Standard Nominal System Voltages

[kingpb]

In reading posts, I have seen a variety of ways in which voltages are being stated. Many seem to have a good handle on this topic, and therefore this post may not apply, however, I think it is of worthy mention to reaffirm how important it is, that as professionals, we do not become complacent in properly stating voltages.

ANSI provides a list of standard nominal system voltages, and voltage ranges, which are reprinted in IEEE Std 242. Also provided are the preferred voltages in bold type. For those who do not have access to these documents, I will provide the basics, for systems up to 480V. The information presented also applies for voltages above 480V.

The difference between 3 ph/3W and 3ph/4W is as follows:

3ph/3W are systems in which only the three current carrying conductors are brought out from the source to serve loads. A 3ph/4W system is one in which the three current carrying conductors and the grounded neutral are brought out from the source to serve loads. Stating the voltages for a 3ph, 3W system you only provide the line to line voltage. For 3ph/4W system you list the line to line voltage first then the line to neutral voltage.

Example: 208V and 480V (3ph, 3W), 480Y/277V and 208Y/120V (3ph, 4W)

Single phase systems are slightly simpler to define. You see them called out as 1ph, 3W, but in reality they are 2ph, 3W. A 1ph/2W system can also be used, but has no neutral brought out. No "Y" designation is used since the voltages are developed by other means than a wye connected transformer.

Example: 120/240V (1ph, 3W), 120V, 240V, and 480V (1ph, 2W)

For either single phase or three phase systems, indicating two different voltages, separated by a slash means a neutral is brought out of the source. If the system is derived from a "Y" system then it is also included. However, as will be demonstared this can be misapplied, and often times is.

Example: 240/120V, by definition is a 3ph, 4W system derived from a delta system, and should not be confused with 120/240V 1ph,3W system.

Other very common errors are the use of 120/208Y in place of 208Y/120V. Technically, 120/208Y is actually a 1ph, 3W system derived from an open wye configuration, whereas 208Y/120V is your common 3ph, 4W, wye system.

You also see 277/480V used in place of 480Y/277V.

Conclusion:
No matter what line of work you are in, when dealing with electrical systems it is very important that you understand the voltages you are working with, and that those voltages are conveyed properly on drawings, in calculations, and in written documents.

 

[jtester]

You need to be certain who you are talking to. When specifying distribution transformers a 7200/12470Y volt primary will get you a 2 bushing high side whereas a 12470Y/7200 will get you a 1 bushing high side with the H2 lead terminated on the tank. Both are correct.

Your post is very informative, but it is amazing how many exceptions to the rule(s) there are.

Jim T

 

[bphgravity]

Nominal voltages are also listed in 220.5(A) in the NEC.

 

[jim dungar]

Excellent post especially the conclusion.

However I take exception to:

Single phase systems are slightly simpler to define. You see them called out as 1ph, 3W, but in reality they are 2ph, 3W.

2ph systems really do exist and the whave nothing to do with single phase.

"Phases" are really determined from the number of line-line (not line-neutral) waveforms

 

[jwelectric]

Single phase systems are slightly simpler to define. You see them called out as 1ph, 3W, but in reality they are 2ph, 3W.

Could you explain this one and how it functions?

 

[kingpb]

The intent of the statement was to try and get people to realize that in a 120/240V system, although historically referred to as single-phase; really takes 2-phases and a neutral, to make it, i.e. 2ph, 3W. A single phase distribution circuit feeding this transformer may be a 1ph, 2W circuit. I will stand corrected if someone can show me a circuit made up of either 1 current carrying conductor (1 phase) and 1 grounded conductor, or 2 current carrying conductors (2 phases) and no grounded conductor, and it be a 3 wire system.

In a 120/240V system, the 2-phases are 180 deg apart, so the vector sum of the voltage phase-to-phase is twice that of the line to neutral voltage. Versus a 3 ph system where each phase is 120 deg apart and the vector sum of the line to line voltage (phase to phase) is 1.732 x the phase to neutral voltage. Ex. 208Y/120V.

Yes, I will agree that there is, historically, an actual 2-phase system, that was one of the first electric systems developed. Although it is now considered obsolete, since the development of three phase.The NEC does still list 2-phase motor FLA in Table 430.249.

Without geting into the technicalities on how a "T" or "Scott T" transformer connection works (Maybe someone can post as a new topic) it is still considered a 2ph, 4W (no neutral) or 2ph, 5W (w/neutral) circuit.
My understanding is that it is still sometimes encountered in control applications. That my be why the 2 phase systems like the 120/240V were called "single phase" But of course that is just a guess.

I can thankfully say I am not old enough to have encountered a "Scott T" application.

Sorry if the reference to 2-phase hurt some of the "old" timers feelings. Afte all it does date back to the late 1890's.

 

[jim dungar]

No, No, No.

There are not two phase 180 degree apart. There are two line to neutral voltages L1-N and N-L2 they are in "phase" with each other. If they are not in phase how can they be added together to get 240V L1-L2?

You are calling them 2-phase because you have decided to use the neutral as a reference point instead of following the convention of numbering phases by counting the number of L-L possibilities. In a 2-wire circuit there is no such thing as a neutral.

