I have a 2 gang box with a light and a duplex in it. The light is 277 and the duplex is, of course, 120. (Yes I know it's illegal but it's been there since '62). Here's the question: When I take a reading between the switch feed and the duplex I get 310 volt. Why? Is it always this? Is this just a coincidence? Will this vary in other locations?
The 277/480 that feeds the light also feeds the xfmr that feeds the 120/208.
Any long detailed explinations will be appreciated.
If you have anything that won't fit here feel free to email me... stevemiller@technologist.com

Sorry people, that should have said a light switch and a duplex (not a light and a duplex)

Steve,

The reading shows that the 120 and 277V legs in question are not in phase.

It's a similar principle to the readings you get on a single 3-phase system. If you measure between the hots of two 120V receptacles that happen to be on the same phase of a 120/208 system, the sinewaves are in phase and the measured voltage between them will be zero. If the two receptacles are on different phases, then there is a 120-degree phase difference between them and you read 208V.

You have a similar thing happening between your two systems, except that it is complicated by the two different voltage levels in use. If the 120 receptacle circuit were exactly in phase with the 277V lighting circuit, then you would just measure the difference, approx. 157 volts (RMS).

If the two are exactly 120 degrees out of phase, the difference comes out at 352 volts. Allowing for the fact that the exact line voltages may not be exactly 120 and 277, and that the xfmr is bound to introduce some phase shift, your reading of 310V is quite reasonable. The highest reading you could get in theory would be if the two lines were 180 degrees out of phase, in which case the difference would simply be the sum of the voltages: 397V.

By the way, is the xfmr wired 277/480 Y or just as a 480 delta primary?


[This message has been edited by pauluk (edited 05-01-2002).]

Thanks for the reply Paul. It's 208Y120 and 480Y277.
Your explaination about a phase shift is just what one of the electronic "wizards" at work suggested.

For and ANSI-standard 480∆-208Y/120V transformer served from 480Y/277V, there is 90° between X1-X0 and H2-N. For 120V and 277V, the difference figures at 302V. If the actual voltages were ~3% high, the difference could be 310V.

Yep, that's partly why I asked if the primary were wired Y or delta.

If the 120, 277 and 310V readings were the exact levels, the phase difference works out as approx. 94 degrees.

(Slow followup…) Paul, probably the most popular dry-type transformer in the US is 480∆-208Y/120 volts in the range of 15-300 kVA. {In Canada…600∆-208Y/120 volt ratings are common.} The high side is usually served from a utility-furnished 480Y/277 volts (…600Y/346V to the north.) The ANSI standard is a 30° lagging secondary with respect to primary. I think that corresponds to IEC Dyn1 designation. It is also common for the primary to have bolted tap connections, allowing up to six 2½ adjustments in voltage ratio without load-current derating.

There have been limited use of wye autotransformers—480Y/277 to 208Y120 volts, but apparently with all the negative press about triplen harmonics, they really don’t seem to be very popular, even though they can be lighter and lower cost. There is no inherent phase shift high- to low-side.

Under 15kVA 3ø, ‘stock’ transformers are 3, 6 and 9 kVA with ‘480∆-208Y/120V’ ratings, but are typically T-T connected as opposed to true ∆-Y. Again, no inherent phase shift high- to low-side.

Your post sent me searching for the Graybar catalog that I have here, as I remember seeing a xfmr section in it, mostly Square D dry types.

I notice that several with 480V primaries specify 2.5% taps, 2 above and 4 below nominal 480.

I see as well that there is a range of single-phase types up to 100kVa or more with a 120/240 secondary and a 240/480 primary. I assume this is so the same version can be hooked-up to a 277/480 Y or a 240 delta supply?

We don't have such an arrangement of different supply systems here. 240/415V Y is the standard LV; if the load is too great for the PoCo to supply at LV, then they get 11kV delta (or 33kV delta if it's a really big place).

pauluk—This continent has a penchant for lots of 2:1 voltage ratings [and fewer 1.7320508:1 ratings.] The idea allows for series/parallel operation of (the coil pairs in) motors and transformers...hence 120:240:480 ratios.

