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Posted By: Reel-Break 3 phase vs single - 10/16/06 03:05 PM
When building a system when would you choose 3 ph over single or vice versa. We`re putting in some equipment into a textile plant where they have 3 phase 600 volt I`ve noticed some three phase 600 to 120/240 transformers and some single phase 600/120/240.We`re installing a small panel with mostly 120 volt loads for small equipment and some lighting. Thanks in advance.
Posted By: earlydean Re: 3 phase vs single - 10/16/06 03:29 PM
Generally speaking, use 3 phase whenever it is available. For the cost of one additional conductor, you increase the capacity of the panel by a factor of 1.73(over single phase).

But, if you consider the cost of the minimum size allowable panel (often 100 amps) versus the cost of a 3 phase 100 amp panel, AND, if the total load to be used can be handled by the single phase 100 amp panel, then that is the most economical choice. (As is the case in residential services.)

In your case, the industrial load may not be sufficient to warrant the additional cost. The question you need to ask is: Who is paying the bills, and do we want to plan for any future loads?
Posted By: George Little Re: 3 phase vs single - 10/16/06 06:39 PM
Unless you need the 3 phase for some equipment there is no point in having a "120/240/" 3 phase system. I say that because you will not be able to use the high leg for anything on 120v. or 240v. single phase. If you have a 208/120 3 phase you could use all three legs.

And since I'm being picky the term is 240/120v. for 3 phase and 120/240v. for single phase per IEEE dictionary.

Finger don't tipe rite. [Linked Image]



[This message has been edited by George Little (edited 10-16-2006).]
Posted By: ShockMe77 Re: 3 phase vs single - 10/16/06 10:03 PM
Quote
the term is 240/120v. for 3 phase and 120/240v. for single phase per IEEE dictionary.

See that? I learn something new everyday here.

Thanks!
Posted By: JBD Re: 3 phase vs single - 10/16/06 10:16 PM
The basic, and informal, method is:
On single phase list the low voltage first, like: 120/240 and 120/208

On three phase list the high voltage first, like: 480/277, 240/120, and 208/120

A letter Y is usually added for wye systems - 208Y/120 or 480Y/277.
Posted By: pauluk Re: 3 phase vs single - 10/21/06 09:58 AM
Quote
the term is 240/120v. for 3 phase and 120/240v. for single phase per IEEE dictionary.

Any idea how far back this convention goes?
Posted By: Eddy Current Re: 3 phase vs single - 10/21/06 11:38 AM
240/120 3 phase? I always thought you can only get 208/120 from 3 phase?
Posted By: Roger Re: 3 phase vs single - 10/21/06 12:53 PM
Eddy, the 240/120 3 phase is a Delta Configuration of windings with one winding center tapped for the grounded conductor whereas the Wye configuration is tapped at the end of three windings.

This is why only two legs/phases of the Delta will give you 120 and all three phases of the Wye will give you 120.

Scott has some good information and drawings of these transformer configurations in the Technical area.

Roger
Posted By: EASports Re: 3 phase vs single - 11/01/06 10:41 PM
"Unless you need the 3 phase for some equipment there is no point in having a "120/240/" 3 phase system. I say that because you will not be able to use the high leg for anything on 120v. or 240v. single phase."

This is partly true. With a high leg delta system you have 240V between any two phases, so you could use the high leg and one other to run a 240V single phase appliance (if the breaker is rated to handle the high leg voltage).

[This message has been edited by EASports (edited 11-01-2006).]
Posted By: George Little Re: 3 phase vs single - 11/01/06 11:06 PM
EASports- Be careful-
Quote
This is partly true. With a high leg delta system you have 240V between any two phases, so you could use the high leg and one other to run a 240V single phase appliance.
You need to look at 240.85, especially the latter part of the text. Unless that breaker you are referring to has a straight 240v. rating, you would not be able to use it on a single phase 240v. circuit using the high leg. Some breakers have a slash rating- 120/240 and they could not be used on the high leg of a 3Ø 240/120v panel if they are accross the high leg. Common violation.