Take a standard 120/240V 3 wire circuit. It can be "generated" by the series connection of 2 single winding transformers, 1 two winding transformer, or 1 center tap transformer. In all configurations the current flow in the windings is always in the same direction (in phase).

If you draw the path for a single winding center tapped transformer feeding a balanced load you will find, the current flow for one 2 wire circuit is out of L1 and into the neutral terminal, then the flow for the other circuit must be out of the neutral and into L2 otherwise the net current on the neutral would not add to zero. This results in a single winding current direction of L2-L1. A single winding current direction, and a single L-L voltage results in a single phase system

 

[mpross]

Phase definition

Phase definition

here we go again on this discussion of what a phase is...

Jim is 100% correct!

Matt

 

[jim dungar]

Without geting into the technicalities on how a "T" or "Scott T" transformer connection works (Maybe someone can post as a new topic) it is still considered a 2ph, 4W (no neutral) or 2ph, 5W (w/neutral) circuit.

You had better get into how a Scott and a standard T transformer work. They are two different beasts.

A Scott connection converts 3-phase into 2-phase (a Taylor connection can even go backwards, 2-phase into 3-phase).
A standard T simply changes the voltage, otherwise we wouldn't need 1-phase to 3-phase converters.

 

[steve66]

Who decided we should list the Line-Line voltage first on a three phase system, but we list the line to neutral voltage first on a single phase system?

 

[jwelectric]

I think that I am going to enjoy this thread should it take off.

We have two opposing thoughts from two different PEs of the electrical field.

Jim, be gentle with this young pup as he has admitted to never seeing one of the old two phase systems. Gently introduce him to the 90 degree out of phase systems.

I shall sit back and read for a while, so the two of you teach the rest of us how this works.

 

[kingpb]

Mike,

Interesting thought, but unfortunately I will not debate theory in public with another PE. It is considerd unethical.

I will however, propose, that Jim do his own experiment to see if PHASE A and PHASE B are in, or out of phase with each other.

Simply, pull a circuit off of your A phase, through a GFI breaker, and pull a circuit off of your B phase through another GFI breaker, and simply wire them together. If the breaker trips, they are out of phase. If the breaker does not trip, well you get the idea.

I don't really recommend doing this, but I think it helps to drive the principle home, that these are out of phase.

I really am not interested in getting this topic off-topic. The point I was originally trying to make was that voltages are consistenly mis-stated, and hopefully some of the people reading these forums could learn something constructive.

If someone is interested in posting a new topic about phase relationships, That would be interesting, and I would be a willing partcipant.

 

[iwire]

If someone is interested in posting a new topic about phase relationships,

I am and I did.

Is it Single or Two Phase?

 

[mpross]

crazy idea

crazy idea

I think that last idea is crazy, and I dont think that it would prove anything.

when you look at two different points of the same phase, they dont necessarily have to be "in phase" with each other. any time that you have an inductor(transformer windings)/capacitor (even in a single phase circuit), you will have a leading or lagging current componets with respect to where you are measuring in the circuit.

kingpb, are you a power engineer?

 

[rbalex]

From The IEEE Standard Dictionary of Electrical and Electronics Terms,
Sixth Edition:

phase

(1) (of a periodic phenomenon f(t), for a particular value of t) The fractional part t/P of the period P through which f has advanced relative to an arbitrary origin. Note: The origin is usually taken at the last previous passage through zero from the negative to the positive direction.
(2) (A) A distinct part of a process in which related operations are performed, as in the shift phase of a shift-and-carry operation.(B) A relative measurement that describes the temporal relationship between two signals that have the same frequency.
(3) A major stage within the generating-plant life cycle.

Definition (1) is the one under consideration for this discussion. I believe it supports Jim because the crucial element is establishing the time origin, not a voltage differential.

Of course, I don?t know the Professional Engineers ethics code of each State; however, neither my home State?s (California) nor those adopted by the National Society of Professional Engineers would forestall public debate.

 

[jwelectric]

The NEC gives us the following voltages to work with:

220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

The only reference to a different type of slash voltage is found in

480.83(E) Voltage Rating. A controller with a straight voltage rating, for example, 240 volts or 480 volts, shall be permitted to be applied in a circuit in which the nominal voltage between any two conductors does not exceed the controller?s voltage rating. A controller with a slash rating, for example, 240/120 volts or 480Y/277 volts, shall only be applied in a solidly grounded circuit in which the nominal voltage to ground from any conductor does not exceed the lower of the two values of the controller?s voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the controller?s voltage rating.

The way that I understand the ?slash? ratings of voltages is:
When the lower voltage leads it is from a delta transformer and when the lower voltage follows it is from a wye transformer and will always have the ?Y? inserted after the higher voltage.

Now I will not stand pat on this because I am just plain not sure of myself (no schooling).
Even without a formal education I will stand pat on the fact that there are two different numbers of phases from either transformer. The are single and three phase.
Simple geometry tells us that we can access one side of a triangle but when we involve another side all three side are involved.