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I'm pretty new to this but I know that in three phase power, when I'm metering line to line its usually at 208V, but I've been told that line to line meters at 240V. I don't know what would cause for it to meter different. Does it matter whether its WYE or Delta? Or is it only 208V when there is a common neutral or ground? I refer to USA and Canada.

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Here are common arrangements in the US and Canada, taken from a cached FAQ from Schneider Electric:

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The 120/208VAC is common in residential complexes and some commercial situations (not ordinary houses, where single phase 120/240 is standard). In industrial installations you find other 3-phase options. For example, in one building I frequent the office lights operate from 347VAC, but phase-to-phase is 600VAC (Canada) so relatively heavy equipment such as screw compressors can be used. 277/480 would likely be used in the US.

Probably some huge houses are wired with 120/208 line-to-line.

As you can see, there are a few delta options with 240VAC.

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    \$\begingroup\$ In the USA, single-phase 120/240 is used in virtually all free-standing and most multi-unit residential occupancies. It is also common for the120 V distribution in commercial spaces and office spaces of industrial facilities. Three-phase 120/208 Y is much less common but common for multi-unit residential buildings in some areas. It is also used somewhat in commercial buildings. Hi leg wye appears to be very rare and never used for residential units. Where is is used, the 208 volt connection is generally prevented from being used. \$\endgroup\$ – Charles Cowie Dec 24 '18 at 23:43
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For three phase, 120V line-to-neutral, the line-to-line voltage is 208.

Normal domestic power in North America is single phase from a center-tapped transformer, with the center tap being neutral. In this case, line-to-neutral is 120 V (half the transformer secondary), and line-to-line is 240V (across the full transformer secondary).

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There are several 3-phase systems used in the US.

208V is always in "wye" configuration (you need to know what that is) and is mainly for powering 120V single-phase hot-neutral loads, though it can power 208V single- or three-phase hot-hot loads as well. 208V is used where you have a lot of 120V loads and yet the power company wants you to load the 3 phases equally. It is the default residential power in New York City, and pretty much nowhere else.

208V can supply 240V resistive heating appliances, though at 3/4 useful output. So for instance a 120/240 split phase dryer will run on two phases of 208V 3-phase.

240V usually shows up as "delta". It is either center-grounded, giving 137V from pole to ground, or sometimes is seen in corner-grounded or "grounded in the middle of one leg".

The latter, "wild-leg delta", allows 120V between a neutral wire pegged at that middle location, and its adjacent hot. Meanwhile it can support 240V motors and the like. The farthest leg, the "wild leg", must use orange wire - that is the only instance where US NEC calls out a specific wire color for a hot.

There are a variety of other voltages, in either wye or delta. Wild-leg only exists in 240V, it would have no reason to exist in other voltages.

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