480V 3 phase to 120/240V single phase Xfmr

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smarteng

Member
Location
Los Angeles
Is 480V 3 phase to 120/240V single xfmr configuration available.

Does utility provide 1 ph, 2 wire, 480V, 400 amp service normally or needs special circumstances.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
smarteng said:
Is 480V 3 phase to 120/240V single xfmr configuration available.

Does utility provide 1 ph, 2 wire, 480V, 400 amp service normally or needs special circumstances.
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You cannot produce single phase from 3 phase. What you can do is put a single phase transformer on 2 lines of the 3 phase source and that is very common.

I have heard talk that such a service might exist, but how common it is, or whether it is currently available is something else.

If you need 120/240V single phase, why not just get a 120/240 V single phase service?
 
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smarteng

Member
Location
Los Angeles
petersonra said:
You cannot produce single phase from 3 phase. What you can do is put a single phase transformer on 2 lines of the 3 phase source and that is very common.

I have heard talk that such a service might exist, but how common it is, or whether it is currently available is something else.

If you need 120/240V single phase, why not just get a 120/240 V single phase service?
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Thank you so much.
 
augie47

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I have inspected jobs where the design was for 480v due to voltage drop and the utility provided 480 single phase by request and/or utility restrictions.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Is this for a huge residence that essentially needs a 1600A 120/240V single phase service?

Much of this will depend upon local POCO policy. The utility at my father in law's house simply put a pad mount near his house for a 400A service. No special hoops; it was just the utility standard for a long driveway in a rural area. In other locations, the utility provides 120/240V at the road, and it is up to you to figure out how to get it to the house with acceptable voltage drop. Some utilities will provide 480V single phase, others will provide MV service but you are on the hook for the MV wiring and transformer losses.

So the very first thing to do is to go to the utility and ask what they provide.

Next: do you specifically need _single phase_ service, or could you actually use 3 phase? If you have a lot of small single phase loads, they can be distributed across all 3 phases. If you really want 240V rather than the 208/120V three phase, you can do that with non-standard transformer setups (eg. three separate 120/240V transformers to provide 3 120/240V sub-systems)

-Jon
 
S

smarteng

Member
Location
Los Angeles
winnie said:
Is this for a huge residence that essentially needs a 1600A 120/240V single phase service?

Much of this will depend upon local POCO policy. The utility at my father in law's house simply put a pad mount near his house for a 400A service. No special hoops; it was just the utility standard for a long driveway in a rural area. In other locations, the utility provides 120/240V at the road, and it is up to you to figure out how to get it to the house with acceptable voltage drop. Some utilities will provide 480V single phase, others will provide MV service but you are on the hook for the MV wiring and transformer losses.

So the very first thing to do is to go to the utility and ask what they provide.

Next: do you specifically need _single phase_ service, or could you actually use 3 phase? If you have a lot of small single phase loads, they can be distributed across all 3 phases. If you really want 240V rather than the 208/120V three phase, you can do that with non-standard transformer setups (eg. three separate 120/240V transformers to provide 3 120/240V sub-systems)

-Jon
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Hi Winnie, thank you for the response. Yes its a fairly large building. The total load is approximately 500amp@240V / 1000 amp@120V. Mostly load is 1 phase 120v, except the AC unit, which is 240V. I am getting enormous cable like like 4 parallel of 500MCM if I use 240V. So clients wants to have a step down xfmr near building and a run of 480V. And one more thing, all panels in the building are 120/240 so 3 phase 120/208 is not really an option ( is it an option? ) that being said we need appx 250 amp@480V and the next standard service size is 400amp.
Avoiding MV since client does not really have qualified people to maintain.
 
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wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
If this is LADWP, have you checked their service guide on what types of services they offer? The first google hit (not sure if it's the latest):


Or if it's another POCO, they should have a service guide as well. PG&E calls it the Green Book.

smarteng said:
Mostly load is 1 phase 120v, except the AC unit, which is 240V. . . And one more thing, all panels in the building are 120/240 so 3 phase 120/208 is not really an option ( is it an option? )
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You can certainly use a 120/240V panel for 3-wire 120/208V, where you run two of the 3 phases plus the neutral, and if all the panel loads are 120V, they won't know the difference. [Although the neutral become a CCC, so you would need to double check if that would matter. E.g. 3 such feeders in a conduit becomes 9 CCCs instead of 6 CCCs.] So with a 3-phase service you could divide the 120/240V panels among the 3 different pairs of phases to balance the 120V loads.

Cheers, Wayne
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I was about to complain that you shouldn't say 500A/1000A; most people would call this a 500A 120/240V service. Yes, it could run 1000A of 120V loads...but only 500A worth on each side. But I made the same mistake going from the 400A 480V number :)

3 phase 208/120 can be used for a residence. It is common in large apartment buildings to feed individual units with 2 out of 3 legs, giving 120V L-N and 208V L-L. The big problem is that large appliances only get 208V and some consideration needs to be made for this. Also the neutral counting as a CCC that Wayne mentioned.

