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In the 208/220v diagrams for 2 pole 3 wire outlets here(http://www.automationdirect.com/static/specs/wiringdevicesnemawiring.pdf) there have an arrow between the two wires that bring in 120v each and label the arrow 250 (or 208) volts. Are they just saying that there is 250v supplied total by these two legs, or that there is 250v between these legs?

I think it is saying that there is 250v supplied total, as I think there's 0v between the legs, unless the outlet is doing something like switching polarity?

Thus reason I am unsure is because the diagrams for 125v use the same arrow to show that there is a 125v difference between the hot wire and ground, so if they are not using the symbol in two different ways then the 250v diagram says that there is 250v between the two legs that have 125v each, which I don't see how that happens unless there is a built in Op amp or something...

Thanks.

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  • \$\begingroup\$ That diagram is something else. Makes me suspect that re-locating machinery between sites will often require a new plug. Little hope of matching socket with so many to choose from but at least one can hope that there is suitable power available. \$\endgroup\$ – KalleMP Dec 17 '15 at 7:19
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The 208/250V is supplied by two phases of a split-phase (250V) or three-phase (208V) utility connection. Ground is not involved in the pair.

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In normal North American power wiring, we get two 120 volt "hot" wires that are 180 degrees out of phase, so if we measure between those two hot wires we will get 240 volts. (Electrical Code rules call these 125 and 250 volt.)

Where there is three-phase service (many commercial and industrial users), normal outlets still get a single phase 120 volt supply. But if you connect between two phases (two separate "hot" wires), you will get 208 volts, as the phases are 120 degrees apart, rather than 180.

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