240v High Leg Question

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drexel

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I am working on a remodel of an existing building with 240/3 phase/4 wire high leg service. The old use was an office/retail, the new use will be a gym. The old service will remain, but we will be adding a lot of single phase in the way of treadmills and stair climbers, etc. My question is in the resulting imbalance of the phases. The high leg will only run the rooftop units, which at full connected load (summer AC running) is only 25% of the total load. In the winter, the 3 phase load will be much less. I have seen a number of these types of set ups, where the phases are unbalanced, and everything is fine, but the thing that bothers me is the magnitude of the imbalance due to the single phase exercise equipment. Is this a case where I should worry about the HVAC motors burning out due to the imbalance, and therefore ask for a transformer (240/3 to 208/3) to provide a more balanced 3 phase load?

The new service for the space will actually be smaller. The old CT was 1100A, our new one is 800A. And they are reusing the rooftop units which will remain 3 phase.

Thanks in advance for any direction.
 
drexel said:
I am working on a remodel of an existing building with 240/3 phase/4 wire high leg service. The old use was an office/retail, the new use will be a gym. The old service will remain, but we will be adding a lot of single phase in the way of treadmills and stair climbers, etc. My question is in the resulting imbalance of the phases. The high leg will only run the rooftop units, which at full connected load (summer AC running) is only 25% of the total load. In the winter, the 3 phase load will be much less. I have seen a number of these types of set ups, where the phases are unbalanced, and everything is fine, but the thing that bothers me is the magnitude of the imbalance due to the single phase exercise equipment. Is this a case where I should worry about the HVAC motors burning out due to the imbalance, and therefore ask for a transformer (240/3 to 208/3) to provide a more balanced 3 phase load?

The new service for the space will actually be smaller. The old CT was 1100A, our new one is 800A. And they are reusing the rooftop units which will remain 3 phase.

Thanks in advance for any direction.
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Did I miss something or did you completely blow past who the 240/120 3ph 4 wire is being feed? There are 2 options, either with a single transformer or 3 transformers, 2 single phase and one 1 ph with a center or lighting tap. The 3ph4w single transformer is not a good application for this but the 3 separate 1ph transformers configured in a delta is where you are able to increase the size of the transformer with the center tap to an appropriate size to serve the 120/240 1p3w load.
Your application should pose to be no problem should you be able to apply three 1ph transformers as having to derate a singe 3ph transformer may prove to be an issue.
 
This is where open deltas are popular with POCO's, small three phase loads, but fair size single phase loads. The transformer that feeds the single phase loads is much larger than the 2nd or 3rd transformer (full delta) load balance is not as important as it is with a wye. The issue may be on the POCO's end, I have had the POCO move loads on their end to help balance their lines. (Rural store, with a large single phase service changed to three phase for new A/C's)
 
Some of the members with POCO connections will be better suited to answer your questions than I am, but I have seen a fair number of these type installations with very limited 3 phase load and heavy single phase load with no particular problems.
I would check with POCO to make sure they are aware of the proposed loading. Locally they now prefer 208Y/120 but many older 240/120 systems are still in service.
As hillbilly notes, the old open delta systems were common for loads such as yours.
 
If this was office spaces before the high leg likely was not loaded all that much then either. Possibly HVAC was the only load on the high leg before. If this is an open delta with a large single phase transformer and a small pot for the high leg, then it was designed for high single phase load and light three phase load to start with.
 
There are actually 3 pole transformers, two of them sized larger than the third. There are actually two services here, so I need to try and go sort out what is what.

I was worried about the problems on my side of the utility transformer. Am I hurting my three phase motors. The contractor has proposed adding a large (300KVA) transformer inside the building to take care of the single phase loads, but I need to do some research on that. That would make all the load on the utility transformers balanced, and like I said, they are unequally sized right now. I'm going to get in tough with the utility and see what info they were given up to now. I'm being pulled on this job after the fact.
 
drexel said:
There are actually 3 pole transformers, two of them sized larger than the third. There are actually two services here, so I need to try and go sort out what is what.

I was worried about the problems on my side of the utility transformer. Am I hurting my three phase motors. The contractor has proposed adding a large (300KVA) transformer inside the building to take care of the single phase loads, but I need to do some research on that. That would make all the load on the utility transformers balanced, and like I said, they are unequally sized right now. I'm going to get in tough with the utility and see what info they were given up to now. I'm being pulled on this job after the fact.
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Are all three transformers part of the same bank, or is one of them supplying another separate single phase system?

I guess it is possible that the two larger ones are connected in parallel effectively making them one larger unit.

Your best bet is contact POCO and find out just how much max load (single and three phase) they want connected to this system.
 
hillbilly1 said:
This is where open deltas are popular with POCO's, small three phase loads, but fair size single phase loads. The transformer that feeds the single phase loads is much larger than the 2nd or 3rd transformer (full delta) load balance is not as important as it is with a wye. The issue may be on the POCO's end, I have had the POCO move loads on their end to help balance their lines. (Rural store, with a large single phase service changed to three phase for new A/C's)
Click to expand...

Now we're getting to the point that I was refering to.
 
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