I have recently relocated to the US and brought European 220V appliances with me. I planed to use power transformers from 110 to 220, but learned that I have NEMA 10-30 outlet in my home, which, from what I understood, is 220V (but with 110V volts on the ground pin?!).

Anyhow, is there (or can I assemble) and adapter from NEMA 10-30 to European 220V and will it work?

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    A NEMA 10-30 will be 240VAC between the two hots, but only 120VAC between each hot and the neutral. The neutral doesn't have voltage "on it." There is also not a ground on a NEMA 10-30, or at least not in the sense I think you are asking. Only the neutral grounded conductor. Here's an excellent visual explanation of North American 240V if what I said doesn't make sense: diy.stackexchange.com/a/33603/38349 – mjohns Jul 11 '15 at 11:45
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    You also have to take into consideration the cycles, or Hz. Much of the EU is 50hZ while we here are 60hZ. This will affect things like motors, clocks and some electronics. IMO is it not a good idea to try and make foreign appliances work. It is typically more trouble than it's worth. – Speedy Petey Jul 11 '15 at 11:49
  • @mjohns your explanation makes perfect sense. From what I understood, the normal 110V outlet is much more "alike" the European outlet (except of the voltage and the frequency), which means that a transformer looks like a better fit, isn't it? – JBaruch Jul 11 '15 at 15:33
  • @SpeedyPetey Since I have nothing to do with the appliances except of connecting them, I'll give it a try. My only question at this point what will be easier? Step up from 110v or try make US 240v work like European 220v. – JBaruch Jul 11 '15 at 15:34
  • Even a transformer will not change the frequency from 60Hz to 50Hz. To do this is very expensive. Some of your European equipment will be OK but many things with motors will run fast and may be damaged. – DoxyLover Jul 11 '15 at 17:38

A NEMA 10-30 is 2 hots and a grounded neutral conductor, with 110V H1-N and H2-N and 240V H1-H2, all at 60Hz. Whether an European appliance will accept it depends on the type of appliance:

  • Appliances with fractional HP motors, or especially AC synchronous (timing) motors, will likely not run correctly due to the frequency difference as motor synchronous RPMs will change as a result.
  • Grounded (IEC Class I, look for the ground symbol) appliances cannot be safely connected to a NEMA 10 outlet due to the lack of an equipment grounding conductor. (They can be connected to a NEMA 14 or NEMA 6 outlet with the correct adapter, though, provided that mains frequency is not an issue.)
  • Some older Class 0 appliances rely on plug polarization and the grounding of the neutral to be safe -- this is common in old consumer audio gear. These appliances also cannot be safely connected to a US 240V outlet of any type (whether it be a NEMA 6, a NEMA 10, or a NEMA 14) due to the lack of a grounded reference conductor in the 240V section of the circuit. I'd junk them anyway.
  • Doubly insulated (IEC Class II, look for the square within a square symbol) appliances (that do not have an AC motor in them) and Class II power supplies (wall warts, laptop bricks, etal) don't care about how the mains is presented to them, and thus can safely be hooked up to a pair of opposite leg hots, even if they are 240V only. The internal transformer will be built more robustly for 50Hz operation than it needs to be for 60Hz, so the frequency change is a non-issue.
  • Older instruments and some other types of (older) electronic equipment intended for worldwide circulation will have a jumper link, back-panel switch (the infamous little red switch), or other documented means to change over from 120 to 240V -- this covers older computer power supplies, for instance. The presence of an IEC-type inlet and detachable (computer type) power cord, instead of a fixed power cord, is a good (albeit not universal) sign to look for a 120-240V changeover function. Again, the frequency change is not an issue in an all-electronic device.
  • Newer Class II power supplies and electronic equipment/instrumentation have "universal inputs", and are capable of 120, 208, or 220/240VAC operation without any twiddling -- this capability will be marked on the device's safety label.
  • some European UPSes are picky about the hot/neutral situation (some instructions say to reverse the plug if it has certain error codes). i suspect these might have an issue if plugged directly into the USA 240 "two hots" type of power. – Skaperen Jul 12 '15 at 8:20

The US supports Euro style 240V with a whole family of receptacle types. They are called NEMA 6 and NEMA 2: grounded and ungrounded, with the latter being largely extinct. These carry only 2 conductors with 240V across them. In America, neutral is in the middle, so neither conductor is neutral.

NEMA 14 adds a neutral to NEMA 6 (grounded). This is useless to you, but socket is usable by ignoring neutral.

NEMA 10 adds a neutral to is NEMA 2 (ungrounded) with a neutral in the middle. You can use that too, but the lack of ground is a potentially big problem.

Ground protects equipment from ESD damage and humans from shock. There is another way to do the latter; use a GFCI (RCD) device on the supply. On 240V circuits, these are generally combined with circuit breakers, and cost about $80.

You still need circuit protection for the loads. I would not put a 9-amp European appliance on a US range circuit fused/breakered at 50A. Even if you put a NEMA 6-50 plug on it, that breaker will trip much too late to save the device or prevent a fire. So I would downsize the breaker accordingly, probably with a 20A.

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