And it gets weirder yet. Used to be that one service voltage--common for overhead service to dairy barns, gasoline stations and convenience stores--was 240/120-volt 3ø 4-wire ∆ service. It measures 240V ø-ø, and 120V ø-ground/neutral on A and C phases, with 208V ø-ground/neutral on Bø. Years ago it was common to have two meters—a 1ø 3-wire meter for “lighting” (typically with a lower electric rate,) and a 3ø 3-wire meter for “power” (motors.) The “drop’ (overhead span of either 6 or 4 leads) was served via three 1ø center-tapped transformers with secondaries delta connected, or two 1ø center-tapped transformers in a 4-wire open-delta secondary configuration; yielding the same voltages. The open-delta arrangement saved money for the utility where the 3ø load was small; possibly even a single ~3-7½hp [refrigeration, air-compressor or vacuum-pump] motor. Nowadays with only one power rate a single meter handles the 4-wire ∆ service, with up to a 400-amp {fuse or circuit-breaker} building service. [Old habits die hard, they say.]

My best guess for the multiple service voltages over here was, with a lower population density in some areas, loads are generally farther apart, making it customary for more but smaller transformers. I understand that in parts of Europe and Australia many homes and businesses may often be served by the same transformer; with what we would term 400Y/230V 3ø 4-wire {50Hz} for businesses and homes with relatively large-kW non-storage water heating, and 2-wire 230V for most other residences.

In the US, a single-phase service tapped from 3ø 4-wire secondary is relatively rare for single-family dwellings, with generally 3-wire 208Y/120V ‘network’ service for multiply-metered apartment/condo buildings. Primates of 120/240V 3-wire secondary transformers may be served ø-ø or ø-neutral from the medium-voltage distribution system.

Sometimes 120/240V 3-wire service—say, for an isolated small office in a plant or warehouse—
is derived from 480V electrical systems can use the 1ø “dry-type” transformer. 208Y/120V 3ø service is most often used in smaller offices. On rare occasions, two or three 1ø 480-120/240V transformers may be used to serve localized 3- or 4-wire Δ loads. Here, 480Y/277V in a residence would surely give electrical inspectors an aneurysm.


[This message has been edited by Bjarney (edited 05-11-2002).]

Many thanks for all the info. I'm always interested in the origins and historical development of different systems.

I ran across your 4-w unbalanced delta here a few months ago in relation to the use of orange to identify the high-leg. I can see how it originated to provide 120 from a simple 240 delta, but it seems quite an odd system from a British perspective. Do you know when these systems were last installed as standard? I think somebody else said that even a regular 240V delta would be rare now and that all new commercial LV supplies are a simple 120/208 or 277/480 Y.

We have some isolated houses on a 1-ph xfmr fed from two legs of an 11kV delta, but the 2-wire residential service here is usually tapped off a 240/415 Y distribution network, the same xfmr often feeding 3-ph power for commercial buildings, as you mentioned for Europe. The 240 nominal became our national standard by the early 1970s; prior to that it varied 200 to 250 in different areas.

Most of Europe standardized at 220/380, although they had some older 127/220 systems in some areas until quite recently. There is now a move toward a common European standard of 230/400.

The main difference in this respect between the U.K. and mainland Europe is that only a very large, power-hungry house would be fed 3-phase here, but on the Continent 3-ph is very common, although often of low power rating (e.g. a lot of older homes in France are fused 20A per phase). Many electric ranges in Europe are 3-ph 5-wire.

I'm not aware of any 1-ph 3-wire AC systems ever being used over here, although 3-wire DC (200/400 to 250/500V) survived in the older parts of some towns until the 1960s.

The "cut-off" point for safety in homes here seems to be regarded as 250V. That was the highest residential utilization voltage under the old 250/440 Y or 250/500 DC systems, and still figures quite prominently in many parts of our code (e.g. 250V max. on regular lampholders).

On the very rare occasions where 3-ph is introduced to a domestic system, the regs. specify quite a number of rules to minimize the risk, such as the requirement that all receptacles in one room be on the same phase.

The use of a greater number of smaller xfmrs in a typical American neighborhood is one of the things I noticed the first time I was over there. Interesting that the primaries on some of these are fed phase-to-neutral; I've not been involved with the HV distribution side of things here, but I'm not aware of any such HV feeds in England.

The 3ph delta with a center tap between a&b we call the wild leg sys its still in use I've seen it in florida and here in indiana many of times and boy if you dont check your panel voltages it can dammage alot of equiment.

