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