CFL

Yeah, I know what he meant. I think I have alot of built up frustration from people I deal with. I hear alot of misinformation coming from people who have been "out of school too long". In some cases I think it's better to keep your mouth closed than to tell a less experienced, impressionable apprentice, for example, what you "think is correct". This is off topic so I'll shut up. No hard feelings, please.

Loose Neutral

Sounds like you have issues. Just like Larry said, sometimes you learn things, don't use the info then forget it. Then even re- learn them and forget again. Use it or forget it. Have a drink and chill.

lectric_hand6855

So in a 208 3-phase panelboard with the high B-leg, connecting a 208 single phase load between B-phase and neutral is the same as connecting it between phases A and C correct ?

InPhase277

No. A 120/208 V system doesn't have a high-leg. A 120/240 V three phase system does. So connecting from B to neutral gives you 208 V, but from A to C you have 240 V.

zgozvrm

This is where some of the confusion comes in...
There is no sine wave on leg A. In other words, you can't supply voltage with a single wire. Rather, you can measure a sine wave from leg A to leg B, from leg A to neutral, or from leg B to neutral. Note that when measuring the 120V sine wave of a 120/240 split phase system with an O-Scope, one typically places the ground clip on the neutral and the probe on one leg or the other. When moving the probe from leg A to leg B, this results in swapping the left-to-right order of the probes and, therefore the polarity. This is why people tend to think that the two 120V sine waves are 180 degrees out of phase.

Again, if they were 180 degrees out of phase,
1) that would mean that they would cancel each other out.
2) there would have to be more than one transformer, since you can't create more than one sine wave (or phase) with a single transformer.

zgozvrm

A 120/208V system is a 3-phase system with a wye-configured secondary and is more commonly called 208Y. The center of the wye (the common point of all three transformers) is grounded, forcing its potential to 0 volts. Any leg measured to ground (in effect measuring the output of only one of the 3 secondaries) will then result in 120 volts. There are 3 legs, thus 3 ways to obtain 120 volts and, like all 3-phase systems, they are 120 degrees apart. When you measure leg-to-leg in such a system, you are then measuring the total output across 2 of the 3 secondaries. This results in 208V due to their 120 degree phase offset. If they were in phase, you'd get 240V. If they were 180 degrees out of phase, you'd get 0 volts. (FYI, the 208V sine wave will be 30 degrees offset from one of the phases and 150 degrees from the other).

A 120/240V 3-phase system (better known as 240 center-tapped delta), is derived from a delta-connected secondary. In this situation, the center of one of the secondaries (usually, the one opposite, or not connected to, phase B) is grounded. This creates a 120/240 split-phase system on that secondary. Any leg-to-leg tap will measure 240V. 208V can be obtained by measuring (or tapping) from leg B (opposite the center-tapped secondary) to ground. This is because you will be measuring 240V that is 120 degrees out of phase from 120 volts.