We have used 690V on some industrial projects.
Single phase transformer fed by delta high leg
- Thread starter Schewe
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ptonsparky
Tom
- Occupation
- EC - retired
Thank you.
I see I misread the original post. For some reason I had read it as the load transformer primary connected high leg to neutral, which would be hokey. It would probably work with no effort but that would be hokey.
"Single phase transformer fed by delta high leg" (I read that fast, it's not high leg to neutral).
Anyway, reading it again OP appears to only inquire about what line pair if any makes a difference, ignoring the neutral.
No it makes no difference. Any line to line pair would work the same, except for trying to balance the load, on the supply winding with the lower load or extra capacity. That's where you would Amp clamp the supply conductors to find two with the lowest load and also look outside at the pole to see if you have a unit with equal supply windings (most common type around here), or a transformer bank built with larger capacity on the 120 / 240 side.
Equally sized supply winding you would avoid the 120 / 240 winding side as that's where the extra load would be and load on one of the 240V only windings for balance.
Most common mistake I see with red leg deltas is the transformer specc'd when they're planning all single phase loads.
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- Licensed Electrician
Where is that, or was that ever common?
There is a large old industrial building I *reluctantly* work on that has a 240V delta (used to be an open delta, I had it converted to full delta) the building has been converted from manufacturing to 'loft space'. Many of the 240V loads are now single phase including 240V lighting, servers, water heaters, kilns, smaller motors (less than 5 HP) etc, the only 3 phase load is a HVAC on the roof and the elevators. Since the elevators, HVAC, parking-lot lighting and other lighting is 240V there is not much 120V load it did not make sense to convert it to a 208Y120. I imagine these type of repurposed buildings are common.
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- Licensed Electrician
That's a far cry from a delta xformer being specc'd and especially one to be used with all single phase loads.
The last 240V delta I installed wasn't specc'd, it was ordered by mistake. The only one I've ever put in that was specc'd was for a printing press.
The last 240V delta I installed wasn't specc'd, it was ordered by mistake. The only one I've ever put in that was specc'd was for a printing press.
The old ones were common for machine shops for full 240V 3 phase. That's probably from the 1950's around here. Usually the utility supply transformer and I've never recommended changing the service when we have something that works. Old machine shops were common around here in large old industrial buildings, probably WW11 era. Lots of those.
On a new drawing new commercial I would run across them on the drawings and I always ask them, how much single phase do you want. Usually all of it and it's obviously office receptacles and maybe some lighting but not sure about that.
Getting those changed to Y transformers has never been a problem, changed on the drawing. Never had one show up misordered, but if it's on the drawing I can see how it happens.
It's not uncommon around here to run into a reg leg delta on the pole, one can I believe. Sometimes it's 3 phase only but checking the triplex it's red leg delta.
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