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As detailed in a previous post, I was planning to use the dryer circuit in my condo as a MWBC. A 120V/20A outlet between one of the two hot conductors and neutral would power my small washer and dryer and a 240V/20A outlet between the two hot conductors would allow me to power some European appliance without the need for an external transformer of questionable reliability.

This plan was based on the implicit assumption that all US residential electrical systems are split-phase, i.e., the two hots are out of phase by 180 degrees. As a reminder that failure is more often due to false certainties than to those critical issues that were identified and properly addressed, the measurements I did today showed 208V between the two hots. This indicates that the utility does not a feed me a split-phase signal but two of the three independent phases of the transformer.

A little reading seems to indicate that this is not too rare, even if uncommon. Since no much fuss gets made about it, I am led to think that all those US home appliances that need 240V (dryer, range) are designed to keep working with 208V.

However, I'm not sure the same can be said for 240V foreign home appliances. In general in Europe the specified working interval is 220V-240V. 208V is well below that. Considering a 1500W appliance, a decrease in supply voltage from 240V to 208V corresponds to an increase in current drawn of 1A, from 6.2A to 7.2A, which appears to be rather benign, well below the 20A nominal limit enforced by the breaker.

Is there any negative consequences from having 208V instead of 240V between the two hots?

UPDATE

Thanks for all the answers. I did not focus on the frequency mismatch because the appliance label indicates a working range from 50 to 60 Hz, which is confirmed by my current use of it via a 110V outlet and a 3000AV step-up transformer with no detectable performance hiccup.

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The label also indicates a working voltage range from 220V to 240, but I believe Simon B might be correct in thinking of this as a nominal range and adding another 10% of tolerance with respect to its center point. I guess the worst can happen would be a slower heating element and a slower motor.

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  • North America uses 60Hz frequencies, but European uses 50Hz frequencies. Single family houses do use split phase, 120/240v, but quite a few commercial/business buildings use three phase.
    – crip659
    Apr 10 at 13:32
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    Your calculation for increased current at lower voltage is correct for a constant-power load. The universal power supply ("charger") for a PC or some such electronic thing could be regarded as a constant-power load because it draws whatever power the PC needs regardless of mains voltage (that's magic of a switch mode power supply). A generic "1500 W appliance" is more likely to be a resistive heating load, not a constant-power load, so it'll draw less current and make less heat when powered at a lower voltage.
    – Greg Hill
    Apr 10 at 16:03
  • Greg, you are correct, that was a mental lapse on my side. Thanks for that explanation of switch-mode power supplies, it really explains a lot of questions I have in regards to the portability of different types of equipment through different supplied voltage ranges.
    – MarcoD
    Apr 10 at 21:09
  • How you considered simply reaching out to the manufacturer and asking them? They would probably say that use might void the warranty, blah, blah blah, but they should be able to confirm "that won't won't" vs "its should/will work but voids warranty.
    – Ian W
    Apr 11 at 2:38
  • The manufacturer wouldn't even sanction the use via a step-up trnsformer, saying that the unit is strictly made for the European system. I am not really concerned about warranty though as that expired a while ago. I am more interested in understanding whether what I had planned still makes sense. It seems it got really borderline
    – MarcoD
    Apr 11 at 4:29

4 Answers 4

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120/208 is stunningly common in multi-unit housing (condo/apartment.)

In most cases (particularly all "dumb resistive heating" cases) the reduced voltage will lead to reduced current and power, not increased current to magically maintain the same power.

If your devices have a motor, they are already in trouble due to 60 hz rather than 50 hz power.

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  • Thanks for your answer. I did not focus much on the frequency mismatch because as noted in my question I currently power said appliance from 110V via a 3000AV step-up transformer and it has been working as supposed, as far as I can tell. It is the habit of reasoning in terms of that transfomer that tricked me into the brain f@rt of writing that line, as if the appliance had, magically as you say, access to a 15% boost transformer
    – MarcoD
    Apr 10 at 21:05
  • Would you believe I've got motors that don't care if you have 50 or 60 hz?
    – Joshua
    Apr 11 at 3:06
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    yes, but many motors DO care, including quite a few in appliances.
    – Ecnerwal
    Apr 11 at 3:48
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    @Joshua AC motors always react differently to 50/60Hz. For motor driven loads that accept both, take a close look at the manuals and specifications - the performance will be different in both cases. Such designs are optimized to be able to accommodate both frequencies, but there is still spread in the ultimate performance and efficiency. Usually both the 50Hz and 60Hz performance take a hit when designing to allow both, and one of the two will generally outperform the other to some degree.
    – J...
    Apr 11 at 13:45
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    @Joshua Ok, but that's also completely irrelevant.
    – J...
    Apr 11 at 15:25
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It's really marginal for European appliances. The standard across Europe has been set at 230V, with a tolerance of ±10%. That wide tolerance is to allow for existing supplies that are really 220V or 240V. Doing the sums, 230V - 10% is 207V.

