Service change 240V--->208

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Npstewart

Senior Member
Hi everyone

Doing a project where we are renovating part of a building do be a shop. The current service is 240-3 open delta. The local POCO will not allow (and for good reasons) to run our 50HP air compressor on an open delta service. They said they are going to bring 208-3 phase to our building instead.

My suggestion was to do a ADDITIONAL service to run our new shop equipment, and keep the open delta service to serve the existing loads that way we wouldn't have to be worried about overloading the existing service when we add on.

The gentleman at the POCO is suggesting (as well as the electrical contractor) that we bring in the new service and run EVERYTHING with the new 208-3 service.

Because this is an existing building however, my main concern is that some of the breakers will now be overloaded because the voltage is 208 now rather then 240. I see this as a legitimate concern, however everyone else seems to think differently, am I over-thinking this?

Thanks
 

Dennis Alwon

Moderator
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Location
Chapel Hill, NC
Occupation
Electrical Contractor
I would not be as concerned about overload as I would be about the equipment being able to run on 208. You may need some step down transformers or new equipment.
 

hurk27

Senior Member
Dennis is right, if that air compressor has a 240 volt motor it will not last on 208, and will need to be switched to a 200 volt rated motor, the other problem is cost, you will most likely pay an upfront cost for the new service and they will take it off the bill as you use electric, by switching all your loads to the new service will allow you to reduce this cost much sooner, (not all POCO's do this)
My choice would be to add a small 240 volt delta service (100-200amp) to pick up this compressor with a little room for a few more loads in the future, and leave the existing service as it is, unless you are adding more loads, by changing the whole service to 120/208 will require checking all 240 volt existing loads to see if they can operate safely on 208, which might cost you much more in the long run if you wind up changing out motors or equipment.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
I often specify buck-boost transformers for existing 240V loads being served by 'upgraded' 208V services.

You really need to know how your loads are presently connected and the voltage they are rated for.
 

kwired

Electron manager
Location
NE Nebraska
..My choice would be to add a small 240 volt delta service (100-200amp) to pick up this compressor ..
The OP has stated that he already has this. The fact that he wants to add a 50 hp compressor is why he is having problems.

My Question is why can't the POCO add another transformer and make this a "closed delta" and keep all the existing equipment as is. Many think that open delta is the only 120/240 three phase system available when in fact it is not. 3 equal sized transformers will allow equal loading on all three phases. A larger center transformer will allow for higher 120/240 single phase loads.
 

Jraef

Moderator
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Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
What are the "other" loads? If they are primarily 120V loads, the 208Y120 system will actually be better, easier to balance. Of the "other" 240V loads (if any), what are they? Heaters? No issue there. Lighting, probably no issue there either. Motors? Maybe, maybe not.

Some "240V" 3 phase motors are really 230V 3 phase, and in fact many are actually 208-230V, which means they are really 220V motors, figuring that the +- 10% tolerance covers from 198-242V.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
My Question is why can't the POCO add another transformer and make this a "closed delta" and keep all the existing equipment as is.
Many utilities are trying to 'eliminate' 240/120 3PH 4W services altogether, so they use any service upgrade as an opportunity to force the customer into a wye system.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
The OP has stated that he already has this. The fact that he wants to add a 50 hp compressor is why he is having problems.

My Question is why can't the POCO add another transformer and make this a "closed delta" and keep all the existing equipment as is. Many think that open delta is the only 120/240 three phase system available when in fact it is not. 3 equal sized transformers will allow equal loading on all three phases. A larger center transformer will allow for higher 120/240 single phase loads.
I agree, why not change to full delta? It's cheaper for everbody, The POCO uses the same meter.(provided the exsisting service is already large enough) They only have to add one transformer, nothing would have to be changed in the panels (same provision)
 

kwired

Electron manager
Location
NE Nebraska
Many utilities are trying to 'eliminate' 240/120 3PH 4W services altogether, so they use any service upgrade as an opportunity to force the customer into a wye system.
What is the problem? They use the same transformers that are used for single phase services so they are readily available.

They will often feed multiple customers from them if they can, both single and three phase.

Wye systems are usually dedicated to a single customer - sometimes multi-tenant but still a single service being fed.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Couple of thoughts:

A change to a full Delta might require upsizing the high-leg conductor size.

If changed to 208v, every panels' loads will likely have to be re-distributed.
 

kwired

Electron manager
Location
NE Nebraska
A change to a full Delta might require upsizing the high-leg conductor size.
No problem from me with that, should be sized the same as the other phases anyhow, if it is not and switching to 120/208 then there is even more reason why all three phases should be the same size.

I have seen many high legs sized smaller than other phases - I have never installed one this way.

If using fuses and a lower amperage fuse is installed, I guess that may be acceptable, but if there are circuit breakers for overcurrent devices , I have yet to see a three pole with one of the poles having a different trip setting than the other poles.
 

hurk27

Senior Member
No problem from me with that, should be sized the same as the other phases anyhow, if it is not and switching to 120/208 then there is even more reason why all three phases should be the same size.

I have seen many high legs sized smaller than other phases - I have never installed one this way.

If using fuses and a lower amperage fuse is installed, I guess that may be acceptable, but if there are circuit breakers for overcurrent devices , I have yet to see a three pole with one of the poles having a different trip setting than the other poles.
Most high leg deltas I have worked on had a small 100 amp 3-phase panel with no neutral, and a second 200 amp single phase panel for all the neutral loads, many of these just fed a small 3-phase load such as air, or pumps, and the rest of the place was all single phase, these would have a larger center tap tank on the pole then the two other tanks, but I have also seen the opposite where the two other tanks were large and the center tap tank was small, and on these most of the loads were 3-phase with only a small office using the neutral, our POCO here will not install anymore 4-wire deltas, so you get either 120/208 Y or straight delta, I think I have only seen two or three 4-wire deltas where all three tanks were the same size, so this is not a very common service set up. but if they are switching to 120/208 Y then all three tanks will be the same size, or they will have a very unbalanced neutral.
Of course our primary feeds are a Y feed so this would be a must.
 
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kwired

Electron manager
Location
NE Nebraska
Most high leg deltas I have worked on had a small 100 amp 3-phase panel with no neutral, and a second 200 amp single phase panel for all the neutral loads, many of these just fed a small 3-phase load such as air, or pumps, and the rest of the place was all single phase, these would have a larger center tap tank on the pole then the two other tanks, but I have also seen the opposite where the two other tanks were large and the center tap tank was small, and on these most of the loads were 3-phase with only a small office using the neutral, our POCO here will not install anymore 4-wire deltas, so you get either 120/208 Y or straight delta, I think I have only seen two or three 4-wire deltas where all three tanks were the same size, so this is not a very common service set up. but if they are switching to 120/208 Y then all three tanks will be the same size, or they will have a very unbalanced neutral.
Of course our primary feeds are a Y feed so this would be a must.
Delta systems are common especially on farms where I live. You have advantage of 120/240 single phase as well as advantage of having three phase for motor loads. If there is a lot of motor load the transformers are all the same size because the capacity is needed. If you run into 120/208 on a farm it is usually a fairly dedicated service to a particular facility, and not like the old small family farm service that feeds the house and all of the outbuildings.

Modern farm buildings or structures are for the most part small industrial facilities anymore.
 
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