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#3251 08/14/01 05:03 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
Some questions for those of you familiar with commercial LV service. Please forgive my ignorance, but my studies of American wiring have been solely on residential 120/240V systems.

I know that 3-phase services are used at 120/208V and 277/480v, and I believe there is also a single-phase 3-wire 240/480V service.

What type of premises usually take each of these types of supplies? Are one or more types only found in certain areas and/or maintained for historical reasons but not installed new any more?

In business premises getting 240/480 or 277/480, how do they obtain 120V for normal office equipment etc.? Is there usually an on-site transformer, or does the utility provide a separate 120/240 or 120/208 supply?


#3252 08/14/01 05:50 AM
Joined: Oct 2000
Posts: 5,392
Hi Paul;
although there is a variety of configurations, the one probably seen used in industry the most is 3 ph 480/277Y, with a customer owned X-former to step down to 3 ph 208/120Y. A dual utility set-up could be arranged depending on the particular utility involved.

I would say that Wye systems are probably the norm, Delta's have had their hey-day, other posters may differ due to their area's utility.

there are some existing 'strange' ones, such as a corner grounded delta that are allowed to be left operating, but this is only for economical reasons. To my knowledge they do not rate any historical classifiction that justifies thier existence.

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#3253 08/14/01 06:55 AM
Joined: Dec 2000
Posts: 4,294
I've not seen a 240/480 1Ø-3W service.
Any services installed within the last 30 yrs or so in this area (So CA)are wyes as Sparky said. We do have some older services that are 120/240 3Ø-4W Delta, with the midpoint between 2 of the phases grounded to provide the 120. This leaves us with a "high leg" which measures 208 volts to ground. Also, some of these delta services are split up at the utility pole, and supplied as a 3Ø-3W 240V power service and a 120/240V 1Ø-3W lighting service. These are being phased out though, it's only a matter of time until they're gone.

#3254 08/14/01 07:34 AM
Joined: Oct 2000
Posts: 4,116
Likes: 4

What I see agrees with the above postings. I don't see too many "High Leg" Deltas, but most of the ones I see should be changed as the buildings' occupants have changed from industrial to commercial (sales) and no longer have much need for 3 phase equipment. They usually bypass the high leg altogether (except for occassional errors) which gives them a very unbalanced service, and doesn't let them use the full capacity.


#3255 08/14/01 11:41 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
Thanks for the info.

To the best of my knowledge we've never used corner-grounded delta or the mid-point delta with "high leg" for public supplies in the U.K.

Our LV commercial 3-phase is 4-wire Wye at 240/415V. Except for a few remote properties which may be fed from a single-phase 240V xfmr, all of our LV distribution uses 240/415V Y with homes tapped off one phase and neutral to get a 2-wire 240V service.

When did most of the old d.c. systems disappear in America?

We had 3-wire d.c. distribution in some towns into the 1950s. That was before standardization at 240V, so the supplies were anything between 200/400 and 250/500V, again with homes getting a 2-wire 200-250V service and heavy commercial getting 3-wire.

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