ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Shout Box
Recent Posts
Burning out of return(white) side of 120volt rec
by crselectric. 06/14/19 02:15 AM
Video door bell causing chime activation of coil
by JoeTestingEngr. 06/13/19 12:43 PM
Troubleshooting Voltage Drop Across Relay
by gfretwell. 06/09/19 11:49 AM
Apple Recalls Three-Prong Wall Plug Adapters
by Admin. 06/05/19 09:06 PM
New in the Gallery:
What is this for?
Plug terminals
Who's Online Now
0 registered members (), 12 guests, and 20 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Commercial service #3251
08/14/01 06:03 AM
08/14/01 06:03 AM
pauluk  Offline OP
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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?


Work Gear for Electricians and the Trades
Re: Commercial service #3252
08/14/01 06:50 AM
08/14/01 06:50 AM
sparky  Offline
Joined: Oct 2000
Posts: 5,372
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.

[Linked Image]

Re: Commercial service #3253
08/14/01 07:55 AM
08/14/01 07:55 AM
electure  Offline

Joined: Dec 2000
Posts: 4,280
Fullerton, CA USA
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.

Re: Commercial service #3254
08/14/01 08:34 AM
08/14/01 08:34 AM
Bill Addiss  Offline
Joined: Oct 2000
Posts: 3,905

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.


Re: Commercial service #3255
08/14/01 12:41 PM
08/14/01 12:41 PM
pauluk  Offline OP
Joined: Aug 2001
Posts: 7,520
Norfolk, England
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.


2017 Master Electrician Exam Preparation Combos
2017 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Member Spotlight
Washington...Not DC
Posts: 233
Joined: March 2005
Show All Member Profiles 
Top Posters(30 Days)
Khombo 2
Popular Topics(Views)
255,737 Are you busy
191,702 Re: Forum
181,482 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.6.1
(Release build 20180101)
Page Time: 0.029s Queries: 14 (0.006s) Memory: 0.9698 MB (Peak: 1.1039 MB) Zlib enabled. Server Time: 2019-06-18 12:58:45 UTC