3 Phase Delta High Leg from 2 transformers?

[aelectricalman]

Hi everyone,

Theres a lot of new faces since my last visit. Looks like the forum is doing well.
I've recently been awarded a job where we have a delta (High leg) system to tap from. Point blank, Ive done a few wye systems but never a delta. I understand the theory but am still learning the hands on. The service will be a 400amp 120v/240v three phase Delta with a single phase 240v/120v panel. The PoCo has done us a favor by letting us take 2 feeders from the coffin handhole to the business, 1 for single phase panel and one for three phase panel. The PoCo will supply feeders via 2 five inch conduits down the pole and into the handhole. I think what we are going to do is run 3 laterals from the handhole to the building - 1 single phase 200A set of feeders (4 a aluminium ) and two parallel sets of 200 a aluminium (4 a aluminium) to the 3phase portion. I have a few questions. 1. The system before the burnout, was the same and I am confused as to why there are only 2 transformers,not three. There appears to be some funny business going on with the PoCo. How did they get a delta from 2 xfmrs? 2. Will this make my high leg or any configurations any different than the standard delta high leg system. 3. Does anyone see this causing any problems during my install. 4. Does anyone see anything I could do to help me save time and money on this install. Thanks for your help. I hope you all had a great Christmas.

 

[russ]

This is an open delta system. The larger of the two transformers will have your 120 /240 load, the second transformer will be joined to the first, to create the third leg. Because the third leg is so far from the grounded conductor in the middle of the first transformer it has a higher voltage to the grounded conductor.

 

[lordofpi]

Yes, open delta. This is something most power companies do to save money by only needing two transformers. For lighting and small appliance loads, it really is not big deal, but for large motors, it can definitely be inefficient since I believe you are limited to about 58% of the normal power capacity. I never really liked this, especially since if you actually were to lose a phase on open delta, you are screwed, whereas on a closed delta you can still function with a three-phase load.

 

[micromind]

Think of an open delta this way; it's a single phase system with a second xfmr added to create the 3rd phase (the high leg).

It's very common to have a single phase panel right next to a 3 phase panel. It looks like your design will work ok. To be code compliant, the high leg must be the B phase, and it must be colored orange. You can use a 2 pole breaker on the high leg and one of the other phases, but it must be rated at 240 volt, not 120/240. This is because if a ground fault develops, and it's the high leg to ground, the 120/240 breaker can only interrupt 120 across one pole, (240 across both poles), and the high leg is 208 to ground.

These systems are quite common in buildings around here where the majority of the load is 120V, and there are a few 3PH A/C units, or other 3PH equipment.

 

[LarryFine]

A, as Micro touched on, the 120/240 portion of the system (normally referred to as the A and C phases) is exactly, and I mean exactly, the same as a simple 120/240v 1ph system. There is a grounded center tap that is properly called the neutral, there's 120v between each line and neutral, and 240v line-to-line. You can supply the usual 1-ph loads as you see fit.

The only difference is that there is a third line conductor, that has 208v to neutral on it. There will be 240 between any two phases, and any 3-phase load can be supplied, as well as any 240v 1ph load between any two phases. You cannot connect a load from B to neutral, though. This could be supplied by a trio of transformers as you seem to be able to picture, or an open Delta, 2 transformers.

How does that work? Let's say you have a 3-transformer Delta supply. Each line is taken from where two transformer winding terminals meet up, the 'corners', and we label them A, B, and C. In theory, you could open up the pair of wires at one corner and you should read zero volts across the opening. Likewise, you could remove one 240v winding and still read 240v across the gap.

As usual, I got long-winded and I have to go now. More later if you need it.