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Jeridiah
 
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Default Three phase question

I am building a new shop and have gotten the power company to commit
to bringing in three phase power(for no cost).

They will be hanging three transformers(2 additional, but three new
ones).
What does this mean to me when I go about getting things wired up? Do
I still have the infamous "wild" leg, or do I even care if I pull all
of my single phase loads off the 2 other transformers?

They will give me 400A three phase service if I want it. At first I
said yes, but now am starting to wonder if it isn't massive overkill.
I don't have anything now that requires a ton of power, other than the
welder which at max current is 100A primary side.

Opinions?

Jeridiah
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Jeridiah
 
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Default Three phase question

Jon Elson wrote in message ...
Peter H. wrote:

They will be hanging three transformers (2 additional, but three new ones).



Then, you'll have *true* three phase.



What does this mean to me when I go about getting things wired up? Do I still
have the infamous "wild" leg, or do I even care if I pull all of my single
phase loads off the 2 other transformers?



No "high leg" problem here.

Do tell the utility if you want 120/240 volt Delta or 120/208 volt Wye, though.


Wait a minute! If he has a 240 V Delta system, there will be NO 120
volt power
available anywhere! There will be 3 wires, with 240 V between any
combination
of them. No neutral will be supplied on the delta system.

The former is good for those cases where you have just lighting on 120 as only
one of the three transformers can be used for 120/240 single-phase.


Unless you have a lot of 240 V single phase loads, or motors that must have
240, and cannot be run on 208 V, you would do best, in most cases, with a
120/208 V Wye system.

If the power company will only install 240 V delta power, you will then
need a
240:120 V step-down transformer. (Since they are going to put in
3-phase FREE,
you may do very well to get a used dry-type stepdown transformer to provide
your 120 V supply for lighting, computers, hand tools, etc. Figure out your
worst-case 120 V load, and size the transformer a little above that. If you
can't find one big one, then get 2 or 3 smaller ones, and you can have
several
separate 120 V feeds. Smaller stepdown transformers in the 2-5 KVA range
can often be had from scrapyards for the scrap metal value.

Jon


Perhaps I need to clarify....

They are putting in a 240V Delta xfrmr, with 2 piggy backed off from
it to provide the single phase service.

My understanding is this - all power will go through the 3 ph xfrmr.
The single phase will be tapped off from the 2 piggy backed xfrmrs.
Can't quite figure out why there are 2 "secondary" xfrmrs. Shouldn't
it be a single 240 single phase and just center tap it to give me my
120 service? Or is it a balancing issue?

As for the main panel... Talking to a electrical supplier, their
recommendation for service cable for 400A was to use dual 3/0. Can I
split this to 2 panels and be legal as far as code? Reason for doing
this would be that I can get 200A 3 ph panels for free. Not sure
about a 400A. I will have to check with my source on that.

Thanks

Jeridiah
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Mike G
 
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Default Three phase question

Hi All: I have three phase open delta which consists of two transformers,
one of which is the normal single phast type 120/240 and one which is used
for the third leg which is 208 ... This is a good one because you have 240
single phase where you need it instead of 208 and you also have three legs
to power your three phase loads... Mike
"Jeridiah" wrote in message
m...
I am building a new shop and have gotten the power company to commit
to bringing in three phase power(for no cost).

They will be hanging three transformers(2 additional, but three new
ones).
What does this mean to me when I go about getting things wired up? Do
I still have the infamous "wild" leg, or do I even care if I pull all
of my single phase loads off the 2 other transformers?

They will give me 400A three phase service if I want it. At first I
said yes, but now am starting to wonder if it isn't massive overkill.
I don't have anything now that requires a ton of power, other than the
welder which at max current is 100A primary side.

Opinions?

Jeridiah



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Gary Coffman
 
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Default Three phase question

On 28 Aug 2003 14:03:50 -0700, (Jeridiah) wrote:
Perhaps I need to clarify....

They are putting in a 240V Delta xfrmr, with 2 piggy backed off from
it to provide the single phase service.

My understanding is this - all power will go through the 3 ph xfrmr.
The single phase will be tapped off from the 2 piggy backed xfrmrs.
Can't quite figure out why there are 2 "secondary" xfrmrs. Shouldn't
it be a single 240 single phase and just center tap it to give me my
120 service? Or is it a balancing issue?


Exactly. The two piggy back transformers are providing a "derived
neutral". The only currents flowing through these transformers is
neutral return currents for 1 ph 120 volt loads. These transformers
will probably be smaller than the 3 ph delta transformer. This is
just a way of providing a center tap on one of the 3 ph windings
when the main transformer doesn't have one.

As for the main panel... Talking to a electrical supplier, their
recommendation for service cable for 400A was to use dual 3/0. Can I
split this to 2 panels and be legal as far as code? Reason for doing
this would be that I can get 200A 3 ph panels for free. Not sure
about a 400A. I will have to check with my source on that.


You'll have to check with an electrician familiar with your local Code,
but the rule today for residential installations in this area is that there
must be an externally accessable positive disconnect for power
entering the premises. The purpose of this is so the fire department
can make the premises cold without having to wait for the power
company to show up.

This disconnect can be a switch rather than a breaker. That'll have to be
400 amp. But you can then split the feed in that switch box and feed two
separate 200 amp panels via metallic conduit. Each of those boxes must
have its own main breakers, but they only have to be 200 amp. The idea
here is that you're only extending and splitting the service drop before it
reaches the breaker boxes.

Gary
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Don Murray
 
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Default Three phase question


Perhaps I need to clarify....

They are putting in a 240V Delta xfrmr, with 2 piggy backed off from
it to provide the single phase service.



Jeridiah,

I suspect what you will be getting is 3 single phase transformers wired
into a 3-phase delta bank. All your single phase load will come off of
one transformer and the other two will only be used for your 3-phase
load. And you will have a high leg. Take a look at the transformer
diagrams on my web page. If you scroll down the page the first one you
come to is a 240V delta secondary like you will be getting.

http://murrayranch.com/Electricity.htm

An open delta bank like Mike says he has will have the same secondary
voltages but an open delta is only good for 86.6% of the rating of the 2
units making up the 3-phase bank. They do make 3-phase transformers in
both delta and wye secondaries with three and four bushing secondaries.
If anyone would like to see wiring diagrams of them, I can post them on
my web page. It doesn't make sense for a utility to hang a 3-phase
transformer and 2 more to get single phase, kinda defeats the purpose of
a 3-phase transformer; having it all in one can.

Some questions you might want to ask the utility; what your rate
structure will be, commercial or residential, is it going to be a demand
meter, if you go 400A are you going to have to get a panel that will
take current coils and a test block?

Don


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Gary Coffman
 
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Default Three phase question

On Fri, 29 Aug 2003 05:55:17 GMT, Don Murray wrote:
It doesn't make sense for a utility to hang a 3-phase
transformer and 2 more to get single phase, kinda defeats the purpose of
a 3-phase transformer; having it all in one can.


It makes sense if that's what they have lying in the yard at the time of the
install. Utilities will often do odd things to use up what they have on hand
rather than having to buy a transformer. Using two small 1 ph transformers
to produce a derived neutral is one of those odd things.

Gary
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Peter H.
 
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Default Three phase question



The main disconnect must be external to the building, and plainly marked. As I
said, it is so the fire department can make the building cold without waiting
for the power company to show up and pull the pole fuses.


Of course, and here in CA it is not uncommon to find six disconnects on the
exterior of a multiple-use building, even though some of the units may not be
"continuously occupied".

We don't use pole fuses here, except on the line side of a large customer fed
from the utility's subtransmisson system.

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