Hello Tom, glad to see you again!
As to your Q', I'll address this in a few parts below.
In my area the pole transformers in an average neighborhood range from 25kva to (i think) 67 or 69 kva. Alot of times there are 6-8 houses tied to each.
I have seen PoCo owned Pole Mounted 1Ø 3 Wire Transformers as low as 10 KVA, and as high as 75 KVA.
The %Z for these (I believe) is between 2% and 5%.
The Primary fuses are typically 600% FLA
On Pad Mount Transformers for Commercial Customers' use, one PoCo in my area considers the Transformer calculated continuous load to be 130% maximum. Higher than this warrants a Transformer KVA size upgrade.
Unsure where the maximum LCL load for Pole Mounted 1Ø Pots becomes crucial for the same PoCo.
The question has to do with starting current for Air Conditioners, and the percent line drop during starting.
Last Residential project I designed + installed Electrical Systems was an addition to existing 1800 Ft² House - resulting in >3000 Ft² Conditioned space.
With this came (of course) the installation of a new larger capacity Service + Underground Feeders.
PoCo's Design Engineer used my load calcs and specs for HVAC equipment, and applied this data to the existing 25 KVA 2.75% Z Pole Mounted Transformer.
Results were a "Flicker Factor" of 4.5 maximum.
Try entering "Flicker Factor" into a search engine for more information.
I realize that these transformers are suppose to be "infinite Buss" but there are limitations especially if someone has a good size A/C unit on one of these.
Actually, they are Infinite Bus when applying a Fault Current Calculation for worst-case scenario (like when performing a "Point-to-Point" SCA calc).
PoCo's Flicker Factor calcs do not consider Primaries as Infinte Bus (AFAIK).
At Hef's Playboy Mansion West (Bel Aire, CA.), the Primary Feeders are so overloaded that when Hef' throws one of those infamous parties, the Primary Feeder's OCPD trips several times per evening. They now run on Generator power to avoid the resulting power loss.
I was just wondering if anyone has run into this problem and what was the solution. I have read of cases where some starting would cause a line drop as large as 23 % of the available line voltage although momentary it still can be an annoyance to the other customers with blinking lights or motors taking a hit during the start up of these large units.
Have run into this problem on a few occasions.
Soft Start methods / equipment are the most common fix on the user's side.
If the problem effects multiple users - and there are not "Huge" unnecessarily large loads being used by the Customers, then the PoCo might consider upgrading the Transformer's KVA size.
If the PoCo can keep the Transformer size low, the problem of frying Service Feeders and excessive SCA can be reduced.
Hope this is helpful information.
Scott35
