single-phase?

single-phase?

  • This is a single-phase service.

    Votes: 26 49.1%

  • This is part of three-phase service feeding single-phase loads, and not actually single-phase.

    Votes: 27 50.9%
  • Total voters
    53
  • Poll closed .
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hardworkingstiff said:
I apologize. It is obvious that I am too ignorant to have posted this survey. :-?
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I'm beginning to think any survey in this forum should always have the "other" option. Sometimes trying to chase down an answer is like herding cats. :smile:
 
quogueelectric said:
No way no how can anyone convince me that 2 xformer windings 120* electrically out of phase can be considdered single phase. It is not so get over it.
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dont.jpg
 
mivey said:
Could you please describe these two calculations and/or what you see to be the difference in these two calculations?
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The reason I ask is that for both cases I would use (assuming L-N loads):
In = I1+I2
V1n = I1*Z1 + In*Zn
V2n = I2*Z2 + In*Zn
 
crossman said:
But, with single phase 240/120 3 wire, the neutral only carries the unbalanced load and doesn't have to be counted as a current-carrying conductor. This is not true of the 208/120 system with 2 ungrounded conductors and a grounded conductor. The grounded conductor has to be counted as a current carrying conductor because the loads do not balance.
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In a 208/120 3-wire circuit the neutral still only carries the unbalance of the 2 ungrounded conductors. But you are correct there is almost always an unbalance (due to the 120? phase difference) so the neutral normally carries current.

Am I correct that you are really basing your definition of single phase on if the neutral conductor current equals zero with a balanced load?
 
My thought is that the "3-phase" or "1-phase" label is symantics.

Yes it's derived from a 3 phase system which means you could consider it a single phase service fed from a three-phase system.

Also, since you cannot run a 3-phase load on this system I guess you could also consider it single phase.

The bottom line is that while it acts as a single phase system it operates similarly to a 3-phase system because the two ungrounded lines are 120 degrees out of phase instead of 180 degrees as you would find in a single phase system.

So for NEC purposes I would consider it a 2 phases of a 3 phase system which would mean applying those specific rules including counting the neutral as a current carrying conductor as specified in 310.15(B)(4)(b).

Just my thought.
 
Within the context of the pole I find these definitions from IEEE 100, The IEEE Standard Dictionary of Electrical and Electronics Terms to be the most relevant:
single-phase circuit … (power and distribution transformers) An alternating current circuit consisting of two or three intentionally interrelated conductors that enter (or leave) a delimited region at two or three terminals of entry. If the circuit consists of two conductors, it is intended to be so energized that, in the steady state, the voltage between the two terminals of entry is an alternating voltage. If the circuit consists of three conductors, it is intended to be so energized that, in steady state, the alternating voltages between any two terminals of entry have the same period and are in phase or in phase opposition.

three-phase circuit … (power and distribution transformers) A combination of circuits energized by alternating electromotive forces which differ in phase by one-third of a cycle (120 degrees). In practice, the phases may vary several degrees from the specified angle.
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Both definitions cite IEEE - C57.12.80-1978, IEEE Standard Terminology for Power and Distribution Transformers as their source.

I note the number of wires in the three-phase definition is irrelevant.
 
mivey said:
Could you please describe these two calculations and/or what you see to be the difference in these two calculations?
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Two places in the code treat this situation differently (310.15B4 and 220.22) maybe more... So when calculating neutral size and current they would differ slightly IMO. And while the calculation may or may not change depending on what you're looking for - the input and output to those calculations will.

For instance: Heating loads and effeiciancy.... ;)
 
You guy's have done it. My head is swimming. At least somebody is right on
both sides. I think it's time to ask an expert.........Calling Bill Clinton.......
 
jim dungar said:
In a 208/120 3-wire circuit the neutral still only carries the unbalance of the 2 ungrounded conductors. But you are correct there is almost always an unbalance (due to the 120? phase difference) so the neutral normally carries current.
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Jim,

Is there ever a time that there would be a balanced load? In other words, would it be appropriate to not have "almost" in that sentence?
 
hardworkingstiff said:
Jim,

Is there ever a time that there would be a balanced load? In other words, would it be appropriate to not have "almost" in that sentence?
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If there were no 120 volt load, then there would be no neutral current.
 
Well, I am now _officially_ confused :)

Rob Alexander posted a set of definitions from IEEE 100 which makes it clear that 120/240 is single phase and 120/208 is _not_ single phase.

In a previous thread, Jim Dungar (I believe, this is from memory) posted links to IEEE standards which make it clear that a 120/208 service with two ungrounded and one grounded conductor _is_ a single phase service.

I'll stand by my gut feeling that such a 'network' service will be _used_ like a single phase service, can reasonably be _named_ a single phase service, but really _isn't_ a single phase service when you get down to the physics of the beastie.

-Jon
 
Yes, but don't expect to see it.

Yes, but don't expect to see it.

hardworkingstiff said:
Jim,

Is there ever a time that there would be a balanced load? In other words, would it be appropriate to not have "almost" in that sentence?
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Stiff,

It theoretically possible but unlikely to have zero neutral current with reactive loads, say one leading and the other lagging, or one lagging by 60 degrees and the other resistive.
 
My vote is three phase, because line and line aren't 180 degrees out of phase, they are 120 degrees out of phase. It is an open "Y". With two transformers connected open "Y" to open "delta" 120/240 volt three phase, or 480 volt three phase can be achieved.
 
jim dungar said:
In a 208/120 3-wire circuit the neutral still only carries the unbalance of the 2 ungrounded conductors. But you are correct there is almost always an unbalance (due to the 120? phase difference) so the neutral normally carries current.
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The real difference between this and a 120/240 volt system is the fact that in the 208/120 3 wire system, if both ungrounded conductors have 15 amps of load, there will also be 15 amps of load on the grounded conductor, while the same loads with a 120/240 volt system would result in no current on the grounded conductor.
 
don_resqcapt19 said:
The real difference between this and a 120/240 volt system is the fact that in the 208/120 3 wire system, if both ungrounded conductors have 15 amps of load, there will also be 15 amps of load on the grounded conductor, while the same loads with a 120/240 volt system would result in no current on the grounded conductor.
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and if any of the loads are non-linear you will quickly build up the current on the neutral, potentially exceeding what the conductor is rated for. Thus the reason many Data Centers ar oversizing the neutral.
 
If you read 117 to ground and 208 ph-ph the source is a three phase Y-connected transformer. Feeding a meter base though I think of it as one winding of a transformer and therefor single phase.:)
 
don_resqcapt19 said:
The real difference between this and a 120/240 volt system is the fact that in the 208/120 3 wire system, if both ungrounded conductors have 15 amps of load, there will also be 15 amps of load on the grounded conductor, while the same loads with a 120/240 volt system would result in no current on the grounded conductor.
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The unbalance in one circuit equals 0A the unbalance in the other circuit equals 15A -> So what? In some situations the neutral counts as a current carrying conductor for derating purposes, but other than that so what?

I know of no 1-phase or 3-phase description that depends on if the grounded conductor is normally carrying current. If one did what would we say about a 120V 2-wire system?
 
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