[This message has been edited by George Little (edited 11-01-2006).]
Posted By: gfretwell Re: 3 phase vs single - 11/02/06 12:32 AM
You also have to be careful using single phase 240v loads across the red leg if this is an "open vee" using 2 transformers. It can drag the open phase down and create an imbalance. You can end up with different voltages across the 3 legs. Single phase loads should only be on the centertapped trnsformer. The red leg is only for 3 phase loads.
Posted By: Dnkldorf Re: 3 phase vs single - 11/02/06 12:44 AM
Do they(manufacturers), or have they, ever made a 240V rated single pole breaker?
Posted By: Roger Re: 3 phase vs single - 11/02/06 12:56 AM
EASports is correct, 240 single phase would be two phases connected to a two pole breaker, it would not matter which two phases are used even in a center grounded Delta.

The High leg is only a High Leg in reference to ground, not phase to phase.

I think the confusion is the difference between "single Pole" and "Single Phase"

Roger
Posted By: George Little Re: 3 phase vs single - 11/02/06 02:34 AM
Roger- You blew it on this one. Look in the 2006 UL White book under "Molded Case Circuit Breaker Marking Guide" in the back of the book on p. 13. This agrees with 240.85. The lower of the two voltages on a Slant-Rated breaker can not be exceeded when measured to ground from either of the 2 phases involved.

[This message has been edited by George Little (edited 11-01-2006).]
Posted By: Roger Re: 3 phase vs single - 11/02/06 02:59 AM
George,
Quote
Roger- You blew it on this one.
I was waiting for that. [Linked Image]

The problem I have with 240.85 is the wording "Solidly Grounded Circuit.

Quote
A circuit breaker with a slash rating, such as 120/240V or 480Y/277V, shall be permitted to be applied in a solidly grounded circuit where the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker's voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker's voltage rating.

A phase to phase circuit is not "Solidly Grounded" is it?

The system may be grounded but the circuit isn't, so there is nothing prohibiting us from using a slash rated breaker on a phase to phase circuit using the high leg for one side of this ungrounded circuit.

Roger



[This message has been edited by Roger (edited 11-01-2006).]
Posted By: George Little Re: 3 phase vs single - 11/02/06 03:43 AM
Not convinced your correct yet Roger- Sorry
Here's the words out of the UL White Book.
Quote
3. Voltage Rating — All circuit breakers are marked with a voltage rating, including: 60, 125, 125/
250, 160, 250, 500 and 600 volts for dc; and 120, 120/240, 240, 277, 347, 480Y/277, 480, 600Y/347
and 600 volts for ac. If the voltage rating does not include the words “ac” or “dc,” the breaker is rated
for both ac and dc voltages. For ac voltage ratings other than 60 Hz, the frequency is marked.
Circuit breakers for use in Communications Equipment may also carry ratings of 30, 65 or 80 Volts dc.
Two-pole independent-trip breakers and single-pole breakers with handle ties that are rated 120/240 V
ac have been investigated for use in line-to-line single-phase circuits or line-to-line lighting and appliance
branch circuits connected to 3-phase, 4-wire systems, provided the systems have a grounded
neutral and the voltage to ground does not exceed 120 V.
Two-pole independent-trip breakers and single-pole breakers with handle ties that are rated 125/250 V
dc have been investigated for use in line-to-line connected 3-wire dc circuits supplied from a system
with a grounded neutral, where the voltage to ground does not exceed 125 V.
Two-pole independent-trip breakers and single-pole breakers with handle ties that are rated 125/250 V
(both ac and dc) have been investigated for use in accordance with either of the above two paragraphs,
as applicable.
Two- and three-pole common-trip breakers rated 120/240V ac are intended for use on 1-phase, 3-wire
circuits, where the voltage to ground does not exceed 120 V.
Two- and three-pole common-trip breakers rated 125/250 V or 125/250 V dc are intended for use on
1-phase and dc, 3-wire circuits, where the voltage to ground does not exceed 125 V.
Circuit breakers with a single voltage rating are intended for use in circuits where the circuit voltage
and the voltage to ground do not exceed the voltage rating of the breaker.
“Slant-rated” breakers with a rating such as 480Y/277 V are intended for use in circuits where the
circuit voltage does not exceed the higher of the two voltages and the voltage to ground does not exceed
the lower of the two voltages.
Based on the preceding paragraphs, “slant-rated” breakers (120/240, 480Y/277 V, etc., as opposed to
240, 480 V, etc.) are not intended for use on “slant-rated” delta systems. For example, a 3-pole, 120/240
V breaker is not intended for use on a 240/120 V, 3-phase, 4-wire, delta system, because on the high leg,
the voltage to neutral is 208 V. In this instance, a 3-pole, 240 V breaker should be used.
Check it out and if I'm wrong- I'm a big boy and will have to rethink my understanding.
Posted By: Roger Re: 3 phase vs single - 11/02/06 04:09 PM
George,
Quote
Check it out and if I'm wrong- I'm a big boy and will have to rethink my understanding
I don't think you're wrong nor do I think I'm wrong. (and no I'm not a politician [Linked Image])

