Load Calculations
Load Calculations
(OP)
When calculating the load for a building, how do you spread the single and three phase loads over the incoming three phase lines? I belive the correct way for 120/208 Wye is that single phase 208 or 240 loads spread equally across two transformers (A&B, B&C, or C&A) whereas for 120/240 Delta, 208 or 240 single phase load is applied to only one transformer (A, B, or C)/phase (just as the 120 loads are only on the A & C phases). The three phase loads are spread equally across all three phases.






RE: Load Calculations
I think you have it. Your 240 volt loads only receive 208 volts. Many small motors are now rated for 230 or 208 volts.
yours
RE: Load Calculations
In 120/240V/1ph/3wr system( now common in some residential and industrial distribution sectors), 120v loads are connected between one of the phases and the neutral point, 240v, loads between the two outboard phase points. In 240v/3ph services there are no 120v. loads, unless a center earth point is established.
RE: Load Calculations
I took the original post to refer to an open delta service, or 4 wire delta.
This is a basic 120/240 volt service with a second transformer added in open delta to support thre phase loads. In some localities a third transformer is added and the service is three phase delta with one winding center tapped and grounded to support the 120 240 volt loads. Caution must be excersised as there is a "Wild" leg or phase with 208 volts to ground. This is not used for 120 volt loads and often not used for 240 volt loads on open delta services.
yours
RE: Load Calculations
I was just talking to some people at my office today about the Dela High-Leg configuration, and there are a few things I don't understand.
-If 240V is obtained between the high leg and another phase, then why would it not be used? I was originally under the impression that the high leg was only utilized for three phase loads, but one of the engineers explained to me that the high leg can also used for 240V single phase loads.
-When a winding is center tapped in a Delta configuration, aren't the 120V loads limited to 5 or 10 percent of the transformer's rating? I recall reading something along those lines in SquareD's catalog, although it could have only applied to a specific transformer. I don't even remember.
Thanks,
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RE: Load Calculations
Two of them are:
1> There is a large single-phase load and a small three-phase load must also be supplied. A small transformer is connected open delta to supply the three-phase load.
The single-phase load may be a normal 120/240 volt local distribution serving an area of residential and commercial customers. For some reason a customer requires three-phase.
It is not uncommon to see a 50 KVA or a 75 KVA or a 100 KVA transformer with a 15 KVA transformer sitting beside it to feed the open delta.
One example I saw years ago was a bowling alley. They were on a 120/240 Volt distribution system supplied with pole mounted, utility owned transformers.
They bought new automated pin-setting machinery and then discovered that the special motors were all three phase. One possible solution was for the utility to add a transformer in open delta and supply them with 4-wire delta.
A common rural installation is a three-phase irrigation pump. If the pump is in a remote location served by a single phase primary, only one additional primary line need be run.
In this case it is common practice to avoid using the "Wild" leg for any but three phase loads. It can be used for 240 volt loads but you must be aware of the loading on the smaller transformer.
2> In a three transformer closed delta system; the "Wild" leg may be used for 240 volt loads. It should not be used for line to neutral loads. The voltage is 208 Volts rather than 120 Volts.
Note; RUS standards warn that in a three transformer Wye-Delta transformer bank the primary neutral must be left floating or transformer burnout may result.
I know of no load limit on the 120/240 Volt loading. Your information may have been for a special application or may have been misunderstood.
In a full delta bank the single-phase load can safely exceed the rating of the center-tapped transformer if the three-phase load is low.
In fact if there is no three-phase load, the single-phase load on a delta bank with equal sized transformers may be double the capacity of one transformer.
That is; A bank of three 100 KVA transformers in delta.
The single-phase load on the 120/240 Volt center tapped transformer may safely be 200 KVA. Any three-phase load and any single-phase load connected to the other transformers will of course reduce the 200% figure.
Before there is a howl of protest over this statement and the 200% load figure, spend a little time researching the conversion factors for supplying single-phase power from a three-phase generator with particular attention to the double delta connection.
respectfully
RE: Load Calculations
RE: Load Calculations
RE: Load Calculations
The 4 wire delta is common in many areas, in other areas it is never seen.
I am wondering why you would choose a delta connection?
What are your loads?
Basically combine your single phase loads so as to be equivalent to one three phase load. Add this to the normal three phase loads.
Subtract this from the capacity of the three phase bank.
Example;
Three 50 KVA transformers
Three phase loads = 75 kva.
Single phase loads combined as one three phase load = 30 KVA
Total three phase load = 105 KVA
Excess capacity = 45 KVA. or 15 KVA per transformer.
The center tapped transformer has 15 KVA of capacity available for single phase loads. The other two transformers will contribute an additional 15 KVA of capacity to the third transformer.
The unbalanced single phase loads in this example may be 30 KVA without over loading the transformer bank. The transformer bank capacity with this combined loading will be;
105 KVA three phase plus 30 KVA unbalanced single phase = 135 KVA on the 150 KVA bank.
A factor that I never see mentioned in load calculations is the power factor of the individual loads. If the loads have widely varying power factors the actual current will be less than indicated by the simple sum. Rather than calculate this, I suggest that you be aware of it and consider it an error on the safe side.
For widely differing loading or differing transformer sizes, do not allow the single phase load to exceed twice the available capacity of the smallest transformer or overloading of the smallest transformer may result.
I prefer Wye.
davidbeach, I spent some time lost in the Square D web site, trying to research your comments. I will be pleased if you can expand on your statments and or give a link where I can research the information. I remember hearing something similar years ago but thought I had misunderstood it, or that it had been mis-stated. Now I am curious. Am I correct in assuming that this is not a factor in building services?
Thank you davidbeach.
respectfully
RE: Load Calculations
I believe the limit is on 3phase unit transformers not the transformer banks.
RE: Load Calculations
If you have a 12 lead generator you can reconnect it for 4 wire delta. No performance problems. Your KVA rating will jump up about 15% as well.
If you have a 10 lead generator, you're out of luck.
yours