Do make sure the appliance concerned will run on 60Hz. It's 50Hz all across Europe.

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  • Thanks Simon, I believe you are correct in considering 220V to 240V as a nominal working interval and adding a 10% tolerance interval to its center point.
    – MarcoD
    Apr 10 at 21:44
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    The other thing to keep in mind is that if you have 208V nominal that's also a nominal value - over the days and seasons it's not uncommon for that 208V to itself drop due to normal circuit loading and grid fluctuation. I've been in places where "208V" was usually closer to 200V most days and would sometimes drop as low as 190V. For motor loads expecting 230V this is really pushing it too far.
    – J...
    Apr 11 at 13:39
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As detailed in a previous post, I was planning to use the dryer circuit in my condo as a MWBC. A 120V/20A outlet between one of the two hot conductors and neutral would power my small washer and dryer and a 240V/20A outlet between the two hot conductors would allow me to power some European appliance without the need for an external transformer of questionable reliability.

I recommend you read this great answer from ThreePhaseEel on the requirements for an MWBC serving both line-to-neutral and line-to-line loads. The TL;DR is you may be limited by the availability of two-pole common-trip breakers with both GFCI and AFCI protection.

Regrding the EU appliance compatibility with 208V 60Hz, check the label. Many modern appliances have universal power supplies which will work from 100V-250V on 50Hz or 60Hz. If yours is not one of those, obey the label.

If the appliance label doesn't say it can accept 208V 60Hz then purchase a new appliance. If it doesn't specify what it accepts then purchase a new appliance. As Ecnerwal mentions in his answer, pay attention to the difference between 60Hz and 50Hz as well as the voltage.

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  • Thanks Jeff, yes the necessity for the 20A/2-pole GFCI breaker was explained to me in my previous post on the subject
    – MarcoD
    Apr 11 at 8:55
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It will probably work but it's marginal.

The numbers on the rating plate are the range of nominal voltages the appliance is designed to work with. There will be a tolerance band beyond that to allow for supply variation and volt drop. This tolerance band is probably of the order of 10%-15% so a supply with an actual voltage of 208V is likely within tolerance.

The problem comes when due to supply variation and/or voltage drop in your installation your nominally 208V supply is not actually 208V. Then you can easilly get outside the range your equipment was designed for. Under voltage is unlikely to be immidiately dangerous but it may well cause equipment to malfunction.

Regarding your specific appliance which seems to be a food processor with heating functionality.

Also be aware that the power output of resitive heating elements will vary significantly with supply voltage. I suspect the quoted 1000W is at 240V, at 208V that will be down to more like 750W. Not a disaster but things may well take longer to heat up then you are used to.

I would watch out for stalling of the motor. Assuming it's a mains powered motor, the motor will have less power on the lower voltage and the higher frequncy may mean it tries to spin faster (depending on what type of motor it is), this could lead to a stall which could lead to the motor overheating.

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  • Thanks, you make some very good points. I was thinking of running the same recipe on the current setup with the step-up transformer and while connected to the 208V supply. I could time the ramp up to a given temperature and try to estimate the motor frequency from an audio recording. And, of course, the final test would be the blind tasting lol. Seriously though, the effect of fluctuation in the 208V supply will be more difficult to analyze unless I can find a Kill a Watt type of device that works around these voltage values
    – MarcoD
    Apr 11 at 4:07
  • This is Thermomix - "smart" appliance with ready programs / recipes. If it uses preprogrammed cooking times, then getting only 3/4 of the power will result in under-cooked food. I'd call that disaster :)
    – Mołot
    Apr 11 at 13:19
  • I'd hope there is some thermostatic control in there to compensate for different heating rates. Apr 11 at 17:09

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