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#9495 - 04/30/02 05:46 PM Ok theorists, help me out
Steve Miller Offline
Member

Registered: 08/30/01
Posts: 322
Loc: Loudoun Cty, VA
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

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#9496 - 04/30/02 05:50 PM Re: Ok theorists, help me out
Steve Miller Offline
Member

Registered: 08/30/01
Posts: 322
Loc: Loudoun Cty, VA
Sorry people, that should have said a light switch and a duplex (not a light and a duplex)

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#9497 - 05/01/02 01:46 PM Re: Ok theorists, help me out
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
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).]

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#9498 - 05/01/02 08:00 PM Re: Ok theorists, help me out
Steve Miller Offline
Member

Registered: 08/30/01
Posts: 322
Loc: Loudoun Cty, VA
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.

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#9499 - 05/01/02 08:23 PM Re: Ok theorists, help me out
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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.

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#9500 - 05/02/02 12:45 AM Re: Ok theorists, help me out
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
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.

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#9501 - 05/06/02 08:08 PM Re: Ok theorists, help me out
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
(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.

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#9502 - 05/07/02 04:35 PM Re: Ok theorists, help me out
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
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).

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#9503 - 05/11/02 06:23 PM Re: Ok theorists, help me out
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
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).]

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#9504 - 05/12/02 01:43 PM Re: Ok theorists, help me out
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
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.

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