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hardworkingstiff

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Wilmington, NC
I've been reading on here that it is very common to install 2-phases of a 208Y/120 distribution system to feed apartments/condos.

I'm curious, is this considered a 3-wire 2-phase system? If not, what is it?
 
From what I've installed the Phase splitting(for lack of better term)is done in the Metering equipment. 208/120 3ph in and 208/120V 1ph out. With balancing to be field adjusted.

Tom
 
This would not be a 2 phase system. That is a completely different animal.
 
In a 2 phase system the two phases are 90 degrees out of phase as opposed to 120 degrees in the 208Y/120 volt system.
 
Here is a set of schematics from ECN's technical reference area:

http://electrical-contractor.net/ubb/Forum15/HTML/000055.html

Basically, the two-phase system is all but dead, as I understand it. All power generated in the US is three-phase, according to posts I've read.
 
This is a single-phase 208/120V 3-wire system.

This is a circuit fed with two LINE conductors not two PHASES. This sounds like nit-picking in my choice of words but it is an important difference.

I believe most of the confusion regarding 1-phase, 2-phase, and 3-phase systems could be resolved if our industry would stop the inaccurate practice of calling a single conductor a "PHASE" instead of "hot" or a "line".

The term phase (as in 3-phase circuit) is properly used to indicate the number of different Line-Line voltage combinations that are available and not the number of Line-Neutral voltages.
 
jim dungar said:
The term phase (as in 3-phase circuit) is properly used to indicate the number of different Line-Line voltage combinations that are available and not the number of Line-Neutral voltages.
Click to expand...

Sounds good to me Jim. But it begs another question. How do you get the 2 Line-Line voltage combinations on a 3 wire, 2-phase system? My understanding is there are two Line voltages and 1 grounded Neutral.
 
hardworkingstiff said:
Ok, but....

So what is a 3-wire, 2-phase system? (See 220.61(A)Exception)
Click to expand...
Interesting question.

In the bottom schematic diagram in the link George posted, the diagram by Scott, on the left, one sees two transformer windings drawn at a right angle to each other. This helps to imply the electrical phase angle between the voltages.

Jim Dungar makes a great point.

Think of the basic 3? set of line conductors, that is, L1, L2 and L3.

The apartment/condo panel you are thinking of in your opening post will have L1 & L2 (or L2 & L3, or L1 & L3). This would be "3-wire, 3?, wye" - not to be confused with 3-wire 3? delta. There is a neutral here that 220.61(A) is concerned about.

A two phase system was commonly four or five wire with a set of four line conductors, L1, L2, L3 & L4. The fifth wire, if present, is the neutral.

The "3-wire, 2-phase" of your question is L1 & L2, or L2 & L3, or L3 & L4, or L1 & L4, plus the neutral. . .any two adjacent lines, not opposite lines (like L1 & L3 or L2 & L4). Because the voltage and current on the two adjacent lines are only 90? electrical degrees apart, line to neutral load currents from different lines will not cancel out.
 
al hildenbrand said:
The "3-wire, 2-phase" of your question is L1 & L2, or L2 & L3, or L3 & L4, or L1 & L4, plus the neutral. . .any two adjacent lines, not opposite lines (like L1 & L3 or L2 & L4). Because the voltage and current on the two adjacent lines are only 90? electrical degrees apart, line to neutral load currents from different lines will not cancel out.
Click to expand...

And since they are out 90 degrees, the neutral current can be about 141% of Line current
 
hardworkingstiff said:
Sounds good to me Jim. But it begs another question. How do you get the 2 Line-Line voltage combinations on a 3 wire, 2-phase system? My understanding is there are two Line voltages and 1 grounded Neutral.
Click to expand...

There is no such thing as "a" line voltage.

You do not have (2) line voltages, you have (2) lines. You have (1) line-to-line voltage and (2) line-to-neutral voltages. (1) L-L voltage is a single phase system.
 
jim dungar said:
There is no such thing as "a" line voltage.

You do not have (2) line voltages, you have (2) lines. You have (1) line-to-line voltage and (2) line-to-neutral voltages. (1) L-L voltage is a single phase system.
Click to expand...

If I understand you correctly, by this definition, there is no such thing as a 3-wire, 2-phase system.
 
hardworkingstiff said:
If I understand you correctly, by this definition, there is no such thing as a 3-wire, 2-phase system.
Click to expand...

I do not have all of my reference books right now but I do believe that is a true statement.

Using my definition of a Vline-line = 1 phase, it becomes easy to describe delta connections, wye connections, and center taps. If you try to use Vline-neutral then it becomes difficult to describe system without neutrals. If you use Vl-l sometimes and Vl-n other times then no one will ever know which one you are talking about.
 
hardworkingstiff said:
If I understand you correctly, by this definition, there is no such thing as a 3-wire, 2-phase system.
Click to expand...
There is definitely a 3-wire, 2-phase system.

With apologies to Jim, and the point Jim's making, I think you are missing the fact that a "3-wire, 2-phase system" is not a complete 2-phase system.

The complete 2-phase system must have four phase conductors (again, apologies, Jim).

The "3-wire, 2-phase system" has two line to neutral voltages that are 90? (that is, a ? cycle) off from each other. When one measures from the voltage on one line to the voltage on the other line, one measures the difference between those two voltages, getting a single voltage, the line to line voltage.

Again, think of the panel in the apartment/condo, where the building has a 4-wire, 3-phase, wye, 120/208 Volt service. When only two "phase conductors" are taken to an apartment/condo, the line to line voltage available in the unit is 208 Volts, and there is only one such voltage. . .a "single phase" voltage of 208 Volts.
 
al hildenbrand said:
There is definitely a 3-wire, 2-phase system.

With apologies to Jim, and the point Jim's making, I think you are missing the fact that a "3-wire, 2-phase system" is not a complete 2-phase system.
Click to expand...

No apologies Al. I admit that without my reference book I was not positive of the 2-phase 3-wire system.

I know old habits are hard to break but I believe my defiant act of calling conductors "lines" instead of "phases" could prevent a lot of misunderstandings.
 
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