As I said before, because of the wording, 240.85 can't be cited if the circuit itself is not a "slant" rated circuit such as a 120/240V dryer or a range, a straight 240 volt circuit is not grounded.

The UL wording is also talking about circuits until the last paragraph, but even this paragraph starts off with "Based on the preceding paragraphs" which were talking about "Grounded Circuits" and "Voltage to Ground"

IMO the violation would have to be 110.3(B) if the manufacturers information said it can not be done, and I know some do, but 240.85 needs some changing to be used.

Roger
Posted By: George Little Re: 3 phase vs single - 11/02/06 08:52 PM
Roger- I do understand your perspective on the issue and the wording is murky at best in the NEC. I would also admit that about 99% of the inspectiors I know, wouldn't even check for this violation. We had a UL rep at one of our IAEI seminars that went into great depth on the issue otherwise I'd not be aware of it. Thanks for the give and take on this one.
Posted By: gfretwell Re: 3 phase vs single - 11/03/06 12:58 AM
This is the same trouble people get in when they are using two pole breakers on corner delta. They have to be rated for full circuit voltage not 120/240
Posted By: Roger Re: 3 phase vs single - 11/03/06 01:16 AM
Quote
Thanks for the give and take on this one.
And the same from me.

Roger
Posted By: Scott35 Re: 3 phase vs single - 11/03/06 02:46 AM
Roger has brought up a great example of yet another possible "Monkey Wrench", for which NFPA may toss into the cogs of the grand machinery.

The Article designates "Grounded Circuit", instead of (IMHO) should have been designated "Grounded System".

To me, the intentions sure point towards the ideal of using dual rated, or slash rated Breakers on Branch Circuitry which would include a Circuit having higher Voltage to Ground than the device is rated (such as a 2 wire single phase 240 volt circuit connected to B & C on a 4 wire delta, or B & C on a 3 wire corner grounded delta).
However, the inclusion of "Grounded Circuits" leaves room for several interpretations!

I see this as a valid argument, and never second-guessed it until now!

Scott35
Posted By: JBD Re: 3 phase vs single - 11/03/06 03:59 AM
You can debate about the use on the poor NEC wording. However, the information from most manufacturers says that the UL testing of slash rated breakers deals with the voltage phase to ground.

So using a slash rated breaker on any circuit that exceeds the voltage to ground rating would be a violation of the manufacturer's instructions.
Posted By: Roger Re: 3 phase vs single - 11/03/06 10:48 AM
JBD, that's just another part of the problem, if the circuit is not referenced to ground there is no voltage from the given conductor to ground.