Hello & Welcome!

Are these systems still being installed in new commercial buildings in Indiana, or are we just talking about old services still in use?

I think it was Scott (Electure) some time ago who reckoned that in his area (so. Cal.) all new services in the past 30 years or so were straight 120/208 or 277/480 Y.

I havent seen any utility install any but I have seen the power co hook up a new service to an existing one that was already there. even after my protest

Is the possibility of accidentally connecting to 208 thinking that it's 120 the main problem with the 4-w delta?

Are there any other drawbacks that you come across regularly with this system?

Hi Paul, Welcome back!!!

In regards to the Delta thingee, the major concern was as you stated - some one mistakingly setting up an L-N circuit between the high voltage to ground line and the grounded center tap [AKA "Neutral"], with the intentions of driving a 120 VAC load.

This will result in two things:

<OL TYPE=1>

[*] 208 VAC impressed upon the load,
and

[*] Smoke leaving said load.
</OL>
<Just wanted to add some humor! >

Anyhow, there are still many 4 wire Deltas in our area [I live near Scott "Electure"]. They can mostly be found in the older Commercial / Industrial areas, along with the areas zoned with Residential and Commercial / Industrial [again, older areas]. These systems are both flavors of Open and Closed configurations.

Funny thing is that the new company I work for, very few people have heard of - let alone worked on - a 4 wire Delta. I get some weird looks when mentioning them! Even weirder when I show the results of the "Orange Line To Ground" on my Wiggy.
P.S. a "Wiggy" is a Solenoid / Neon lamp type voltage tester which is usable from 120 to 600 Volts - AC or DC. They are the most common volt meters in the field. They have low accuracy [208 falls above the "240 VAC" markings!] and have a low input Impedance - which makes them useful as GFCI testers and when you run across an ungrounded Delta!

Lastly, my thinking on the 310 VAC [+/-] was that the tester is "Bridging" between two coils [one of 277 VAC potential, one of 120 VAC potential]- creating an Autotransformer / Reactor setup, similar to an HID Ballast.

I have seen readings like this. First was by use of a Wiggy, just to see what the voltage between Lines of 2 different panels [one 480Y277, the other 208Y120], the other[s] were with a DVM. I shunted the leads with a 22K Resistor to lower the input Z.

Felt nice to ramble again! Been really BZ [busy].

I'll be flying to San Fransisco on Monday, to do some surveys - then back to So Ca for a few days of work and colation, then back on Da Plane! again.

Scott S.E.T.

P.S.S. Looks like maybe next thursday our E-Mail address will change.
We are shooting for "stat113" at pac bell dot net. That will be confirmed later...

Scott

Safe to say that in most parts of the US, the only likelihood of being able to even request 4W&#8710; would be for small loads in older overhead areas.

Con Ed [the electric utility in New York City] lists 2-phase and DC available {for older elevators} for as long as their serving equipment lasts, and then you gotta update. [Imagine somewhere there is an apartment building with 90-year-old tenants that has a motor-generator set in its basement…that probably won't restart after the next power outage.]



[This message has been edited by Bjarney (edited 05-18-2002).]

Hey Scott,

Long time no hear!

On the 4-w deltas, didn't somebody here come up with the idea of painting the phase B blanking plates on the panel orange as a warning? Can't remember who it was, but I guess it could save the next person to work on that panel a lot of trouble.

It could be worse I suppose. Just imagine a (non-technical) U.S./Canadian tourist coming over here, buying a simple plig adapter and hooking up to our 240V. Even more smoke than on 208....

Off-topic....
"BZ" reminds me of the people here who look at U.S. software with something like "EZ Install" on it and ask what the "ee zed" means. Cue explanation that in America it's pronounced "ee-zee."

BJ,
Does Con-Ed still actually distribute DC or 2-phase, or do they just own/maintain conversion equipment at the customer's premises to convert? I assume we're talking about real 2-ph (90 deg.) here and not just two legs of normal 3-ph.

paul-- There used to be a link to an older {but still in service} New York turnstile bridge that used two 2-phase motors, but the link seems to have dried up.
http://www.coned.com/documents/elec/018-024.pdf See: §III. 2. (D and (E) for DC and 2-phase servce.
http://www.powerlineman.com/pictures/2001/march/g_good/good.htm Poster refers to it as “scotts system.” It looks like it is a true utilty-owned Scott-Tee transformation. A rare bird I’m sure.