If you can get your service at 480V 3 phase, then you are talking 150A/phase with correspondingly smaller conductors.


-Jon
 
S

smarteng

Member
Location
Los Angeles
wwhitney said:
If this is LADWP, have you checked their service guide on what types of services they offer? The first google hit (not sure if it's the latest):


Or if it's another POCO, they should have a service guide as well. PG&E calls it the Green Book.


You can certainly use a 120/240V panel for 3-wire 120/208V, where you run two of the 3 phases plus the neutral, and if all the panel loads are 120V, they won't know the difference. [Although the neutral become a CCC, so you would need to double check if that would matter. E.g. 3 such feeders in a conduit becomes 9 CCCs instead of 6 CCCs.] So with a 3-phase service you could divide the 120/240V panels among the 3 different pairs of phases to balance the 120V loads.

Cheers, Wayne
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So if I install a 3 phase 120/208 panel, and feed all 1 phase, 120/240V panels (as subpanels) from there. It should work. I will see 208V in between 2 poles and 120V on all single poles on the subpanels. So as long as AC is rated for 208 V 1 phase we are good. Is it common to draw 120/240 1 phase from 208V 3-phase ?
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
smarteng said:
So if I install a 3 phase 120/208 panel, and feed all 1 phase, 120/240V panels (as subpanels) from there. It should work. I will see 208V in between 2 poles and 120V on all single poles on the subpanels. So as long as AC is rated for 208 V 1 phase we are good. Is it common to draw 120/240 1 phase from 208V 3-phase ?
Click to expand...
you can't get 120/240 from 208 3 phase without a transformer and running 208 3 phase 1500 feet is a huge problem at the kind of load you mentioned
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
smarteng said:
So if I install a 3 phase 120/208 panel, and feed all 1 phase, 120/240V panels (as subpanels) from there. It should work. I will see 208V in between 2 poles and 120V on all single poles on the subpanels. So as long as AC is rated for 208 V 1 phase we are good. Is it common to draw 120/240 1 phase from 208V 3-phase ?
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Providing units with 120/208 1ph 3w is very common in apartment complexes.
 
M

MD Automation

Senior Member
Location
Maryland
Occupation
Engineer
As has been said, very common to send 120/208 single phase to apartments from a 208Y/120 3 phase service. In NYC, ConEd even does this to single family houses , sending 2 hots and a neutral from a 3 phase service that runs down the street.

"Dumb" resistive loads will simply take a bit longer to do their thing. Electric clothes dryers, water heaters, stoves, etc. They will all work fine, just takes a little bit longer to boil the water for your spaghetti.

You'd want to check nameplate info for proposed AC units, but again, this is a common service.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
smarteng said:
So if I install a 3 phase 120/208 panel, and feed all 1 phase, 120/240V panels (as subpanels) from there. It should work. I will see 208V in between 2 poles and 120V on all single poles on the subpanels. So as long as AC is rated for 208 V 1 phase we are good. Is it common to draw 120/240 1 phase from 208V 3-phase ?
Click to expand...

It is common to feed panels _rated_ for 120/240V as two legs from a 208/120V three phase system. But of course you are not getting true 240V, and the reality is that _most_ 120/240V rated panels are also rated for 120/208V.

I don't know if this is true for all 120/240V panels, so in the fine print of the panel label you will need to confirm that it is also rated for 208V.

In addition to the AC using 208V, also watch out for electric heated devices such as ovens, cooktops, dryers, etc. Pure resistive elements only put out 75% of their 240V rating when used at 208V.

If you really want both 3 phase in _and_ 120/240V, I can describe the transformer connection. But do you really want to use a non-standard transformer from some random dude on the internet. :)

-Jon
 
S

smarteng

Member
Location
Los Angeles
MD Automation said:
As has been said, very common to send 120/208 single phase to apartments from a 208Y/120 3 phase service. In NYC, ConEd even does this to single family houses , sending 2 hots and a neutral from a 3 phase service that runs down the street.

"Dumb" resistive loads will simply take a bit longer to do their thing. Electric clothes dryers, water heaters, stoves, etc. They will all work fine, just takes a little bit longer to boil the water for your spaghetti.

You'd want to check nameplate info for proposed AC units, but again, this is a common service.
Click to expand...
Thank you. I will check AC voltage range and see if they have option for different voltage setting ( 240 and 208V with different taps) . I will keep posted. Thank you so much for help.
 
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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
winnie said:
It is common to feed panels _rated_ for 120/240V as two legs from a 208/120V three phase system. But of course you are not getting true 240V, and the reality is that _most_ 120/240V rated panels are also rated for 120/208V.

I don't know if this is true for all 120/240V panels, so in the fine print of the panel label you will need to confirm that it is also rated for 208V.
Click to expand...
Panels are rated for Maximum voltage.
 
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