And to further the problem, read the definition of "Voltage to Ground" in the NEC.

Roger
Posted By: JBD Re: 3 phase vs single - 11/03/06 03:34 PM
Roger,

IMHO, if there is a ground in the system then all circuits have a reference to ground regardless if any particular circuit actually contains a ground.

This is also an issue with the testing and listing of a single pole interruption of the breaker not just with the wording of the NEC. For example, Square D says their 120/240 slash rated breakers can only be used where the "Maximum voltage to ground is 120Vac".

The NEC definition of "voltage to ground" says that if the circuit does not have a ground then you look at the voltage between any two of the ungrounded conductors.
Posted By: Roger Re: 3 phase vs single - 11/03/06 04:09 PM
JBD,
Quote
The NEC definition of "voltage to ground" says that if the circuit does not have a ground then you look at the voltage between any two of the ungrounded conductors.
it actually says
Quote
for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.
So with a two wire ungrounded 240 V circuit what is the greatest voltage between the two conductors? It is the same voltage if you measure A-B, B-C, or C-A.

So, with that being the case we would have to consider how much voltage is dropped at the center of a 240 V circuit from each phase of the circuit, and that would be 120 V

Roger



[This message has been edited by Roger (edited 11-03-2006).]
Posted By: JBD Re: 3 phase vs single - 11/03/06 07:27 PM
Roger,

How are you coming up with this?
Quote
So, with that being the case we would have to consider how much voltage is dropped at the center of a 240 V circuit from each phase of the circuit, and that would be 120 V

Are you saying that you have a 120/240 3W system that is not grounded (in violation of 250.20(B) unless allowed in 250.22)? Regardless the greatest voltage between two conductors would be 240V so a 120/240 breaker is not allowed.
Posted By: Roger Re: 3 phase vs single - 11/03/06 08:09 PM
JBD, you keep saying system, this is not the problem, the problem is where the NEC and UL specifically use the word "Circuit".

We are not questioning whether the system is grounded, it is, the "circuit" is not.

Roger
Posted By: George Little Re: 3 phase vs single - 11/03/06 09:26 PM
I used to have a contractor friend of mine who would not install a single phase breaker in a three phase panel and he would install two panels- One for single phase loads and one for 3 thase loads when he dealt with a Delta 240/120 3 phase service. Not cost effective I agree but this was his standard and I'm not to sure he was wrong. He's gone to his reward now but I always knew why he installed things this way and he never would change. That being the case we can argue all day about system/circuit/ground. He never had any issues doing it his way. My final comment on this issue is a quote from the White Book:
Quote
Based on the preceding paragraphs, “slant-rated” breakers (120/240, 480Y/277 V, etc., as opposed to
240, 480 V, etc.) are not intended for use on “slant-rated” delta systems. For example, a 3-pole, 120/240
V breaker is not intended for use on a 240/120 V, 3-phase, 4-wire, delta system, because on the high leg,
the voltage to neutral is 208 V. In this instance, a 3-pole, 240 V breaker should be used.

Edited for spelling

[This message has been edited by George Little (edited 11-03-2006).]
Posted By: JBD Re: 3 phase vs single - 11/03/06 11:41 PM
Roger,
You keep focusing on the word circuit in 240.85 like it requires a grounded conductor or else it doesn't apply.

Can't you run an ungrounded circuit in a grounded raceway? If so don't you have a voltage to ground? If you say no, then you need to determine which two circuit conductors have the greatest voltage between between them (see the definition of Voltage to Ground).
Posted By: Roger Re: 3 phase vs single - 11/04/06 12:08 AM
JBD, you are thinking of the EGC, which is not a part of the circuit in function as a current carrying conductor.

Looking at a 240 volt phase to phase (ungrounded) circuit, do you think the high leg (referenced to ground) actually provides 208 volts and the other phase will see this and shed 88 volts so it will only provide 32 volts to the load? [Linked Image]

Both of the phases will drop 120 volts each, so neither is a higher voltage than the other is it?

