3phase transformers

[ceb58]

We have a communication building at a site with 2 buildings. The site is fed with 120/208 3 ph service to a meter bank. The building that we have our equipment in is going to be moved out and we are going to move one of our prefab buildings in. Our prefab buildings are 120/240 sp. I have priced a 3 ph to 1 ph transformer for the building but am now being ask if the building can be hooked up 208 sp. It is not a problem with our A/C units, they are slash rated, the UPS and the microwave rectifier. My concern is what the unbalance will do/can do the the utility transformers. The other building on the site will remain 3 ph. The load on our building will be around 80-100 amps. What do you think?

 

[fmtjfw]

We have a communication building at a site with 2 buildings. The site is fed with 120/208 3 ph service to a meter bank. The building that we have our equipment in is going to be moved out and we are going to move one of our prefab buildings in. Our prefab buildings are 120/240 sp. I have priced a 3 ph to 1 ph transformer for the building but am now being ask if the building can be hooked up 208 sp. It is not a problem with our A/C units, they are slash rated, the UPS and the microwave rectifier. My concern is what the unbalance will do/can do the the utility transformers. The other building on the site will remain 3 ph. The load on our building will be around 80-100 amps. What do you think?

Gee, I'd check with power company, but an 80--100 amp load sounds like a big residence. Lots of apartment buildings are now 120/208, with single phase to each unit. Isn't buying a 3-phase to 1-phase xfmr a lot to spend?

 

[ceb58]

Gee, I'd check with power company, but an 80--100 amp load sounds like a big residence. Lots of apartment buildings are now 120/208, with single phase to each unit. Isn't buying a 3-phase to 1-phase xfmr a lot to spend?

Transformer is in the $2000.00 range. I just need to know if we are looking at a melt down.

 

[Dennis Alwon]

This is something I don't quite understand either. Does a 3 phase to 1 ph trany actually use the 3 phases so that there is a load on the line side of all 3 phases. It would not seem necessary if that were the case. If it did then you should be ok---

Obviously I have no idea just thinking aloud.

 

[fmtjfw]

Transformer is in the $2000.00 range. I just need to know if we are looking at a melt down.

What is the size of the total load and what is the size of the utility transformer(s). Bear in mind that Utilities do not follow NEC in load calculations. If you have 100A 240V that is 24KVA, or 12KVA per xfrmr A & B. if you use a Scott connected transformer that can't go below 8KVA xfrmer A, B, & C. You are only saving 4KVA per transformer. I doubt if the Utility would worry about that.

 

[fmtjfw]

What is the size of the total load and what is the size of the utility transformer(s). Bear in mind that Utilities do not follow NEC in load calculations. If you have 100A 240V that is 24KVA, or 12KVA per xfrmr A & B. if you use a Scott connected transformer that can't go below 8KVA xfrmer A, B, & C. You are only saving 4KVA per transformer. I doubt if the Utility would worry about that.

see http://forums.mikeholt.com/showthread.php?t=143585

There may be no benefit. I'm only thinking of a theory about a black box into which 3 phase goes and 1 phase comes out. It may be that the inherent imbalances in practical solutions don't get you what you want. For instance a 3-phase generator connected zig-zag for single phase gives you 66% of capacity. The referenced thread talks about imbalance in transformers.

 

[Dennis Alwon]

The utility transformers are not an issue per se but they do not like an unbalance load. If that were the norm on their distribution it would cause problems I believe.

 

[mivey]

The utility transformers are not an issue per se but they do not like an unbalance load. If that were the norm on their distribution it would cause problems I believe.

They do not really care as long as you do not overload the winding. The problem with unbalance is more of an issue for you. So it is not so much of a question as to what the unbalance will do to the transformer but what the resulting voltage unbalance will do to your motors.

This voltage unbalance is mitigated somewhat by putting larger transformers on the portion serving the single-phase loads. If all three phases have the same size transformer, and the single-phase load is significant, you may have a problem with sensitive/heavily-loaded motors.

With all that said, the single-phase load you quoted is less than 15 kVA per phase. How big is the serving transformer and how big is the 3-phase load at the rest of the facility? If you have a bank of (3) 15 kVA units and some sensitive motors, that might be a problem for you, not for the transformer. However, if you have a bank of (3) 100 kVA units, I doubt the voltage unbalance would be an issue unless the motors were very sensitive (I have not crunched the numbers).

 

[mivey]

With all that said, the single-phase load you quoted

Actually that was ceb. Don't worry Dennis, you are in good company.