seen a lot of different service voltages supplied to buildings. First I
am not an Electrical Engineer although I have always been the go to guy
on theory questions that the trainees and apprentices come up with. So I
'll just go ahead and admit that I don't understand how you could get a s
ix phase output out of a three phase input. If it is a delta secondary i
t can only be corner or center of one phase grounded as the code specific
ally requires those as the only place the bonding jumper can be connected
. I have worked in manufacturing plants were the delta was ungrounded
but elaborate measures were taken to insure that any ground fault was det
ected well before a second such fault on a different phase had any chance
to occur. You simply cannot connect any delta system to ground on two s
eparate phases or you have a dead fault. I have yet to see any supply sy
stem that allows me to take half phase loads from more than one phase.
You can have 3 single phase transformers with center tapped secondaries
connected on the primary to the HV side (delta preferred) and secondary
center taps connected to ground so that there would be 120V on each
pair of adjacent legs but 120/240 between opposite legs. This idea has
been used for many years for rectifier supplies
Technically speaking it is a 6 phase star (the center tapped single
phase setup (edison) can be considered as a 2 phase star (although it is
called this only in countries that don't use it.
The idea of doing this fordistributioncan have advantages- at higher
distribution voltages as the clearances can be reduced.
In some areas this is worth while- chopping big holes through heritage
trees is not a good idea. .
in a star configuration that they would have to be supplied from six pha
se distribution or two of each of the secondary windings would have same
primary winding exciting them and would therefore be functionally paralle
l with no phase difference or voltage between them.
20 center tapped phase delta with the resultant high leg on one phase wou
ld do what is needed. Either would give you Three phased power for eleva
tor motors, air conditioning chiller or whatever the bigger Three phased
load is.
se, I have yet to see a situation were that is actually needed. Any Thre
e phase heat pump, air conditioner, or other motor load that will run on
240 volt three phase that I have had to install could be re-tapped to run
on 208 Three phase or the motors were labelled as 240,208 Three phase th
at would run on either without modification. Even single phase home appl
iances will run on either in most cases. Why then would one need to have
actual 240 volts in single or Three phase all the way to the individual
unit. I have never actually seen Three phase power run to the individual
units of an apartment building.
s small apartment building; although no one has explained why that might
be true; then you will need either a single phase supply or a separate br
anch or feeder to each unit for the 240 volts. I have seen existing buil
dings with Fourteen units with single phase supply but I have not seen an
y new construction wired that way in the last several decades.
I live in an area where air conditioning loads are light and the supply
to the transformers on the street is 7200V line to neutral- only one
feed from the nearby 3 phase line is taken down this street- hence
120/240 single phase supply to each home.
There are rural lines that are single phase. 13-15 KV line to neutral,
ground return (actually a grounded neutral wire is also used) is
common. You are right in that generally new construction will have
120/208 in the units- as the load density is sufficient to warrant it.
40 volt Three phase then the designer should compare the cost of installa
tion and operation of either two separate services to supply different vo
ltages of Three phase or instal a boost transformer off of the 208 volt T
hree phase to derive the needed 240 volt Three phase. Keep in mind that
most utilities charge additional monthly fees for a second service to the
same premise even if it is only in the form of a fee for reading a secon
d meter each month.
In the case of a larger load- I see no need (technical or economic(
for a special 240 3 phase supply
that is in the regulatory tariffs maintained by the state regulatory body
. Since the derived system transformers that would produce the 120/240
volts from the 480/277 are not part of the distribution network they are
beyond the utilities power to control. Like anything else on the load s
ide of the demarcation point it is the exclusive province of the Authorit
y Having Jurisdiction (AHJ) over the enforcement of the locally adopted e
lectrical code. The only exceptions I have encountered in almost Fifty y
ears is with publicly owned utilities because some of those do the electr
ical code enforcement.
- posted
7 years ago
