Convert 240/120 V Delta Service to 208/120 wye service

[hbendillo]

I got a call from an electrician and building owner who had already started work to convert an existing 240/120 volt, three-phase delta service to 120/208-volt wye. The power company wanted to get rid of the old transformers and service and provide a new pad mounted transformer for the building. Existing service uses three, pole mount type transformers mounted on timbers at ground level within a concrete block enclosure next to the building. The main panel for the building is mounted on the outside wall next to the enclosure. 1000 amp main breaker and sub breakers feeding panels within. Service runs along wall from main panel through the wall of the enclosure, terminates with weather heads and ties to secondary of existing transformer. They want to eliminate that service and run a new one underground to a pad mounted transformer, 120/208 volt three phase.

Now I am left with considering any implications for making the conversion. The only time I have ever made a conversion like this is when the entire building is being renovated and distribution system redone for the most part with new electrical and mechanical system. What are the implications that I have to consider?

 

[waross]

1> Phase rotation.
2> Lower voltage to 240 Volt loads.
Almost all heating loads will be content with 208 volts. The time to come from cold to operating temperature will be longer but once up to temperature there is seldom a problem. The on periods of the on-off cycle will be longer.
Motors. It is good practice to examine and evaluate every motor for suitability for operation on 208 Volts. Potential problems will be motors running at close to full load. A current check on motors running on 240 Volts will give some indication as the the loading. Some single phase motors may be reconnected for 120 Volts.
A few motors may have to be replaced.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[hbendillo]

Thanks. Let me add a complication to this situation that I should have explained before. The contractor had already begun the work of replacing the service with the blessing of the power company. The old service has been removed. Existing transformer enclosure still there for now but secondary from existing main panel completely removed. New underground conduits have been run to the pad mounted transformer location. No transformer installed yet. So no power to the building at this time. The building is not being used for anything at this point. This is an old Coca-Cola bottling plant that was used prior to this by a small manufacturing company. The new owner bought this property as pure speculation to either use as a warehouse or if he gets a better offer to rent or sell the property.

The city stopped the contractor short when he informed them of what he was doing and told him they needed engineering drawings to inspect and approve the installation. They told him they just wanted a one-line showing the work being done. They would then grant permit and inspect when done. Of course they want this done by Wednesday to present to the city. We went into this thinking that was all the engineering we had to do. Didn't know the existing service was a delta with high leg. I want to make sure I can do what the contractor asks or is more work involved. Is it incumbent on me to physicals check all the loads before making the changeover? I have already decided that all panelboards in the building need to be labeled indicating the service voltage.

 

[stevenal]

Some may say that 208 lies within +/- 10% allowed by NEMA for a 230 nameplate motor. I would remind those folks that 208 itself is a nominal rating subject to variation, and the ANSI low limits on 208 are below 230 less 10%.

Old transformers likely also means old motors that are unlikely to be dual rated 200/230.

 

[X49]

For loads that will not work well on 208V (most non-heating loads), you could consider installing buck-boost transformers for each load instead of replacing the equipment. They're a relatively economical way to boost the voltage from 208V to 240V. In many cases it will also be easier than sourcing replacement equipment on your tight schedule.

Even if a 230V motor is not fully loaded as measured, I would not run it with 208V unless it is dual rated 208/230V. It is likely a code violation.

 

[itsmoked]

All refrigeration compressors will be damaged. To prevent this use X49's suggestion.

Yes, you should check EVERYTHING in the building. You're putting your name at risk otherwise.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

This reminds me of an plant we serviced years ago. The parent company was building a new plant.
The motors at the existing plant were 460 Volt rated.
The utility had changed the standard supply voltage from 480V to 600V.
The parent company owned a plant in Eastern Canada where 600/347V was the standard and wished to ship out a quantity of 575V machines.
The design engineers had a unit service/power distribution center built.
This had a 600/347V service disconnect as per utility specs. There was a 600/347V section to supply the 575V equipment. There were then two transformers connected in an open delta auto-transformer configuration to supply 480 volts for the 460 Volt motors.
There the planning broke down. Underground conduits ran from the 600V section to the 575V motors. Underground conduits ran from the 480V section to the 460V motors.
Then the customer started changing 460V machines for 575V machines. There was no easy way to route conductors between the 480V section and the 600V section.
We solved that by using small transformer pairs at individual motors.
Sizing is easy. The low voltage winding must carry full load current so select based on the amp rating of the low voltage winding.
For instance a 120:12 Volt, 500 VA transformer will boost the voltage 10% from 208V to 220.8V and has a low voltage current rating of 42 Amps.
A 120:24 Volt, 500 VA transformer will boost the voltage 20% to 250 Volts and has a low voltage current rating of 21 Amps.
The best would be a 120:18 Volt transformer (rated for boost applications). This will give a 15% boost to 239 Volts.
Keith; Did you ever use the auto-transformer boost in your application? How did it work?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Keith; Did you ever use the auto-transformer boost in your application? How did it work?

Sure. Worked great! Thanks for your help with it.

Keith Cress
kcress -

http://www.flaminsystems.com