwhile, you're likely to be in for quite a struggle to find white wire with stripes on a supply house shelf anywhere nearby.
This is bad news for those thousands of facilities that have been following the customary practice, as well as the inspectors who are in an untenable position. Indeed, the current inspector representative and chair of CMP 5 supported efforts to recognize gray in the 1999 NEC, but to no avail. We, reluctantly, encourage local rule-making on this topic, and we strongly support a concerted effort in the 2002 code-making cycle to modernize this aspect of the Code.