208 volt high leg

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arits74

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dixie arkansas
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working owner electrician
does using the high leg from a 120/240 three phase system to supply 240 volt single phase loads like a welder or an air compressor cause premature equipment failure in yalls opinion
 
arits74 said:
does using the high leg from a 120/240 three phase system to supply 240 volt single phase loads like a welder or an air compressor cause premature equipment failure in yalls opinion
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Your talking line to line right? Yeah, no problem there.
 
If the equipment stays within the manufacturer's voltage specs: no problem. The equipment does not care if it comes from a high-leg or not as it just sees a voltage. The high-leg will deliver an adequate supply for the load as long as the transformer is not over-loaded.
 
arits74 said:
no,just a general question on the use of a 208 volt high leg to supply single phase 240 volt loads
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The only issue seemed to finding the breaker with the appropriate rating, which I think some did in another thread (GE, I think).
 
If you are asking about feeding the 240 loads from the high leg and one additional leg you are fine. If the supply is an open delta you do have to watch the load on the high leg because the poco frequently uses a much smaller transformer. If you are asking about feeding the 240 volt loads using the high leg and the grounded conductor you should not do that.
 
mivey said:
Do you have a good reason other than it might be hard to find the right breaker?
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Or that the high leg voltage might be outside the manufacturer's recommended supply voltage range?

Add: That sounds leading so I'll save you the trouble of looking: There is nothing physically wrong with using the high-leg voltage. As with any single-phase loading on a three-phase bank, you want to be sure you do not over-load the transformer.
 
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I'm not strong in the theory department but I have read that you should not use the high leg with the grounded conductor for single phase loads. It seems like you are loading 2 transformers (1/2 of the main transformer plus the high leg transformer) to make it work which doesn't seem like a good idea but I can't back up my thought. None of the manufactures make a 240 volt breaker for standard 120/240 distribution equipment so you would need to use one pole of a 240 volt multi pole breaker or use 277/480 rated equipment.
 
curt swartz said:
I'm not strong in the theory department but I have read that you should not use the high leg with the grounded conductor for single phase loads. It seems like you are loading 2 transformers (1/2 of the main transformer plus the high leg transformer) to make it work which doesn't seem like a good idea but I can't back up my thought. None of the manufactures make a 240 volt breaker for standard 120/240 distribution equipment so you would need to use one pole of a 240 volt multi pole breaker or use 277/480 rated equipment.
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There is no problem with it working. You just make sure the transformer is not over-loaded.

You have a load connected highleg to ground. The current splits at the ground and 50% goes through one 1/2 of the winding and 50% through the other half. Each 1/2 winding is in series with another full winding. See the diagram here where I detailed the splitting of the current:
http://forums.mikeholt.com/showpost.php?p=1154933&postcount=114
 
Other then the breaker rating being wrong, I would say if any 240 equipment has 120 volt control circuits or loads in or on the equipment, using the high leg will burn up these loads or controls. been there done that:mad:
 
The OP didn't say if this was a closed or open delta service.

If this is an open delta service, then the voltage across the 'open jaw' is less stable than the voltage across one of the closed legs.

Additionally, the transformers in an open delta service are often differently sized, with different impedance. Generally they will have been sized so that all of the needed single phase capacity is on the grounded transformer, and the 'stinger' is just for the three phase loads.

-Jon
 
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