One 25 KVA transformer is mounted to the utility pole which serves several homes and a small machine shop. The machine shop is looking into converting from single phase power to three phase power. The three phase choices from the power company are 120/240 Delta and 120/208 Wye. 75 KVA total three phase should be enough for the machine shop and allow some future expansion. Are either of these choices capable of using the current transformer and just adding only two more to make up the other two phases. The prefered way would be to choose either the Delta or the Wye so that the power company cannot utilize this existing transformer as one for the three phase bank. Meaning that the machine shop would not have to share the full capacity of the three phase transformer bank with any of the homes. The homes would only share the current single phase transformer. The end result (if possible) would be a utility pole with either four single phase transformers on it or one single phase transformer and one three phase transformer.
I am suprised that the utility offered 208 volt service. They may be planning on installing a pole close to the shop with the transformers installed there. If so then either voltage is available. Why would you care if the homes shared the transformer capacity? the utility will provide what is necessary. If you get the 208 volts service what are you going to do about the 240 equipment you now have?
#130367 - 03/15/0610:41 PMRe: Three Phase Utility Transformers
Well, if they (PoCo) utilize the existing 25KVA Transformer for a new 3Ø setup, it could be used, in addition to 2 more 25KVA Transformers, for a 75KVA 208Y/120V 3Ø 4 Wire Wye system.
For a Delta (which is what used to be commonly setup for Residential / Commercial shared customer power), the 25KVA Transformer might become the "Kicker" Transformer on an Open Delta Vee 4 wire setup - with like a 75KVA Transformer used for the "Main" Transformer.
If a closed Delta, they may use the 25KVA on an "End", with an additional 25KVA on the "Other end" - and place maybe a 37.5 or 50KVA in the center.
Chances are they will setup a 4 wire Wye. They might do an Open Delta Vee (around here, this is only for special conditions). The Electrical Service / Design Manuals from SCE, PG&E, SDG&E, LADWP + Anaheim Water & Power describe that anything in the "Delta Flavors" for non-industrial / heavy commercial, or coming close to a residential / light commercial shared system, is not going to happen. They will only give 4 wire Wye systems - even though there will be 1Ø 3 Wire Residential customers using this system too.
There are only a few exceptions to this "rule", and they all cover replacements of existing systems.
So to sum things up, I would expect the PoCo to create a 4 wire Wye system, then connect everyone on the Residential side to this, give your shop a 4 wire Wye service drop + meter, and be done with everything.
They can use the existing 25KVA Transformer, and add two more, which will create a 75KVA 208Y/120V 3 Phase 4 Wire Wye system.
How much do they (the PoCo) want to do this upgrade?
Are there any poles close to your shop, which have a 3 Phase system currently setup?
Do your Load Calcs call for 75KVA or higher?
P.S. Unless the Houses which will share the system are very large - all electric, and each one has a home wood working shop that is used a lot, don't worry too much about them effecting the total capacity of the new Polyphase setup.
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
#130368 - 03/16/0612:58 AMRe: Three Phase Utility Transformers
I have already spoken with the Power Company about my choices. I can chose which three phase service I want whether it be the Delta (closed or open) or the Wye. There will not be any cost to me whatever I choose since the person in engineering that sets up everything is a friend and will make sure it costs nothing. The homes that are connected to the current transformer do not have electric heat.
#130370 - 03/16/0611:12 AMRe: Three Phase Utility Transformers
Just my opinion here, but I'd go with 240V delta myself....
Most of the machine shops I deal with use alot of 230V stuff, and the new machines are coming in 230V also. Whoever originally did the plant, used 208. The office loads are small, but the machine loads are great.. The 208 needs transformers everywhere, where the 240 could be used directly..
#130373 - 03/16/0609:19 PMRe: Three Phase Utility Transformers
I have already spoken with the Power Company about my choices. I can chose which three phase service I want whether it be the Delta (closed or open) or the Wye.
That changes everything! Go for the Delta system definitely. If the PoCo is willing to give you a Full Closed Delta, take that option.
This will be best for both yourself (having 230 Volts 3Ø available for Machinery), and the existing Residential Customers (they will not experience lower Electric Heating values for 230 Volt rated equipment - if the new 3 Phase system were to be a wye, then the Heating Value for them would be somewhat lower - possibly un noticed!)
There will not be any cost to me whatever I choose since the person in engineering that sets up everything is a friend and will make sure it costs nothing.
Choice of installed Transformer setup AND no cost for upgrade!!! How about rubbing some of that good luck my way!!!
Seriously though, it has been maybe 15 years since I last heard any Utility Design Engineers mention the phrase "No Cost To The Customer". Must be the fields and client base I deal with.
There are three phase lines on the pole with the exisiting transformer, so that is not a problem.
I was going more towards obtaining service from an existing 3 Phase bank, which would have been a lower cost (but cost is no issue on this upgrade)
An existing 240/120v 1ph x-former has a grounded, center-tapped 240v secondary, while a 208/120v 3ph x-former has a 120v secondary with one end grounded.
If you mean that the POCO can open the can and reconfigure the secondary, paralleling the two 120v windings, then yes, they can re-use the x-former, along with two more.
10-4, I was thinking of the commonly used Split Coil Secondary pole mounted Transformer - the ones with 3 Secondary Bushings visible on the front - of which the PoCo would reconfigure the Secondary jumpers from Center Tapped Series (240V), to Paralleled windings (120V).
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!