Phase A and Phase B in single phase wiring diagrams

[teej]

I have seen in the wiring diagrams of single phase 120/240 V 3 wire systems, that the two hot legs of the circuit are labled phase A and phase B. I thought it was single phase power? It is confusing to me that the two hot legs are labled phase A and phase B. Does this nomenclature refer to the fact that the single phase power is split out from two phases of a 3 phase system?

For instance, when wiring a 2 pole 240V circuit breaker, the two hot legs of the circuit are connected to the two poles of the circuit breaker. Now, does it matter which pole of the circuit breaker is connected to phase A and B? In other words, aren't the two hot legs of the 120/240V circuit interchangeable since it is single phase power? Thanks in advance.

 

[charlie b]

Re: Phase A and Phase B in single phase wiring diagrams

Let's just write this off to the "What's in a name" philosophy. Call it phases A and B, or call it hot wires 1 and 2, or call it red and black, or call it Peter and Paul; it does not matter. It's still single phase. And when you connect to a 2 pole breaker, it does not matter which wire connects to which pole.

 

[George Stolz]

Re: Phase A and Phase B in single phase wiring diagrams

Originally posted by teej:
Does this nomenclature refer to the fact that the single phase power is split out from two phases of a 3 phase system?

No, it is gotten from only one phase of a 3? system. One ungrounded phase and one grounded conductor.

As this diagram from Scott Thompson (found here) shows:

 

[LawnGuyLandSparky]

Re: Phase A and Phase B in single phase wiring diagrams

I agree, "phase" should only refer to legitimate phases. The 2 hot "legs" of a center-tapped transformer should be referred to as legs, not phases.

Otherwise, the 5 or 6 different "taps" from a multi-tap transformer should be considered phases as well.

 

[George Stolz]

Re: Phase A and Phase B in single phase wiring diagrams

But LawnGuy, the two legs of this transformer are 180? out of phase with each other.

"Phase" may be a slightly misleading term, but it's still applicable, IMO.

 

[charlie b]

Re: Phase A and Phase B in single phase wiring diagrams

I agree, George. (Don't worry, I won't let that happen very often. )

 

[George Stolz]

Re: Phase A and Phase B in single phase wiring diagrams

Charlie, I certainly hope not! Things would get terribly boring 'round here.

 

[Rick Kelly_home]

Re: Phase A and Phase B in single phase wiring diagrams

High Guys... I was out of town doing a round of inspections in one of my communities when this came up and did not have a chance to reply then. Please excuse the tardiness of this reply.

Originally posted by georgestolz:
But LawnGuy, the two legs of this transformer are 180? out of phase with each other.

"Phase" may be a slightly misleading term, but it's still applicable, IMO.

IMHO... There is no phase difference in the voltage waveform of the standard 120/240 Volt three wire systems we use. Though I must admit it does look like there is. In reality, what we have is two 120 volt windings in phase with each other. Those winding are then connected in such a way so that their polarities are additive.

The diagram below shows what I mean...

Could it not be misleading to state that there is 180 degree phase difference in play here between the two hot legs of a split phase system?

 

[George Stolz]

Re: Phase A and Phase B in single phase wiring diagrams

I'd be interested to see what A-N & B-N look like on an oscilloscope. Your thinking bears consideration.

 

[jwelectric]

Re: Phase A and Phase B in single phase wiring diagrams

Originally posted by georgestolz:
I'd be interested to see what A-N & B-N look like on an oscilloscope. Your thinking bears consideration.

George it would depend on where you connected the leads.

Look at it outside the box. I have three people standing in a straight line; Amanda, Nancy and Beth.
When Amanda looks at Beth and Nancy she looks to the right in a straight line.
When Beth looks at Amanda and Nancy she looks to the left in a straight line.
When Nancy looks at either Amanda or Beth she has to turn her head from the left to the right.
Nancy can never see both Amanda and Beth at the same time yet they all three stand in ONE straight line.
Amanda is point (A), Nancy is the grounded (neutral) point and Beth is point (B) of a single phase of a Delta transformer.

 

[Rick Kelly]

Re: Phase A and Phase B in single phase wiring diagrams

Hi George, and everyone else

Originally posted by georgestolz:
I'd be interested to see what A-N & B-N look like on an oscilloscope. Your thinking bears consideration.

There would be no difference at all from either wave form. Remember... we have two different in phase sources. As such, the wave form has to be identical. This is the only way possible that when seried the resultant voltage equals 240. If there was any difference in phase angle relative to each other the resultant voltage would not equal 240 volts.

Though as JW mentioned... where you put your scope leeds impacts what you would see. Consider the scope leeds to be "tip" and "shield". To look at the windings properly, for winding #1, you would need to put the "tip" on L1 and the "shield" on the Neutral, for winding #2, you would need to put the "tip" on the Neutral and the "shield" on L2. Placing the "shield" on the Neutral and then looking at either L1 or L2, with the "tip" would do nothing other then "mirror" the waveform and would not be a true visualization of what is going on.

[ February 20, 2006, 09:47 AM: Message edited by: Rick Kelly ]

 

[eric stromberg]

Re: Phase A and Phase B in single phase wiring diagrams

<you would need to put the "tip" on L1 and the "shield" on the Neutral, for winding #2, you would need to put the "tip" on the Neutral and the "shield" on L2.>

You could do this, philosophically, but please don't try to do it in the real world. The shields of the oscilloscope are connected together. If you connect them this way, you are connecting them across a voltage. Either fuses will blow, or there will be smoke, fire, and jello. Plus, the shields of the scope leads are connected to the equipment "ground" of the three pin plug. You are also connecting the shield to the system neutral point.

