Why is 10 missing in NEMA nomenclature

Question

Well, I know that NEMA 10-30 would indicate 240-volt, 30 amp plug/receptacle. But when I started looking at NEMA literature, I see that they don't mention "10" at all!

Is this not correct literature?
Can someone shine some light?

Answer 1

NEMA 10 connectors are ungrounded, 125/250V connectors which have been outlawed since 1966 when grounding came in.

Because of appliance industry lobbying, exception was made until 1996 for ranges and dryers only. So we are dealing with really ancient codes here. The exception required that electrical wiring be 4-wire as soon as old stocks of ungrounded cable were used up*, but that the legacy NEMA 10 connectors could be used for ranges and dryers only.

The idea was that by about 1970, legacy stocks of 3-wire cable would be gone, and all cabling would provide 4 wires, and a person with a NEMA 10 connector could simply remove the connector and find a 4-wire connection fit for a NEMA 14.

Unfortunately many broke the law and used "/2 w/ground" cabling for dryers and ranges, which was never legal in the first place, is not grandfathered, and must be killed on sight.

If you are not making ranges or dryers, NEMA 10 has been irrelevant to you for 55 years.

Because there are still legacy NEMA 10 receptacles out there in the wild, new range and dryer manufacturers provide a means to wire in a NEMA 10 connector, effectively by bootlegging ground off neutral (which is just as bad an idea as it sounds). Nobody else is allowed to do this.

China ignores the rules

Of course, if you search the bottom of the barrel, anyone will make a connector for anything. So either some fella in Kentucky will cheerfully build you a NEMA 10 to NEMA something-else cheater cable and mail order it to you... or naturally China will build anything you want and mail order it to you including through Amazon warehouses, with no accounting to the law.

Such things would be illegal to sell in reputable retail stores who answer to the FTC and CPSC. And if Customs were adequately funded, they would also intercept those objects at the border. Amazon uses "free trade zone" legal chicanery to avoid enforcement actions.

So you talked in comments about cheap Chinese EVSE's that you can buy with NEMA 10 plugs. That is some of that illegal stock.

UL will not list anything relating to NEMA 10, except for receptacles (intended ONLY for replacement of broken receptacles), plugs and cords (intended for use on legacy receptacles), and same-to-same extension cords.

UL will never approve any adapter cord of NEMA 10 to anything else. Nor NEMA 10-30 to 10-50 or vice versa. They will never approve any appliance that is not a range or dryer to be shipped with a NEMA 10 cord.

UL is the author of the standards. Several companies, called NRTLs, can perform the "UL Listing" part of certifying your item as legal.

I see some people who perceive a "gap" in the marketplace and rush to fill it. They build their masterwork (easy pickings, they wonder why no one else has done this, and this should be a red flag, but they are stubborn). Home Depot says "we'd love to sell it, but get a UL listing please" and they go "how hard could it be?" And then UL red-tags it because their entire approach is illegal and unsafe. Turns out, that's WHY there was a hole in the market there.

The lesson is to learn UL requirements first, so that they are front-of-mind when your product is at the "scribbling on the back of a bar napkin" stage. Such a product will breeze through UL listing presuming RU recognized components are used.

Whatever you hope to do with NEMA 10, stop.

If you feel there is some market to be targeted using NEMA 10 plugs, then use correct, grounded plugs (NEMA 6 or NEMA 14) and advise your customer to install proper circuits. Leave it to the consumer to obtain illegal "cheater" cords.

Or better yet, counsel them how to either

• make use of the ground wire that's actually hiding behind their NEMA 10 receptacle, or
• how to retrofit a ground wire to make it properly NEMA 14, or
• how to fit a NEMA 14 using the GFCI loophole. (GFCI can replace ground for legacy circuits).

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* The "stocks to be used up" were ungrounded /3 NM or UF type cable, because UL required manufacturers to wind up their production of that. And ungrounded SE (Service Entrance) cable in 30A-50A sizes, which manufacturers were expected to stop making, since the minimum service size in America had been 60A for some years. With SE and ungrounded NM/UF gone, the only wire for a range or dryer connection would be /3+ground cable.

Wires in conduit were exempt from that rule because it's easy to throw a ground wire into conduit, and in 1966 virtually all conduit was metal, and that is itself a legal ground path.

Answer 2

NEMA 10 isn't included, because you are not supposed to design any new equipment using it! NEMA 10 was for 125V/250V ungrounded receptacles. Except allowed to combine ground with neutral in certain circumstances.

In general, all 125V, 250V or 125V/250V equipment and wiring is now supposed to be grounded, and has been for many years. That includes 125V NEMA 1. However, NEMA 1 (and apparently NEMA 2 as well, but I have never seen any of that myself) is still included in the NEMA guide because while no new wiring (receptacles) has been allowed since 1962, plenty of equipment is still designed with a NEMA 1 plug, which allows for compatibility with older non-upgraded NEMA 1 receptacles (I've been slowly replacing those in my 1950s house). The difference though is that a typical well-insulated (ideally "double-insulated") device (clocks, radios, lamps, some power tools, etc.) is perfectly safe without a ground, that is not so much the case for a large appliance with a metal case, particularly the 125V/250V appliances of NEMA 10/14 land.

The end result is that all new major appliances (ovens, cooktops, clothes dryers) that historically used NEMA 10 have been gradually moving to NEMA 14 over the past 30 years or so. But nothing new should be designed that way, so NEMA 10 is not included in the current NEMA literature.

Replacement items - cords/plugs and receptacles - and auxiliary items - extension cords - can be legitimate and safe (as safe as NEMA 10 can be) if approved by UL, ETL, etc. But one of the key criteria for considering NEMA 10 to be "safe enough" is avoiding heavy use (insertion/removal) of NEMA 10 plugs/receptacles. A typical dryer might get unplugged once or twice a year for maintenance, and a typical oven even less frequently than that. But if you use a NEMA 10 for an EV charger or, even worse, swap between using the same NEMA 10 receptacle for a dryer and a charger, then you put a lot of stress on the plugs and receptacles and that leads to problems.

There are plenty of items available through Amazon and other sources that are not UL or ETL listed. That is fine for some things, but I really don't recommend it for 30A @ 250V - that's, literally, playing with fire.