Sometimes referred to incorrectly as 2-phase—a 3-wire 208Y/120-volt utility-furnished service tapped from two phases and a neutral is correctly called “Network Service” in the US.

Lastly, I have no idea why, but Table 430-149 in the 1999 NEC deals exclusively with 2-phase motors. I once mentioned in an IAEI chapter code-change session that it may be of limited use and possibly time to request its deletion, but the suggestion was not on the agenda so maybe I should use proper parliamentary decorum; wait 3 years and resubmit it for the next code cycle. [Unbelievably but sadly, I lost interest.]

Even Farther Off Topic...

www.ci.nyc.ny.us/html/dot/html/bridges/bridges/macombs.html mentioned being powered by two 2-phase motors, but link is dead.

www.nycroads.com/crossings/macombs-dam/ doesn't talk about motors.

Looks like many 2-phase motors were 4-wire, nominally 220-240 volts across opposite phases {for 90° displacement.} Probably usually grounded at the neutral point for “5-wire”...but there were likely ungrounded versions, too. Can you imagine floating pairs of voltages to ground, and maybe even pairs of connections floating with respect to adjacent terminals, too?

Musta' been cool! <What this has to do with 2-gang boxes and 310 volts I'll never know.>








[This message has been edited by Bjarney (edited 05-20-2002).]

Interesting about the bridges even if nothing whatever to do with power! One of the links reports "being revised," so I'll take another look sometime soon.

Re the floating adjacent terminals, just imagine if high-impedance DVMs had been around when these systems were being installed. That could have provided hours of amusement for someone trying to figure out what was going on!

Is there any particular reason for the "network service" designation? It seems a rather vague term to me, although I suppose it's shorter than "two phases from a three-phase service."

Another totally unrelated query, but as we're talking about Con Edison, do they still distribute their waste steam under NYC's streets for heating?


[This message has been edited by pauluk (edited 05-20-2002).]

Paul— About the term “network service,’ in the US, ANSI C84.1 60Hz-voltage ratings states in the footer notes: {Referring to 208Y/120-volt 3ø 4w} “A modification of this three-phase four-wire system is available as a 120/208Y-volt service for single phase, three-wire, open-wye applications.” I suppose it’s somewhat of a slang term by utilty meterguys and line crews, because it’s derived from a (potentially quite large) 4-wire network.

From one US electric utility’s electric rule book: “In locations where PG&E maintains a {4-wire} 120/208 volt secondary system, 3-wire single-phase service normally shall be limited to that which can be supplied by a main switch or service entrance rating of 200 amperes.” From: pge.com/customer_services/business/tariffs/doc/ER2.doc

An example might be a metropolitan area surrounded by multi-story office buildings, with a little old cigar store and its small electric service; i.e., 60 amperes, with only lighting loads so 208 V (versus 240) is not of concern.


[This message has been edited by Bjarney (edited 05-23-2002).]

There is at least one utility in Wisconsin that still offers 120/240V 3phase 4wire new services. We also can get 240V 3phase 3wire delta, if we want to.

Paul, you asked about other drawbacks on these wild-leg systems.

1) Utilities always have the high (orange) leg as phase C, while the NEC requires the facility to have the high leg as phase B. This means that the wiring is different on the load and line side of the service connection point.

2) Most of our 2 pole breakers are listed for a maximum voltage to ground of 120V, and so can fail violently if connected to the high leg. Non-standard 240V to ground breakers have to be used.

3) Load balancing can be a concern. Most (but not all) transformer banks are not sized to provide single phase (240V) connection with one connection being the high leg.

Thanks for all the info. I've just been trying to get an idea of how widely these "unusual" systems have been used and for what types of buildings.

I think somebody mentioned the high-leg phase B/C conflict before. Are we just talking about a matter of terminology, or is it more practical than that? For example, three panel lugs in a row, does the NEC actually require the high-leg to be on the middle lug?

Somewhat related question: How common is a corner-grounded delta system?


[This message has been edited by pauluk (edited 05-24-2002).]

JBD,
It is correct that the utility metering equipment requires that the high phase be landed on the "C" phase position at the meter, but it still is "B" phase as far as the rotation sequence.