Now, I'm going to try to follow Georges lead and let this be my last post here.

No promises though. [Linked Image]

Roger
Posted By: JBD Re: 3 phase vs single - 11/04/06 04:52 PM
Roger,

I still dont' see how you are coming up with statements like:
Quote
Both of the phases will drop 120 volts each, so neither is a higher voltage than the other is it?
. To where are they dropping the voltage? And, why it is important?

If the circuit is ungrounded and there is any combination of any two circuit conductors that exceeds the lower voltage then a slash breaker cannot be used. The reason for this is that one of these conductors may accidentally become grounded.

I would include the failure to follow manufacturers instructions and warnings when noting a violation.

[This message has been edited by JBD (edited 11-04-2006).]
Posted By: winnie Re: 3 phase vs single - 11/05/06 02:59 AM
Roger,

I disagree with this point:
Quote

A phase to phase circuit is not "Solidly Grounded" is it?

The system may be grounded but the circuit isn't, so there is nothing prohibiting us from using a slash rated breaker on a phase to phase circuit using the high leg for one side of this ungrounded circuit.

If your _circuit_ consists of two ungrounded conductors from a grounded _system_, I contend that this is in fact a _grounded_ circuit. The voltage between these ungrounded conductors and ground will be well defined, and this remains true even if the circuit does not include a grounded conductor.

I agree that the code is not clear in that the term solidly grounded _circuit_ is used, rather than 'circuit derived from a solidly grounded _system_' or some similar verbiage, however what other meaning would you apply to 'soildly grounded circuit'?

-Jon
Posted By: Scott35 Re: 3 phase vs single - 11/05/06 09:30 AM
< memory book = open >

Just a quick response to George's post:

Quote

I used to have a contractor friend of mine who would not install a single phase breaker in a three phase panel and he would install two panels- One for single phase loads and one for 3 Phase loads when he dealt with a Delta 240/120 3 phase service.

When I was very green in the field, having starry eyed dreams of being an EEE-Lecktrickle Injun-ear (and really not seeing the connected craziness that comes with the job,... but that's another story ~:-O...), every new and existing power system was a beautiful lesson at hand.

In the more designated Industrial areas (as opposed to Commercial/light Industrial) - such as Gardena, Santa Fe Springs, and the portion of East LA known as "Bandini" (remember "Bandini Mountain"?), Delta systems covered the earth.
Many 3 Wire Deltas, and a large 1Ø 3 Wire system used for multiple Customers.

Where convenient or economical, customers were serviced with 4 Wire 240/120VAC 3Ø 4 Wire systems - typically warehouses and low demand machine shops.

The thing I found to be extremely bright, was the use of 1Ø 3 Wire Panelboards where L-N or smaller L-L single phase loads where connected, and 3Ø 3 Wire Panelboards (fed without the Grounded Neutral Conductor) where the heavier L-L loads (and, of course, the 3 phase loads) would be connected.

Made complete sense to me, and thought it was like a "standard" technique done by eee-lecktrikle injunears (sorry, I just can't stop jokingly mis-spelling the word "Electrical Engineer").


Quote

Not cost effective I agree but this was his standard and I'm not to sure he was wrong. He's gone to his reward now but I always knew why he installed things this way and he never would change.

Never got any straight answer myself, if this was "Bad Practice" or not - as even though it made 100% sense to me, it still may have been a poor choice somehow.

That was circa 1984 to 1987, and by the beginning of 1988 I was taking on the first (of what turned out to be many) Design/Build Projects - as the King Dingdong in charge of creating the Electrical Power and Lighting systems + generating the Construction Documents (drawing the plans).

When any 4 wire delta systems were encountered, I took the "Old School Approach" so commonly seen, and designed using the "1 Phase / 3 Phase" Panelboards concept.

