Why delta overhead

[Toros]

Hi
i like to know in los angeles/ calif
overhead high volt utility transmission line get step down to 4- wire delta that contains wild nasty leg
is it too costly for utility comp to modify that to 120/208 Y system??

what is reason they send delta to habitants of ...??
Can not they step the mega down to balance 3- phase through transformer sub stations located through out the city???

thank you

 

[augie47]

Not sure about there, but locally the 4 wire delta was common where the load consisted of a majority of single phase 240/120 with limited 3 phase and, yes, the reason was the utility could supply the load with a smaller 2nd transformer to satisfy the limited 3 phase load.

 

[mbrooke]

Hi
i like to know in los angeles/ calif
overhead high volt utility transmission line get step down to 4- wire delta that contains wild nasty leg
is it too costly for utility comp to modify that to 120/208 Y system??

what is reason they send delta to habitants of ...??
Can not they step the mega down to balance 3- phase through transformer sub stations located through out the city???

thank you

Delta is economical where 120/240 single phase is needed along with 240 volt 3 phase.

Because 9 times out of 10 the primary is delta in California the secondary can either be wye or delta with no issue. If there is only 1 3 phase motor in a building say an a/c or elevator motor in an apartment 3 phase power can be provided via only 2 cans "open delta".

 

[kwired]

If all three primary phases are available, it really isn't that much more difficult or difference in cost to build a wye secondary or a full delta secondary of similar kVA rating.

What is more cost effective is refining it to what is needed by the load. If you have a facility as others have mentioned that has only one three phase load such as an elevator or an AC unit, and everything else is single phase - it may be more cost effective to install an open delta transformer bank as the third "wild leg" will have limited load on it, but they will install a larger transformer for the 120/240 portion of the system that does carry the bulk of the load.

For an installation where there is mostly motors or other heavy 240 volt loads the full delta secondary may be preferred over the 208 volt wye secondary - you get a little more bang for your buck so to speak with the slightly higher voltage.

If your load is mostly 120 volt loads then the wye secondary very well may be preferred so you can balance that load across all three phases.