Delta load calculation using 208 High leg for single phase 240 circuits

[11bgrunt]

Looking at a load calculation provided by EC for a plant upgrade. The service will continue to be 4 wire, 240 three phase from a POCO closed delta overhead bank. His calculation sheet is using L1-120, L2-208 and L3-120.
On this calculation sheet the EC is using L2-208 as one leg to serve half of 240 volt single phase circuits. I have never done delta calculations this way. I do not believe this type of service was intended to be used like this. I have always served single phase from the two 120 volt legs and the only time the 208 high leg was used was when a three phase breaker was used that covered all three legs. I am not aware of a physical block that will keep him from using the 208 leg as proposed. Is there an NEC rule that would allow/not allow this? If I am sizing transformers to serve this load, his calculation method will force the EC to get it right and the POCO will have a hard time sizing transformers to serve the load correctly.
ActionDave and infinity have written in separate post that the EC proposed method is acceptable. I have never even considered his method until today and always loaded 2/3 of the single phase load on the lighter transformer.

http://forums.mikeholt.com/showthread.php?t=173696&highlight=Delta+load+calculation+using+208+High+leg+for+single+phase+240+circuits
http://forums.mikeholt.com/showthread.php?t=170172&highlight=Delta+load+calculation+using+208+High+leg+for+single+phase+240+circuits

Do I have it all wrong?

 

[ptonsparky]

The load and the transformers will not care. The load is looking for 240 volts and the transformers are supplying it.

The CBs need to be properly selected to be used with that system, but that's about it.

 

[kwired]

As Tom mentioned you need to watch the rating of the breakers - they need to be rated straight 240 and not a 120/240 volt rated breaker. But otherwise nothing wrong with that. If you have a open delta or a larger lighting pot and smaller wing pots you may need to consider how much load the high leg can handle from the source, but with three of the same size pots, why wouldn't you want to try to balance loads - especially if you had significant amount of 240 volt single phase loads?

 

[11bgrunt]

As Tom mentioned you need to watch the rating of the breakers - they need to be rated straight 240 and not a 120/240 volt rated breaker. But otherwise nothing wrong with that. If you have a open delta or a larger lighting pot and smaller wing pots you may need to consider how much load the high leg can handle from the source, but with three of the same size pots, why wouldn't you want to try to balance loads - especially if you had significant amount of 240 volt single phase loads?

I have never had three pots of the same size in a closed bank when serving a large single phase load and smaller three phase. The outside transformers were to make the three phase work and the one in the middle was for the single phase. The loading was always 2/3 of single phase - 1/3 of three phase on the lighter and 1/3 of single - 2/3 of the three phase assigned to the two outside transformers. What I have always done, and never thought any more about it not being the right way. In a panel I would never stab a double pole breaker in the 208 slot. Not because it wouldn't work, I just wouldn't do it because i knew how the bank was sized. I guess I can start to see that the load balance by the EC is really no different than if it was a wye secondary. At this POCO, wye is the standard issue today for new customers requesting three phase. If a delta is requested it requires an Engineering waiver.

Thanks for the responses.

 

[kwired]

I have never had three pots of the same size in a closed bank when serving a large single phase load and smaller three phase. The outside transformers were to make the three phase work and the one in the middle was for the single phase. The loading was always 2/3 of single phase - 1/3 of three phase on the lighter and 1/3 of single - 2/3 of the three phase assigned to the two outside transformers. What I have always done, and never thought any more about it not being the right way. In a panel I would never stab a double pole breaker in the 208 slot. Not because it wouldn't work, I just wouldn't do it because i knew how the bank was sized. I guess I can start to see that the load balance by the EC is really no different than if it was a wye secondary. At this POCO, wye is the standard issue today for new customers requesting three phase. If a delta is requested it requires an Engineering waiver.

Thanks for the responses.

I see a lot of 240 delta with high leg systems that have three equal sized transformers for the supply. Nearly all of them are sites with a majority of motor load. If one has large lighting pot and smaller high leg pot(s) then one should consider that when connecting loads to the high leg, but if they are all same size and you want to get more out of the system using the high leg for single phase loads does work. It would be no different to the system then if you applied the same loads to a corner grounded system.