Never had any issues with Plans Examiners or Inspectors (had many complimenting the idea as "Thinking Outside The Box").

So far, after searching the NEC, querying the ideas to building departments + inspectors, and asking a few "open minded EEs" (explained later), all signs point towards "Good and Ethical Design Practice".

Currently - and maybe as long as 10 years current, a 4 Wire Delta is rarely seen, let alone being involved with one of my designed projects.

What seemed to be so "normal" back then, is more or less nostalgic, and in a way - legendary stuff, when looked at in the field today.

Just felt really compelled to mention this after seeing George's reply, as just recently I was describing these very same things to field personnel - those of which were in Elementary School at the time I was first starting to perform designs involving the 4 wire delta systems.

< memory book = closed >

Scott35

btw:

"open minded EEs"

Referring to those EEE-Lecktrikkle Injun-ears, who refused to place themselves in Ivory Towers, and kept their minds open to remember how things in the field went; - like remembering the simple fact of how rapidly smoke is lost from equipment, intended to be connected to a 120V circuit, when it's connected to a circuit having a potential in the 185 - 215 Volts range!

Most had Trade Knowledge (worked as Electricians before going the EE route), a few didn't - all had excellent skills, nonetheless.

Rant is over [Linked Image]
Posted By: gfretwell Re: 3 phase vs single - 11/05/06 05:16 PM
Scott, that is exactly the way the computer installations I saw were wired. They brought the 3p panel into the computer room without the grounded conductor, only grounding for the equipment. The regular office space and the single phase (lights and 5-15s on the wall) in the computer room came from a separate single phase panel. The only thing we were careful of on the 3P panel was that the 2 wire L/L loads were not on the red leg.
Posted By: Almost Fried Re: 3 phase vs single - 11/10/06 04:01 AM
The first inspectors I came in contact with required seperate panels because of the idiot factor. When I was an electrical knee high, I found out about the wild leg the painful way, but them lights sure was bright for a while...
I have run into several open delta services and the pocos like AEP/SWEPCO and ENTERGY have it available, but so many installations are specing pad mount xfmrs, which, to my knowledge either aren't available or more probable, the pocos dont keep them in stock and should one fail, they could not get your building up without a special order. I like 240 personally, as opposed to 208, but ya get what ya get. The Master who was my first boss must have had leather for skin because he would walk up to a switchboard and touch the bus with his two fingers, rocking them back and fourth, and say, "boys, this'un is 208, not 240..."
Posted By: trobb Re: 3 phase vs single - 11/14/06 03:26 AM
In reply to an early part of this post, according to the 1968 EE manual I own (for a look back, I wasn't even born then) the convention at the time was 277/480 style, not the reverse.
Posted By: JBD Re: 3 phase vs single - 11/14/06 02:48 PM
That obsolote terminology may have been in effect back in the late 60's (my 1969 handbook also used it). However, the 1993 edition (the oldest one I have)of the IEEE Red Book discusses ANSI C84.1-1989 as the standard that defines the nominal voltages in the US.

Usage of obsolete voltages (i.e. 110 and 220) is discouraged when refering to supply systems.
Posted By: pauluk Re: 3 phase vs single - 11/15/06 11:50 PM
Looks like I lost track of this thread for a while......

My take is that a 240V 2-wire circuit (without neutral) does not have a solidly grounded conductor as part of the actual circuit, but the circuit itself is still quite solidly referenced to ground by the grounded neutral at the xfmr.

So I'd say the circuit is solidly grounded, it's just that it doesn't include a conductor which is directly connected to ground.

Quote
In reply to an early part of this post, according to the 1968 EE manual I own (for a look back, I wasn't even born then) the convention at the time was 277/480 style, not the reverse.

Thanks for that. That's the similar convention to the way we would normally show 3-phase voltages here, e.g. 240/415V.

Quote
Do they(manufacturers), or have they, ever made a 240V rated single pole breaker?

Use 'em all the time over here...... [Linked Image]
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