Open Delta 240v "B" leg knockdown

Status
Not open for further replies.
N

Npstewart

Senior Member
Hi guys,

I designed a project for 208-3 phase. Its for a small pizza place in Florida. I received pictures of the jobs site to design with, and basically went with that. The electrician finished the project and went to go turn the power on. Long story short he blew out 8 lights because they were connected to the B leg. I asked him if it was marked with red tape and he said it wasn't (if it was he probably ripped it off).
I told him we need to add another panel to give us enough room for the equipment because we would be losing so many 120V spaces. He said he that he suggests putting in a transformer to knock the B leg down to a usable voltage (120V) so he wouldn't have to re-wire it. The transformer would be about $3000.00.

I never heard of doing this, has anyone else? It doesent even seem possible! Also, is this legal? I always thought open delta was bitter sweet because you could get 3 phase power out of single phase, but the down side is that you lose a 3rd of the panel respectively.

Any opinions would be greatly appreciated...!!:-?
 
I designed all my calculations of 208-3 phase. Its an existing service... So on my drawings I put on the bottom of my load calc, 208-3 phase, and also on my panel schedule.

come to find out, it was 240v open delta, the electrician never verified. The POCO was never even involved.
 
Npstewart said:
...I designed a project for 208-3 phase.
... Long story short he blew out 8 lights because they were connected to the B leg. I asked him if it was marked with red tape and he said it wasn't (if it was he probably ripped it off).
... he suggests putting in a transformer to knock the B leg down to a usable voltage (120V) so he wouldn't have to re-wire it.

... is this legal?
Click to expand...
Bummer on the damaged equipment. Such be the perils of designing by telephone and drawings - and never actually looking at the site.

I don't know of any transformer that will correct just B phase. I suspect you want a 240D/208Y transformer - big enough to feed the panel.

cf
 
Around here, the POCO tells you what is availble and you design to that or pay BIG $$$$ to get what you want. The EC is at falt here. He should have known better. Just set a small sub panel for the lighting. FPL POCO doesn't care about the high leg. The electrician is required to mark it orange and put it on the right in the meter and move it to the center in the panel.
 
Npstewart said:
I designed all my calculations of 208-3 phase. Its an existing service... So on my drawings I put on the bottom of my load calc, 208-3 phase, and also on my panel schedule.

come to find out, it was 240v open delta, the electrician never verified. The POCO was never even involved.
Click to expand...
Kinda weak on the part of the EC. Do you have any 240 volt 1ph loads to load up the high leg with. I once built a 3ph service for a welding shop with the idea of using all 3 phases. POCO built 240 open Delta with a small pot for high leg. I burnt it off the pole and POCO flipped out. I explained the 3ph service was built specifically for the welders with the intent to use all three and I did. Nuff said . They fixied it.
 
It should have marked with orange tape, that being said, a red flag should of been thrown by the electrician when he saw nearly every third space was not used. If your equipment is rated for 240 volts single or three phase, and does not require a neutral, using the high leg is not an issue. Since it is a small pizza restaurant, a 100 amp single phase loadcenter would be sufficent for the 120 volt loads to free up space for the larger loads in the main panel.
 
So what I gather here is you guys are suggesting I feed a single phase 100A load center from the 3 phase 240v open delta panel to feed my single phase loads?

Would it be cheaper to just feed another 3 phase panel from the existing panel, and just skip every B load?
 
If you want to serve 3 phase loads with 208/120V, and have a 240V high leg service, then you will need a 240 to 208 delta to wye transformer, as CF mentions above.

Such 3 phase loads would include any 208V load that needs a neutral, as well as 120V multi-wire branch circuits.

It is _possible_ to use a transformer just on the high leg to create a 120V leg, however this would have several problems:
1) This 120V leg would not be balanced with the other 120V legs; L-L voltage would be about 170V and the current would not properly balance on the neutral, so you couldn't safely share a neutral.
2) Any 3 phase loads served would be wildly unbalanced, and would not operate efficiently.
3) You would be placing a single phase load on the wild leg, which can have unstable voltage to ground.
4) Any transformer solution will involve continuous losses in that transformer.

Many 3 phase loads are happier at 240V than at 208V, but some are only rated for the lower voltage. IMHO you should go with a subpanel for all single phase loads.

-Jon
 
Do you have a drawing of the panel schedule. Maybe the loads could be rearranged to "bypass" the problem or provide enough open space for a small LC to be put in.
 
I have a drawing of the panel schedule however the panel that I originally specd is FILLED. I have about 4 spaces left out of 42.


I understand the use and science behind open delta 240v 3 phase services, however they arent very common. I know this type of service is everywhere, but as far as design goes, its kind of odd thing to run across.


If I was to feed a single phase panel (ill refer to as panel "N") from the 3 phase 240v open delta panel (Panel "F"), I suppose I would just connect panel "N" to 2 circuits that are utilizing the A & C leg, (Say circuit 18 & 20) correct? I couldn't use 2 circuits that are connected to the high leg correct?
 
Npstewart said:
I understand the use and science behind open delta 240v 3 phase services, however they arent very common. I know this type of service is everywhere, but as far as design goes, its kind of odd thing to run across.
Click to expand...
It's easy to forget that electrical supplies have evolved over time. We didn't start out with 208Y/120v (or 277Y/480) 3ph services. They became necessary as larger, 3ph motor loads gained in popularity.

The high-leg Delta began as a 3ph modification to existing 1ph services in industrial buildings. It helps to remember that the 120/240v section of a high-leg service is exactly the same as a 1ph supply.

One can have an existing 1ph service and convert it to 3ph by adding one (open Delta) or two more transformers. New services in existing areas are simply designed to work with existing POCO equipment.

If I was to feed a single phase panel (ill refer to as panel "N") from the 3 phase 240v open delta panel (Panel "F"), I suppose I would just connect panel "N" to 2 circuits that are utilizing the A & C leg, (Say circuit 18 & 20) correct? I couldn't use 2 circuits that are connected to the high leg correct?
Click to expand...
Correct. Line-to-line loads don't care which, if any conductor is grounded, only the voltage between them. Line-to-neutral loads, on the other hand, do care, so A-N-C is the way to go for your needs.

Again, your 1ph sub-panel will be exactly the same as any 120/240v 1ph sub-panel. Simply ignore the high leg,
 
Status
Not open for further replies.
Top