Phase rotation from the utility is meaningless, except for parallel systems. The exact phase identification at the service entrance is unique to each service. Of course once the phase rotation has been established for a service it need to be maintained throughout the facility

Utilities alway identify the C phase as the high phase and the NEC requires the middle conductor (B phase)to be the high leg and colored orange (99NEC 384-3e & f)

[This message has been edited by JBD (edited 05-24-2002).]

Utilities in this area always maintain the ABC phase rotation on the service. The high leg is B pahse in the rotation and landed on the C phase position in the meter.
Also the code in 408.3(E) and 430.97(B)require that the phase rotation be ABC top to bottom or left to right.
Don(resqcapt19)

Hey Paul,

Thanks for the reply!

Not too sure who made that post, but it's a good idea. I did a similar thing by placing Orange tape across the "B" position filler plates and / or K.O.'s on the dead front. Also tagged or "Highlighted" the "B" position[s] circuit spots Orange when printing out Panel Schedules and Panel Directories.

<Quote>
It could be worse I suppose. Just imagine a (non-technical) U.S./Canadian tourist coming over here, buying a simple plig
adapter and hooking up to our 240V. Even more smoke than on 208...
<Quote>

LOL!!! That's called "Crash Course In European Electrical Systems 101"

EZ-Install, huh? That's an Oxy-Moron <joke>

To me, all Software Hacked [Engineered] since 1996 is EZ-Install! Remember when everything had to be done in DOS [or X Tree Gold if you were a real Computer Geek!], and the normal situation was MD [or MKDIR], then XCOPY the Floppy[s] from A: to C:
[Syntax example: XCOPY A:\*.* C:\DOOM\ /s /a /e ].

Bjarney,

When I was younger [we'll not go there just yet ] I had seen an older area of Long Beach / San Pedro where one customer still had 2 phase 4 wire service.
The setup was from a 3 Phase Delta to 2 Phase 4 wire configuration.
Very old Pots!!! This was the last one in the area, and I personally haven't seen anymore since then.

Those 120/208 1 Phase 3 wire services are quite abundant in the San Diego area!
When I first ran across one, I thought someone messed up pulling subfeeders! There was 208 VAC between the Ungrounded Lines [with 120 VAC to Ground], but only 3 wires at the Subpanel! Was expecting 240 VAC L-L, so went out to the Utility pole to investigate. Saw a typical 4 wire Wye configuration with only 3 wires dropped to the Building's service [this was a Commercial Location].
Looked at the Main Gear [Service / Meter Section] and it was clearly labeled and tagged "120/208 1 phase 3 wire, 400 Amps Max"

My first visit with that type of system!

I guess it could be figured as an Open Wye since that describes it well, but [to me] looking at a Pictorial of an Open Wye just looks like an Open Delta which has been turned around!

Now for some trivial pursute stuff

I have drawn the Scott T setups, plus the Delta / 2 Phase setups and posted in the Tech Reference section, along with a few DC 3 and 5 wire setups. For anyone interested in these, feel free to check them out Be sure to view ALL Posts, otherwise you will never find them!

Scott S.E.T.

P.S. In Regards to the Utility Co. landing Line C as the High Leg, I was thinking that this was done in the Meter section just to keep it "Away" from the "Usable" Lines, which run through the KWH Meter.

If you notice the KWH Meter used on the 4 wire Delta, it is a Single Stator Meter - typical of the common 1 Phase 3 Wire setups used in Residential Metering for 200 Amps and lower, or the ones driven by CT's in applications of 400 amps and higher.

Just my thoughts - not necessarily the method to the madness!

Scott S.E.T.

[If anyone can find out the real reason this is done, please post your findings here!]

Well, we've gone quite a way off the original topic of this thread, but thanks for the background info.

Scott,
I still use 20-year-old software on an old XT for accounts, word-processing etc. I don't like "EZ-Install" modern stuff that doesn't let you make your own installation choices. But then I grew up using an ASR33 teletype linked to a PDP-11!

"Bø or Cø Meter Stinger/Red/Orange/Purple/Power/Wild/Bastard/High Leg?" is a new thread now.


[This message has been edited by Bjarney (edited 05-25-2002).]

Scott35/pauluk-- A self-contained layout is covered at: themeterguy.com Theory/Four%20Wire%20Delta.htm

See: "The Form 15 is a 2½ element meter..."