Eric Stromberg, P.E.

 

[Rick Kelly]

Re: Phase A and Phase B in single phase wiring diagrams

Originally posted by eric stromberg:
You could do this, philosophically, but please don't try to do it in the real world. The shields of the oscilloscope are connected together. If you connect them this way, you are connecting them across a voltage. Either fuses will blow, or there will be smoke, fire, and jello. Plus, the shields of the scope leads are connected to the equipment "ground" of the three pin plug. You are also connecting the shield to the system neutral point.

Eric Stromberg, P.E.

I did not mean to imply that you should, or even could, view both windings at the same time. If I did... it was not intentional. As well, maybe the use of the word "shield" was not appropriate,
"reference lead" might have been a better choice.

[ February 21, 2006, 10:28 AM: Message edited by: Rick Kelly ]

 

[hardworkingstiff]

Can you get a 2-phase 120/240-volt service and how would it differ from a 120/240-volt single-phase service?

 

[charlie b]

A 2-phase system has a 90 degree phase difference between the phases. That is as opposed to a 3-phase system, in which each of the three phases is separated from the two others by 120 degrees. A single phase system has only one phase, so it makes no sense to talk about phase separation.

I don't know if any utility would provide a 2-phase anymore. I can't think of a good reason that it was ever invented, unless it had something to do with motor starting and running characteristics.

 

[johnny watt]

Re: Phase A and Phase B in single phase wiring diagrams

Could it not be misleading to state that there is 180 degree phase difference in play here between the two hot legs of a split phase system?

Hi everyone I hope it?s okay for me to just jump in on this topic.

Correct me if I?m wrong, but the diagram shows the ground at 120V potential.
It also shows L2 at zero volts
By convention ground is at zero potential and L2 is at 120V.
To correct that the L2 should be labeled -120V @ 0 degrees
Negative @ 0 degrees is the same as positive at 180 degrees

Therefore the voltage signals L1 and L2 are 180 degrees out of phase with each other with respect to ground.

Due to the pragmatic definition of 2-phase, these two out of phase voltages are still considered single phase.

In researching 2-phase I have seen many definitions that call for exactly 90 degrees and an occasional definition that calls for anything that can drive a motor without starting capacitors and such.

 

[George Stolz]

Re: Phase A and Phase B in single phase wiring diagrams

Could it not be misleading to state that there is 180 degree phase difference in play here between the two hot legs of a split phase system?

No, there has to be a phase difference, otherwise there would be no potential (voltage). It is a 180? difference, because there are two halves of the transformer sitting at opposite poles. Think of an armature spinning around inside two magnets set opposite one other. When the positively charged armature is at 0? (dead-center of one magnet), then the negative side of the armature is sitting at dead center of the other magnet, 180? of the circle away.

Correct me if I?m wrong, but the diagram shows the ground at 120V potential.
It also shows L2 at zero volts

It doesn't really show the relationships clearly. The " 120V @ 0? " is supposed to be the label between L2 and neutral.

Therefore the voltage signals L1 and L2 are 180 degrees out of phase with each other with respect to ground.

Not exactly; they're 180? out of phase with each other, irrespective of ground.

Think of it this way: A 240V water heater has no reference to ground, it's a line-to-line load. Yet it always will recieve 240V because the two lines will always be 180? out of phase with each other.

 

[hardworkingstiff]

George,

I have to disagree with you. The phasing perspective is all about reference to ground (neutral). If you were able to have another tap in the transformer that was half-way between A and Neutral (lets call it Y), then A would look like it was 180 degrees out of phase with Neutral when referenced to "Y".

If you used A and Neutral for a 120-volt circuit, would you say they were 180 out? If you use A & B for a 240-volt circuit, I don't think it is correct to say they are 180 out. (They appear 180 out when referenced to the mid-point, aka neutral).

 

[hardworkingstiff]

Re: Phase A and Phase B in single phase wiring diagrams

Think of it this way: A 240V water heater has no reference to ground, it's a line-to-line load. Yet it always will recieve 240V because the two lines will always be 180? out of phase with each other.

A light bulb receives 120V, how far out of phase are the two conductors bringing the 120V to the light bulb?

 

[johnny watt]

Not exactly; they're 180? out of phase with each other, irrespective of ground.

I get your point?similar to DC where one line has to be negative and the other positive with respect to each other in order for any potential to exist.

But the definition of in or out of phase requires two signals to be contrasted. If you are using a single point to describe the signals you must be describing the current, and I don?t believe the currents are out of phase. That is why I said with respect to ground (I should have said neutral) because I am contrasting voltage signals.

In my mind, the fact that we always use the legs on the hot side of the load, defines how the scope should be hooked up when comparing the waveforms. Just like your stereo when you were a kid, the A/B channels were not in phase or out of phase with each other UNTIL you hooked them to the speaker loads.

It may be stretching it to say the neutral is at 120V in the diagram but I will say this: Plus and minus signs ?mean something? even in AC (which reminds me, I don?t like using arrows to describe the currents).